C:Documents and SettingslisacLocal SettingsTemp~hh9web.gps.caltech.edu/gislab/HowTo/ESRI - What is...

A geographic information system (GIS) is a system used to describe and characterize the earth and other geographies for the purpose of visualizing and analyzing geographically referenced information.

Many have characterized GIS as one of the most powerful of all information technologies because it focuses on integrating knowledge from multiple sources (for example, as layers within a map) and creates a crosscutting environment for collaboration. In addition, GIS is attractive to most people who encounter it because it is both intuitive and cognitive. It combines a powerful visualization environment—using maps to communicate and visualize—with a strong analytic and modeling framework that is rooted in the science of geography.

This combination has resulted in a technology that is science based, trusted, and easily communicated using maps and other geographic views.

As you get started using ArcGIS, it will be important to understand a few GIS fundamentals and how ArcGIS brings these to life when you use the software. In this section, you will read about some key aspects of GIS and how geographic information models build on a series of key map concepts.

ArcGIS combines series of fundamental aspects of GIS:

A GIS utilizes a layer-based geographic information model for characterizing and describing our world.

ArcGIS models geographic information as a logical set of layers or themes. For example, a GIS can contain data layers for the following:

Streets represented as centerlines

Land-use areas that represent vegetation, residential areas, business zones, and so forth

Administrative areas

Water bodies and rivers

Parcel polygons representing landownership

A surface used to represent elevation and terrain

An aerial photo or satellite image for an area of interest

Geographic information layers such as those described here are represented using a few common GIS data structures:

Copyright © 1995-2010 ESRI, Inc. All rights reserved.

ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10 ArcGIS 10

Key aspects of GIS

What is GIS?

of 36What is GIS?

9/22/2010file://C:\Documents and Settings\lisac\Local Settings\Temp\~hh92C4.htm

Feature classes: Each feature class is a logical collection of features of a common type (such as the four feature types shown here).

Raster datasets: Rasters are cell-based datasets used to hold imagery, digital elevation models, and other thematic

data.

Attributes and descriptive information: These are traditional tabular information used to describe features and

categories about the geographic objects within each dataset.

Like map layers, GIS datasets are geographically referenced so that they overlay one another and can be located on the

of 36What is GIS?


earth's surface.

See Overview of geographic information elements for more information about modeling and representing geographic information.

A GIS uses maps to visualize and work with geographic information. Each GIS includes a set of intelligent, interactive maps and other views (such as 3D globes) that show features and feature relationships on the earth's surface. Various map views of the underlying geographic information can be constructed and used as "windows into the geographic database" to support query, analysis, and editing of geographic information. Maps can also be used to access geographic modeling tools that are used to derive new information.

GIS maps are interactive and help to communicate vast amounts of information. You can reach "through" an interactive map to present any set of information that helps your end users meet their missions and do important work.

See How maps convey geographic information for more information about mapping and visualization.

A GIS has a comprehensive set of analytic and data transformation tools to perform spatial analysis and data processing.

GIS includes a large set of geoprocessing functions to take information from existing datasets, apply analytic functions, and write results into new result datasets. There are numerous spatial operators, such as the Buffer and Intersect tools shown here, that can be applied to GIS data.

of 36What is GIS?


Each geoprocessing tool takes existing information as input and derives a new result, which can be used in subsequent operations. This ability to string together a logical sequence of operations so that you can perform spatial analysis and automate data processing—all by assembling a model—is one of the key elements of GIS.

See Geoprocessing—Computing with geographic data for more information about geoprocessing.

Georeferencing is about using map coordinates to assign a spatial location to map features. All the elements in a map layer have a specific geographic location and extent that enables them to be located on or near the earth's surface. The ability to accurately locate geographic features is critical in both mapping and GIS.

Describing the correct location and shape of features requires a coordinate framework for defining real-world locations. A geographic coordinate system is used to assign geographic locations to objects. A global coordinate system of latitude-longitude is one such framework. Another is a planar or Cartesian coordinate system derived from the global framework.

Maps represent locations on the earth's surface using grids, graticules, and tic marks labeled with various ground locations—both in measures of latitude-longitude and in projected coordinate systems, such as UTM meters. The geographic elements contained in various map layers are drawn in a specific order (one on top of another) for the given map extent.

GIS datasets contain coordinate locations within a global or Cartesian coordinate system to record geographic locations and shapes. In this way, multiple GIS data layers can be overlaid onto the earth's surface.

Latitude and longitude One method for describing the position of a geographic location on the earth's surface is using spherical measures of latitude


Georeferencing and coordinate systems

of 36What is GIS?


and longitude. They are measures of the angles (in degrees) from the center of the earth to a point on the earth's surface. This type of coordinate reference system is often referred to as a geographic coordinate system.

Longitude measures angles in an east–west direction. Longitude measures are traditionally based on the prime meridian, which is an imaginary line running from the North Pole through Greenwich, England, to the South Pole. This angle is longitude 0. West of the prime meridian is typically recorded as negative longitude, and east is recorded as positive. For example, the location of Los Angeles, California, is roughly plus 33 degrees, 56 minutes latitude and minus 118 degrees, 24 minutes longitude.

Although longitude and latitude can locate exact positions on the surface of the globe, they do not provide uniform units of measure for length and distance. Only along the equator does the distance represented by one degree of longitude approximate the distance represented by one degree of latitude. This is because the equator is the only parallel as large as a meridian. (Circles with the same radius as the spherical earth are called great circles. The equator and all meridians are great circles.)

Above and below the equator, the circles defining the parallels of latitude get gradually smaller until they become a single point at the North and South Poles where the meridians converge. As the meridians converge toward the poles, the distance represented by one degree of longitude decreases to zero. On the Clarke 1866 spheroid, one degree of longitude at the equator equals 111.321 kilometers, while at 60° latitude, it is only 55.802 kilometers. Since degrees of latitude and longitude don't have a standard length, you can't measure distances or areas accurately or display the data easily on a flat map or computer screen. Using many (but not all) GIS analysis and mapping applications often requires a more stable, planar coordinate framework, which is provided by projected coordinate systems. Alternatively, some of the algorithms used for

of 36What is GIS?


spatial operators take into account the geometric behavior of spherical (geographic) coordinate systems.

Map projections using Cartesian coordinates A projected coordinate system is any coordinate system designed for a flat surface, such as a printed map or a computer screen.

Both 2D and 3D Cartesian coordinate systems provide the mechanism for describing the geographic location and shape of features using x- and y-values (and, as you will read later, by using columns and rows in rasters).

The Cartesian coordinate system uses two axes: one horizontal (x), representing east–west, and one vertical (y), representing north–south. The point at which the axes intersect is called the origin. Locations of geographic objects are defined relative to the origin, using the notation (x,y), where x refers to the distance along the horizontal axis and y refers to the distance along the vertical axis. The origin is defined as (0,0).

In the illustration below, the notation (4,3) records a point that is four units over in x and three units up in y from the origin.

3D coordinate systems Increasingly, projected coordinate systems also use a z-value to measure elevation above or below mean sea level.

In the illustration below, the notation (2,3,4) records a point that is two units over in x and three units in y from the origin and whose elevation is four units above the earth's surface (such as 4 meters above mean sea level).

Properties and distortion in map projections Since the earth is spherical, a challenge faced by cartographers and GIS professionals is how to represent the real world using a flat or planar coordinate system. To understand the dilemma, consider how you would flatten half of a basketball; it can't be done without distorting its shape or creating areas of discontinuity. The process of flattening the earth is called projection, hence the term map projection.

A projected coordinate system is defined on a flat, two-dimensional surface. Projected coordinates can be defined for 2D (x,y) or 3D (x,y,z) in which the x,y measurements represent the location on the earth's surface and z would represent height above or below mean sea level.

of 36What is GIS?


Unlike a geographic coordinate system, a projected coordinate system has constant lengths, angles, and areas across the two dimensions. However, all map projections representing the earth's surface as a flat map create distortions in some aspect of distance, area, shape, or direction.

Users cope with these limitations by using map projections that fit their intended uses, their specific geographic location, and extent. GIS software also can transform information between coordinate systems to support integration of datasets held in differing coordinate systems and to support a number of critical workflows.

Many map projections are designed for specific purposes. One map projection might be used for preserving shape while another might be used for preserving the area ("conformal" versus "equal-area" projections).

These properties—the map projection along with spheroid and datum— become important parameters in the definition of the coordinate system for each GIS dataset and each map. By recording detailed descriptions of these properties for each GIS dataset, computers can reproject and transform the geographic locations of dataset elements on the fly into any appropriate coordinate system. As a result, it's possible to integrate and combine information from multiple GIS layers regardless of their coordinate systems. This is a fundamental GIS capability. Accurate location forms the basis for almost all GIS operations.

Learn more about map projections.

Fundamental GIS concepts are closely related to maps and their contents. In fact, map concepts form the basis for understanding GIS more fully. This topic explores some fundamental map concepts and describes how they are applied and used within GIS.

Maps A map is a representation of spatial or geographic information as a series of thematic layers of information for an area of interest. A printed map also includes additional map elements laid out and organized on a page. The map frame provides the geographic view of information while other elements—for example, a symbol legend, scale bar, north arrow, descriptive text, and a map title—around the map collar help you to understand, read, and interpret the map's contents.


How maps convey geographic information

of 36What is GIS?


People also work with computer maps—interactive images on computer screens with tools that allow you to interrogate and interact with the map's underlying geographic information.

Common to all maps is the set of thematic layers that represent the real-world features.

Layers Geographic entities are presented as a series of map layers that cover a given map extent—for example, you can view map layers such as roads, rivers, place-names, buildings, political boundaries, surface elevation, and satellite imagery.

Geographic elements are portrayed in maps through this series of map layers.

Map layers are thematic representations of geographic information, such as transportation, water, and elevation. Within each map layer, symbols, colors, and text are used to portray important information that describes each of the individual geographic elements. Map layers help convey information using the following:

Discrete features such as collections of points, lines, and polygons

Map symbols, colors, and labels that help to describe the objects in the map

Aerial photography or satellite imagery that covers the map extent

Continuous surfaces, such as elevation, which can be represented in a number of ways—for example, as a collection of contour lines and elevation points or as shaded relief

Map layout and composition Along with the map frame, a map presents other information using an integrated series of map elements laid out on a page. Common map elements include a north arrow, a scale bar, a symbol legend, and other graphic elements. These elements aid in map reading and interpretation by defining the meaning of each map symbol and often by providing messages and insight

of 36What is GIS?


into the map's contents.

This information enables each map to communicate more, simply by portraying large amounts of information in a systematic, intuitive way. This in turn helps each map reader visualize and understand interesting facts critical to their work.

Spatial relationships in a map Maps help convey geographic relationships that can be interpreted and analyzed by map readers. Relationships based on location are referred to as spatial relationships. Here are some examples:

Which geographic features connect to others (For example, Water Street connects with 18th Ave.)

Which geographic features are adjacent (contiguous) to others (For example, the city park is adjacent to the university.)

Which geographic features are contained within an area (For example, the building footprints are contained within the parcel boundary.)

Which geographic features overlap (For example, the railway crosses the freeway.)

Which geographic features are near others (proximity) (For example, the courthouse is near the State Capitol.)

The feature geometry is equal to another feature (For example, the city park is equal to the historic site polygon.)

The difference in elevation of geographic features (For example, the State Capitol is uphill from the water.)

The feature is along another feature (For example, the bus route follows along the street network.)

Within a map, such relationships are not explicitly represented. Instead, as the map reader, you interpret relationships and derive information from the relative position and shape of the map elements, such as the streets, contours, buildings, lakes, railways, and other features. In a GIS, such relationships can be modeled by applying rich data types and behaviors (for example, topologies and networks) and by applying a comprehensive set of spatial operators to the geographic objects (such as buffer and polygon overlay).

Maps play a special role in GIS:

They portray logical collections of geographic information as map layers.

They are at the heart of how GIS is used.

They provide an effective metaphor for modeling and organizing geographic information as a series of thematic layers.

In addition, interactive GIS maps provide the focal point for using geographic information and bringing that information to life. GIS maps are the way that GIS content is shared among professional GIS users and with everyone online.

This topic provides an important context for the role that maps play in delivering GIS to many new users.

Maps are important, and almost everyone understands and appreciates good maps.

And mapping encompasses a lot—from traditional printed maps and imagery to new media maps that are used on computers, across the Web, and on mobile devices.

A new kind of map is a GIS map, and each GIS map is more than a static map presentation. It is an interactive window into all geographic information and descriptive data, and into rich spatial analysis models created by GIS professionals.

GIS maps are:

How you communicate and share GIS

How GIS content is compiled and maintained

How geographic information is designed and organized using thematic layers


How maps are used in GIS

of 36What is GIS?


How you derive new information using geoprocessing and, subsequently, how you visualize, summarize, analyze, compare, and interpret analytic results

How you share geographic information for use on the Web

Examples of map use

For communication and understanding

Maps are used to communicate and convey overwhelmingly large amounts of information in an organized way. Humans, as spatial thinkers, are able to view a map, associate map locations with real-world phenomena, and perceive and interpret critical information from the sea of content that is contained within each map display.

For finding patterns

Maps are used to discover and investigate patterns such as the characteristics of a population across a city or the movement of antelope between winter and summer habitats. In GIS, many maps can be dynamic and generate reports and views about multiple features and changes across time frames.

Maps convey large amounts of information yet still manage to communicate that information effectively and clearly. You can begin to understand and gain insights by using maps.

of 36What is GIS?


This is a key point. GIS maps provide interactive reports of the information behind the map—not solely lists of attributes but also charts, reports, photos, and virtually any relevant content (for example, a link to a Web site). Defining how features are reported and what you access through a map feature is one of the key specifications that you design and capture when you create a GIS map.

You can also define and capture map interaction properties for time-aware layers as part of your GIS map definition. For example, here is a dynamic map that shows animal movements from GPS tracking devices. You can use the time slider tool to control the display of animal locations on various days. Clicking forward moves to the next day's observations.

Maps visually convey patterns. This map shows the age distribution of populations across parts of Southern California. Darker colors represent areas with older populations. You can click a feature on the map to show the age distribution for the selected block group.

In this way, you are using the map as a window into richer sets of geographic and tabular information.

of 36What is GIS?


For deriving new information using analysis

GIS maps combine powerful visualization with a strong analytic and modeling framework. Analytic models in a GIS are used to generate model results that can be added to your map display as new derived map layers.

Just like you can use each map layer as a window into rich information about features, you can use the map as a window into rich analytic results. You essentially use your GIS map to access analysis models and display their results as a new map layer, which can have the same types of feature reporting, visualization, and animation capabilities that are described above.

Here are four snapshots from a dynamic online map of pronghorn antelope locations from four particular days (out of 300). These dots represent daily movement of pronghorn over a 10-month time span from October 2002 through August 2003. The Time Slider tool in ArcMap is used to put

this dynamic map into motion.

A "heat map" showing criminal activity. The hotter colors represent higher crime rates. Image courtesy of the Philadelphia

Police Department (http://www.phillypolice.com/).

of 36What is GIS?


Spatial analysis is one of the more interesting and remarkable aspects of GIS. Using spatial analysis, GIS users can combine information from many independent sources and derive an entirely new set of information (results)—applying a large, rich, and sophisticated set of spatial operators. GIS professionals use Geoprocessing to "program their own ideas" in order to derive these analytical results. In turn, these results are applied to a wide variety of problems.

To get status reports

On the Web, maps can be used to communicate status and keep teams up-to-date on events. GIS information is dynamic and, for many layers, is updated on a frequent basis. Dynamic maps are an effective way for everyone to see a common picture of the latest information.

This map shows predictions for malaria outbreaks in Africa. Darker colors represent a higher projected density of malaria cases. Image courtesy of

Adaptation Atlas (http://www.adaptationatlas.org/).

This map illustrates three routes used to optimize travel time between stops for three vehicles in a fleet. Organizations that use network analysis to optimize their vehicle

routes typically save 20 percent or more on their annual delivery costs.

of 36What is GIS?


A very common application for GIS is the use of operational dashboards that present data feeds and status for a particular set of users. The information layers in dashboards are targeted to a specific audience and their operational needs, enabling them to work more effectively and responsively.

To compile geographic information

Maps are used to compile and edit features and other data, which are managed and maintained in geodatabases. The best GIS maps for editing present the specific types of features that you want to add to your maps along with the relevant editing tools and attribute properties.

ArcGIS enables users to define and share these editing properties as part of a layer design.

This status map from the United Nations Office for the Coordination of Humanitarian Affairs (OCHA) shows humanitarian response efforts for the Haitian earthquake in January 2010. image courtesy of

OCHA (http://ochaonline.un.org/).

Many Web maps act as operational dashboards that communicate status information across work teams to keep everyone informed and up-to-date. This is an operational dashboard for a water utility. Up-to-the-minute information coming directly from the field,

from the operations center, and from the call center is displayed within this operational dashboard.

of 36What is GIS?


To communicate ideas, concepts, plans, and designs

Maps help to communicate ideas, plans, and design alternatives. Effective layer display, combined with interactive feature reporting, provide an important mechanism to visualize, communicate, and understand various alternatives.

In this map, a feature palette of land-use types is used to lay out land-use zones. You can sketch in (outline) proposed zones for visualizing and analyzing various land-use alternatives.

Mobile GIS maps can be used for collecting data in the field and for receiving and viewing status reports on a

mobile map.

of 36What is GIS?


To openly share geographic knowledge

As illustrated by these map examples, maps are both effective and efficient for visualizing geographic knowledge. Great maps are how GIS users communicate and how geographic information and knowledge are shared.

Like a map, a GIS is layer based. And like the thematic layers in a map, GIS datasets represent logical collections of individual features with their geographic locations and shapes as well as descriptive information about each feature stored as attributes.

Prior to GIS, mapmakers created a series of map layers that were used to geographically describe and characterize a location. They often used transparencies that could be overlaid on a light table. These integrated displays were used to visualize spatial relationships and gain insight about relevant characteristics of a place. Practitioners would use these to make interpretations and to draw interesting conclusions.

One visionary who used this process for planning was Dr. Ian McHarg, a landscape architect and renowned writer on regional planning using natural systems. His seminal book was published in 1969 and articulated concepts for ecological planning, which applied these map overlay principles. You can learn more at Wikipedia about Ian McHarg and his work.

Around this same time, Dr. Roger Tomlinson, known as the "father of GIS," developed his early ideas for GIS. Among other aspects of GIS, he further articulated the concept of thematic layers and overlays as a cornerstone for GIS.

These early GIS practitioners thought about how geographic information could be partitioned into a series of logical information layers—as more than a random collection of objects. They envisioned homogeneous collections of representations that could be managed as layers. These GIS users organized information in individual data themes that described the distribution of a phenomenon and how each theme should be portrayed across a geographic extent. They found that they could use relatively simple GIS data types (points, lines, polygons, and rasters). These simple data layers could be combined through location—that is, georeferencing enabled datasets to be combined in a map or overlaid using geoprocessing operations such as polygon overlay.

These pioneers also provided a protocol for data collection and how to manage these collections as geographic data layers. Here is one example for representing soils.

Each and every area (polygon) in a specified extent could be assigned a dominant soil type, and the soil types could be consistently classified and described using properties or attributes of each polygon. In the case of soils, very involved sets of properties are typically recorded for each soil polygon.

A theme could be defined to delineate various areas representing the dominant soil type (that is, a layer collection of soil type polygons and their descriptions as attribute values).

Here are a series of 2D and 3D maps that are used to develop and present design alternatives and some of the analysis used as inputs into the design decisions.


An overview of geographic information elements

of 36What is GIS?


This organizing principle of geographic layers became one of the universal GIS principles that provided the foundation for how GIS systems represent, operate on, manage, and apply geographic information.

All of the rich GIS behavior for representing and managing geographic information is based on three fundamental representations or expressions of geographic information:

Features (collections of points, lines, and polygons)

Attributes

Imagery

Each type is described here.

Features—points, lines, and polygons Geographic features are representations of things located on or near the surface of the earth. Geographic features can occur naturally (such as rivers and vegetation), can be constructions (such as roads, pipelines, wells, and buildings), and can be subdivisions of land (such as counties, political divisions, and land parcels).

Although there are a number of additional feature types, geographic features are most commonly represented as points, lines, or polygons.

Points define discrete locations of geographic features too small to be depicted as lines or areas, such as well locations, telephone poles, and stream gauges. Points can also represent address locations, GPS coordinates, or mountain peaks.

Lines represent the shape and location of geographic objects too narrow to depict as areas (such as street centerlines and streams). Lines are also used to represent features that have length but no area, such as contour lines and administrative boundaries. (Contours are interesting, as you'll read later on, because they provide one of a number of alternatives for representing continuous surfaces.)


Three fundamental representations of geographic information layers

of 36What is GIS?


Polygons are enclosed areas (many-sided figures) that represent the shape and location of homogeneous features such as states, counties, parcels, soil types, and land-use zones. In the example below, the polygons represent land parcels.

Attributes Maps convey descriptive information through map symbols, colors, and labels. For example:

Roads are displayed based on their road class (such as line symbols representing divided highways, main streets, residential streets, unpaved roads, and trails).

Streams and water bodies are drawn in blue to indicate water.

City streets are labeled with their names and often some address range information.

Special point and line symbols denote specific features such as rail lines, airports, schools, hospitals, and incidents of various types.

In a GIS, descriptive attributes are managed in tables, which are based on a series of essential relational database concepts. Attribute tables provide a simple, universal data model for storing and working with attribute information. They are inherently open because their simplicity and flexibility enables support for a broad range of applications. Key concepts include the following:

Descriptive data is organized into tables.

Tables contain rows.

All rows in a table have the same columns.

Each column has a type, such as integer, decimal number, character, and date.

Within relational databases, these concepts are extended to include a series of relational functions and operators that can be used to operate on the tables and their data elements. This is known as Structured Query Language or SQL.

Imagery Imagery in GIS often refers to a number of types of cell- or pixel-based data sources—for satellites, aerial photography, digital elevation models, raster datasets, and so on.

Imagery is managed as a raster data type composed of cells organized in a grid of rows and columns. In addition to the map projection, the coordinate system for a raster dataset includes its cell size and a reference coordinate (usually the upper left or lower left corner of the grid).

of 36What is GIS?


These properties enable a raster dataset to be described by a series of cell values starting in the upper left row.

Typical image sources include cameras capable of capturing aerial photographs that can be georeferenced and corrected to ground locations (such as digital orthophotography).

Imagery is also used to collect data in both the visible and nonvisible portions of the electromagnetic spectrum. One system is the multispectral scanner carried in Landsat satellites that records imagery in seven bands (or ranges) along the electromagnetic spectrum. The measures for each band are recorded in a separate grid. The stack of seven grids makes up a multiband image.

In addition to the GIS data types, features and rasters often participate in relationships with other features and with attribute values held across multiple tables. These spatial and attribute relationships, as well as certain behaviors, can be modeled by extending the three fundamental types of geographic information.

Here are a few examples of spatial and attribute relationships.

Networks

Each cell location can be automatically located using a reference coordinate for the origin, the cell size, and

the number of rows and columns.


Spatial relationships and behavior

of 36What is GIS?


Some linear features are connected. For example, street segments connect in a road network, pipes connect in a water network, stream lines connect in a hydro network,and electrical lines connect in an electrical network. Networks can be traced—for example, to find the fastest travel route, to identify the valve to turn off in a water network, or to trace river flow downstream.

Topologies Many features are adjacent to one another and have coincident geometry; for example, adjacent counties, parcels, and other administrative areas share coincident edges.

Related tables Often, much of the descriptive information about features is held in separate attribute tables. Attributes from these separate attribute tables can be associated with each feature using standard relational database methods.

of 36What is GIS?


Summary All types of geographic information—features, rasters, and attributes—can participate in these spatial and attribute relationships. In a GIS, such relationships are modeled using extended data types such as topologies and networks.

In addition, many types of spatial relationships can be discovered and identified by applying a series of spatial operators to the geographic objects. For example, you can create buffer zones of a given distance around features and perform a polygon overlay with another dataset to identify features that are near to others in your GIS.

This example illustrates how you might model surface elevation in a GIS. This is a useful example because you'll typically need to use multiple datasets for representing and analyzing surface elevation. Most of these are derived from a small set of data sources.

Typically, you will need the following:

A digital elevation model (DEM) dataset stored as a raster. DEMs are used in surface analysis applications and for deriving other datasets for surface display.

A triangulated irregular network (TIN). TINs are derived from data sources such as lidar and other 3D points, as well as 3D lines (such as water body shorelines, ridgelines, and so on).

Contour lines and elevation points. These feature classes are derived from your DEM and are used for displaying surface elevation.

A shaded relief raster that is derived from your DEM and used in your surface elevation display as well.


Example: Representing surfaces

of 36What is GIS?


Continuous surfaces in GIS

A continuous surface describes an occurrence that has a value for every point on the earth. For example, surface elevation is a continuous layer of values for ground elevation above mean sea level for the entire extent of a dataset. Other surface type examples include rainfall, pollution concentration, population density, and subsurface representations of geologic formations.

Surface representation is somewhat challenging. With continuous datasets, it is impossible to represent all values for all locations. Various alternatives exist for representing surfaces using both features and rasters.

Often, a GIS will need to use multiple surface representations and datasets to depict surface elevation. For example:

Contour lines and elevation points—Contours are lines that represent locations having an equal value, such as surface elevation height. Elevation points can be used to represent key mountain peaks.

Raster datasets—Each raster contains a matrix of cells where each cell value represents a measure of the continuous variable. For example, DEMs are frequently used to represent surface elevation and are often portrayed using shaded relief maps.

TINs—A TIN is a data structure for representing surfaces as a connected network of triangles. Each triangle node has an x,y coordinate and a z or surface elevation value. TINs can contain additional information for modeling surface elevation such as ridgelines, drainage lines, building pads, and water boundaries. Like a DEM, TIN representations can be used to estimate the surface value for any location using interpolation.

of 36What is GIS?


All geographic information is represented and managed using three primary GIS data structures:

Feature classes

Attribute tables

Raster datasets

These three fundamental data types can be extended with additional capabilities to manage data integrity, model geographic relationships (such as network connectivity and flow), and add important geographic behavior.

Each GIS has a collection of datasets Typically, a GIS is used for handling several different datasets where each holds data about a particular feature collection (for example, roads) that is geographically referenced to the earth's surface.

A GIS database design is based on a series of data themes, each having a specified geographic representation. For example, individual geographic entities can be represented as features (such as points, lines, and polygons); as imagery using rasters; as surfaces using features, rasters, or TINs; and as descriptive attributes held in tables.

In a GIS, homogeneous collections of geographic objects are organized into data themes such as parcels, wells, buildings, orthoimagery, and raster-based digital elevation models (DEMs). Precisely and simply defined geographic datasets are critical for useful geographic information systems, and the design of layer-based data themes is a key GIS concept.

GIS datasets are logical collections of geographic features A dataset is a collection of homogeneous features for each theme. Geographic representations are organized in a series of datasets or layers. Most datasets are collections of simple geographic elements such as a road network, a collection of parcel boundaries, soil types, an elevation surface, satellite imagery for a certain date, well locations, or surface water.

In a GIS, spatial data collections are typically organized as feature class datasets or raster-based datasets.

Many data themes are best represented by a single dataset such as for soil types or well locations. Other themes, such as a transportation framework or surface elevation, are often represented by multiple datasets. For example, transportation might be represented as multiple feature classes for streets, intersections, bridges, highway ramps, railroads, and so on. The table below illustrates how surface elevation might be represented using multiple datasets.

Raster datasets are used to represent georeferenced imagery as well as continuous surfaces such as elevation, slope, and aspect.

Common GIS representations


Theme Geographic representation

Streams Lines

Large water bodies Polygons

Vegetation Polygons

Urban areas Polygons

How a GIS represents and models geographic information

of 36What is GIS?


Thematic layers become datasets. This is the key organizing principle in a GIS database. Each GIS will contain multiple themes for a common geographic area. The collection of themes acts as a stack of layers. Each theme can be managed as an information set independent of other themes. Each has its own representation (as a collection of points, lines, polygons, surfaces, rasters, and so on).

Because layers are spatially referenced, they overlay one another and can be combined in a common map display. In addition, GIS analysis tools, such as polygon overlay, can fuse information between data layers to discover and work with the derived spatial relationships.

Any effective GIS database will adhere to these common principles and concepts. Each GIS requires a mechanism for describing geographic data in these terms along with a comprehensive set of tools to use, manage, and share this information.

How GIS users work with geographic information Users work with geographic data in two fundamental ways:

As datasets, which are homogeneous collections of features, rasters, or attributes, such as parcels, wells, buildings, orthophoto imagery, and raster-based digital elevation models

As individual elements or subsets, such as the individual features, rasters, and attribute values contained within each dataset

Working with GIS datasets

In ArcGIS, homogeneous collections of geographic objects are organized into datasets about common subjects, such as

Road centerlines Lines

Administrative boundaries Polygons

Well locations Points

Orthophotography Rasters

Satellite imagery Rasters

Surface elevation DEM rasters

Contour lines

Elevation points

Shaded relief rasters

Land parcels Polygons

Parcel tax records Tables

of 36What is GIS?


parcels, wells, roads, buildings, orthophoto imagery, and raster-based DEMs.

Many of the operations that users perform in ArcGIS work on datasets as inputs or create new datasets as results. Datasets also represent the most common method for data sharing among GIS users.

Datasets provide the primary data sources for each of the following:

Maps, globes, and 3D scenes: These views provide the principal display of geographic information as a series of map layers. Each map layer references a specific GIS dataset and is used to symbolize and label the dataset. In this way, map layers help bring your GIS datasets to life in your GIS.

Map layers in 2D maps and 3D scenes are used to symbolize and label GIS datasets. This map has layers for cities, highways, state and county boundaries, water bodies, and streams. Each of these layers is used to portray a GIS dataset.

Geoprocessing inputs and derived datasets: GIS datasets are common data sources used for geoprocessing and are useful for automated data processing and GIS analysis. Datasets are used as inputs and new datasets are derived as results for various geoprocessing tools.

Geoprocessing helps you to automate many tasks as a series of operations so that they can be run as a single step. This helps to create a repeatable, well-documented data processing workflow.

Users also work with ArcGIS datasets to perform spatial analysis.

of 36What is GIS?


This model illustrates how to identify and rank potential sites for new parks. Good candidate locations must have high population counts and not be too close to existing parks.

Working with individual features and elements in datasets

In addition to working with datasets, users also work with the individual elements contained in datasets. These elements include individual features, rows and columns in attribute tables, and individual cells in raster datasets. For example, when you identify a parcel by pointing at it, you're working with the individual data elements in a dataset:

You work with individual data elements when you edit features—as in this example for editing road centerlines:

of 36What is GIS?


When working with tables, users work with descriptive information contained in rows and columns, as illustrated here:

GIS users work with interactive maps. There is a series of GIS map applications that provide the primary interfaces through which users work with ArcGIS. Map documents (ArcMap MXDs) and ArcGIS Web Maps are used to encapsulate maps that are used and shared across these applications.

Many kinds of GIS map applications GIS maps provide the primary user interface for many GIS applications. Users can point to map features to display information about them, discover new relationships, perform editing and analysis, and efficiently communicate results using geographic views.

A number of alternative GIS map applications can be deployed in various application frameworks to support many types of users and tasks. They include the following:

Professional GIS maps

ArcGIS Desktop users (users of ArcView, ArcEditor, ArcInfo, and often custom ArcGIS Engine applications) employ a rich set of mapping applications, such as ArcMap, to perform their daily work. ArcGIS Desktop provides professional GIS applications for map authoring and map use, working with 3D scenes and globes, data compilation, running GIS analysis, and publishing GIS information products for use by others in your organization.


Map applications provide the primary user interface for GIS

of 36What is GIS?


Typical tasks in professional GIS map applications include these:

Author maps.

Edit data.

Perform spatial analysis.

Visualize results.

Animate geographic information.

Chart data.

Publish and share as desktop maps, Web maps, and ArcGIS Explorer maps.

Print maps.

Web maps

Web maps are used by a wide-ranging audience from citizens to field-workers, operations staff, managers, and executives and are published in concert with ArcGIS Server. Web GIS applications have a user experience much like many of the consumer Web maps such as Google Maps and Microsoft Bing Maps.

You can create and share your own custom Web map applications powered by powerful, back-end map and geoservices hosted by ArcGIS Server. You can easily combine (that is, mash up) Web maps with one another.

Web applications

A number of application program interfaces (APIs) are available for building and deploying Web maps with ArcGIS: JavaScript, Adobe Flex, and Microsoft Silverlight. Web mapping applications are useful for combining information from a range of Web maps and GIS servers.

of 36What is GIS?


of 36What is GIS?


Here is a list of some typical tasks that are used in custom applications:

Map use with focused tasks—identify, query, summarize, and so on

Simple editing (for example, enabling citizens to compile and share their their own information as a member of a community)

Attribute and forms-based updates

Access to status maps (situational awareness)

Fusion of information from multiple servers

ArcGIS Explorer

ArcGIS Explorer is a free application from ESRI that you can share with any of your users. Explorer is used to build interactive Web maps that provide 2D and 3D views of geographic information. These GIS maps can integrate many information sets. ArcGIS Explorer typically relies on ArcGIS Web and other Web services (KML, WMS, and so forth), but can also use local datasets.

Here are some ArcGIS Explorer characteristics:

It can access any 2D or 3D basemap published using ArcGIS Server along with the ability to overlay just about any GIS service (KML, OGC, Bing, and others).

It supports advanced GIS tasks and access to geoprocessing services.

The ArcGIS Explorer presentation mode is very effective at using maps and GIS to tell stories and communicate with and visualize your community.

It helps users answer many questions that go beyond simple Web mapping: Where are my customers?

Where should I put new stores or facilities?

Who is impacted by this emergency? Where are the first responders? Where are the vulnerable populations, and how many are there? Where are the elderly who are affected? The children? Where should we place evacuation

of 36What is GIS?


centers?

What is the best way to respond to a power outage? Can we pinpoint the outage location?

What are the most congested traffic areas of a city?

What is the projected tax base for land parcels under this proposed plan alternative?

What is the environmental impact of a new development?

What is the air quality impact on children near major roads?

What happens if the water level rises one meter?

It's free to download and use.

Mobile GIS applications

The wide adoption of mobile wireless devices (advances in cell phones) and the universal use of the Global Positioning System (GPS) are fueling the growth in mobile GIS. ArcGIS includes mobile capabilities that work on wireless devices (using the mobile Web) as well as devices with advanced GPS capabilities for field data collection and mapping applications. Mobile wireless appliances use standard Web map frameworks, while Tablet PCs and professional GPS devices use special maps that you can take to the field on your mobile appliance using ArcGIS Mobile technologies.

Maps provide the mechanism for how GIS is deployed and shared. Each interactive GIS map is a specification for how geographic information is brought together and portrayed.

When you create a map, you specify its map layers and their drawing order in the map. Each layer is used to symbolize and label a particular dataset. You also define what information can be accessed through the features in each layer and what additional operations can be performed on the layer, such as spatial analysis or editing.

You can save and share your map definitions as map documents. For example, you can give a copy of your GIS map to other users. This enables users to deploy the shared map in many applications and across many organizations.

ArcGIS users work with and share two primary interactive map types—ArcMap documents (and their layers) and GIS Web maps.

ArcMap documents are the way that Desktop users share their professional GIS work with other Desktop users. Map documents are also used to publish maps and their underlying geographic information as map services and other GIS services using ArcGIS Server.

Web maps are the mechanism for how map services can be shared and used in many clients online—for example, you can open and use Web maps in ArcGIS Desktop, ArcGIS Explorer, the iPhone, and other mobile clients. Both Desktop users and novice users can create and share Web maps.

Each type of map document is described in more detail here.

ArcMap documents ArcMap documents and layers are created and used in ArcMap, which is the primary mapping application in ArcGIS.


ArcMap documents and Web maps

of 36What is GIS?


Once an ArcMap document is created and its various map properties are defined, all the properties of the GIS map are saved as well. This captures many layer properties and interaction settings—the data source; how the geographic data is symbolized, labeled, and visualized; which map scales are used and the layer appearance at each scale; specifications for tools (such as how a layer is edited and in what datasets the new features are stored); properties for working with attribute information; and so on.

These properties are captured and encapsulated in map packages and layer packages.

The map can also encapsulate exactly how features are edited, what attributes are used, and how they are displayed in pop-ups. The map package specifies the geodatabase that holds the data, the geoprocessing models (in other words, tools) that are used to derive new information sets, and the related tables that are used to connect additional attribute information to the map. All these settings are captured as part of the map.

Among the key information elements in a map document are the map's layers, and often users want to share and encapsulate layer information in an independent layer file or layer package as well.

Users can then share their maps and layers with one another, enabling many to view and use geographic information in a common way. Any ArcGIS Desktop user can get a copy of someone else's map or layer package, then simply double-click to open and work with these documents in ArcMap.

When you receive a map package (or layer package) from another user, you can download that package onto your computer, and your ArcGIS Desktop is transformed by the package—it can do all the same work that the user designed and built into

Each ArcMap document contains the complete map specification—the set of map layers, their display properties, editing rules, analytical models, attribute access and reporting, and so on.

Each ArcGIS map document can capture both the cartography as well as all other elements of geographic information—the geodatabase, editing templates and rules, analytic models, how you work with tables and

charts, and so on.

of 36What is GIS?


their shared map package or layer package. Everything that the other user was able to do, you can now do as well.

In addition, these ArcGIS documents and packages can be published as map services on the Web. Using ArcGIS Server, users can turn any map, geodatabase, or model into a GIS Web service for sharing in a workgroup, throughout an enterprise, or openly in the cloud.

Encapsulating knowledge within maps and sharing maps and layers using ArcGIS Desktop

See What is ArcMap? for more information about map documents and layers.

ArcGIS Web maps An ArcGIS Web map is an organized set of map service layers that can be opened and used together as a single map. Web maps can be shared on the web and opened in any ArcGIS client application—for example, in ArcMap, ArcGIS Explorer Online, ArcGIS.com, iPhones, etc.

Web maps are how ArcGIS users share and disseminate their geographic information as Web map layers that reference rich GIS services. Individuals use ArcGIS Desktop and ArcGIS Server to create map services and other GIS services to share their rich information—map services, image services, editing services, geoprocessing services, and so forth. Once published, these can be discovered and used to create ArcGIS Web maps and used anywhere in the ArcGIS system.

Using this approach, each GIS organization can make its information available to nonspecialists. This also enables the integration of information across organizations, providing a strong basis for collaboration.

Document Key properties Shared as

Map document

Map name, summary, description, and so on

List of map layers

Geodatabase

Geoprocessing tools

Image services

Properties for each layer

Map document (MXD)

Map package with its data (MPK)

Layer Layer name, summary, description, and so on

Properties (name, metadata, map scales, data source, transparency, and so on)

Attributes: Visible fields, alias names, display expressions, read-only versus update, and so on

Symbology

Labeling

Editing properties

Attachments to features

Identify and pop-up properties

Time-aware properties

Layer file (LYR)

Layer package with its data (LPK)

As one of the layers in a map document/package

You can use a web browser to combine Web maps for creating and sharing your own web maps.

Key properties Shared as:

Map title, summary, description, and so on

A set of one of more map service URLs that comprise the basemap

The ordered list of Web map URLs used as operational layers

A list of tasks (for example, whether a layer can be

Web maps, which can be shared and used in all ArcGIS clients:

ArcGIS Desktop

ArcGIS Explorer Online

Mobile applications (for example, iPhone, Windows Mobile)

of 36What is GIS?


See Using Web maps at ArcGIS.com for more information.

Working with Web maps Each Web map is essentially an organized set of map layers, and each map layer references a Web map service. This simple paradigm can enable delivery of many advanced GIS capabilities on the Web. For example:

Web maps support feature pop-ups and interactive reports

You can use the Web map to click and pop up information about features. Essentially, you reach through the map to access important information. This can be through simple attribute reports or rich experiences for information access using graphics and dynamic charts.

Web maps support analytic functions

GIS supports a comprehensive set of rich analytic tools for performing sophisticated geographic analysis. All model results can be viewed and brought to life as map layers. Analytic results might be generated interactively or they may be precomputed. The primary point is that rich GIS analysis can be shared and visualized by anyone using Web maps. It's simple to reference the results in Web maps.

Web maps support editing and data compilation

Users can digitize and enter features on Web maps. This enables many within a community to contribute rich content and

queried)

A set of widgets to use the operational layers (for example, for editing or for time-aware layers)

Web applications (JavaScript, Flex, and Silverlight)

SharePoint Web sites

Images courtesy of Philadephia Police Department and Adaptation Atlas, respectively.

of 36What is GIS?


observations for many interesting scenarios.

You can create your own Web maps by combining a set of published Web map services. First, you specify the set of Web maps that you want to use as a basemap. Then you can specify the set of Web maps that will be your operational overlays, how you interact with them, and what tools or capabilities will be included (editing, tools for working with time-aware layers, and so on).

Geoprocessing is the methodical execution of a sequence of operations on geographic data to create new information. The two fundamental purposes are to help you perform modeling and analysis and to automate GIS tasks.

Spatial analysis Spatial analysis is the process of modeling, deriving results by computer processing, then examining and interpreting the model results. Spatial analysis is useful for evaluating suitability and capability, estimating and predicting, and interpreting and understanding.

For example, spatial analysis can be used to study the relationships between air quality in urban settings and childhood asthma.

Spatial analysis is one of the more interesting and remarkable aspects of GIS. Using spatial analysis, GIS users can combine information from many independent sources and derive an entirely new set of information (results)—by applying a large,

In this fire response map example, incident commanders can easily sketch in their operational status and plans and rapidly share this information using Web maps.


Home addresses of children with asthma can be geocoded against a streets layer. Major roads (such as multilane roads and highways) can be selected and buffered—say by a distance of 150 meters. These layers can be overlaid for studying this spatial relationship and its impact on the incidence of

asthma. GIS includes many more sophisticated operators (such as spatial statistics tools) for studying these relationships.

Geoprocessing—Computing with geographic data

of 36What is GIS?


rich, and sophisticated set of spatial operators. GIS professionals use Geoprocessing to "program their own ideas" in order to derive these analytical results. In turn, these results are applied to a wide variety of problems.

Automation using geoprocessing Geoprocessing is certainly used for spatial analysis, but it is also used for much more. With geoprocessing, users automate many GIS tasks—data preparation and conversion, creation of a set of automated tests to perform integrity checks of your data against a set of business rules, coordinate management, automation of other data management workflows, map production, and much more.

Geoprocessing is repeatable. In fact, many users create a series of automated workflows that help perform tedious, repetitive work. These workflows are repeatable and self-documenting. They can be shared with many users. They can be placed into a server framework and used for all kinds of GIS tasks, not just for analysis.

The spatial analysis process Spatial analysis is the process of applying analytic techniques to geographically referenced datasets to extract or generate new geographic information to address a particular question or objective.

Steps in the spatial analysis process

In practice, this analysis process is iterative. A review is conducted at each step, providing the opportunity to incorporate new knowledge and insight gained along the way. The analysis process is one part modeling and one part the generation of—and working with—a series of maps, summary reports, scientific and statistical charts, and summaries for analysis.

During analysis, a model is built based on the analysis objectives. A set of results (output data and map views) is generated. Then that information is analyzed—the results are mapped, compared, visualized, interpreted, modified, updated, calibrated, reexecuted, and so forth.

Users explore and interpret the results and use these to draw conclusions and make decisions.

1 Establish an objective and frame the questions you want to answer.

2 Gather, organize, and prepare the data for analysis.

3 Build the analysis model (typically performed using geoprocessing but could be as simple as a few mouse clicks in ArcMap).

4 Execute the model and generate results.

5 Explore, evaluate, chart, summarize, interpret, visualize, understand, and analyze the results.

6 Make conclusions, arrive at decisions, and document your results.

7 Present your results and findings.


of 36What is GIS?


C:Documents and SettingslisacLocal SettingsTemp~hh9web.gps.caltech.edu/gislab/HowTo/ESRI - What is...

Documents

Transcript of C:Documents and SettingslisacLocal SettingsTemp~hh9web.gps.caltech.edu/gislab/HowTo/ESRI - What is...