Basic Spatial Analysis

May 30, 2013Institute of Space Technology, Karachi

Spatial Analysis

Involves applications of operations to coordinates and related attribute data

Chapter 9 of your text book.

Spatial Data Analysis

Spatial analyses are applied to solve problems related to geographic decisions

Examples: Identify high crime areas Generate a list of road segments that need repaving Select a best location for a new business

Input Layer Spatial Operation Output Layer

Sequence of Spatial Operations

SINGLE SPATIAL OPERATIONS

Challenge!!!! Selecting appropriate spatial

operations and applying them in the appropriate order

Spatial Data AnalysisApplication of operations to coordinates

and related attribute data

Non Spatial Queries (standard Database queries) How long is River Indus? Population of Pakistan? etc.

Spatial Queries Neighboring countries of Pakistan? List of provinces through which Indus river flows?

Spatial Data Analysis Since attribute data is related to spatial

objects therefore we can not separate operations on attribute data from operations on coordinate portion of spatial data

Spatial Operations Applied to one or more input layers to

produce one or more output data layers

One to One Conversion of raster data into vector

One input – Many outputs Slope and aspect produced from raster elevation model

Many inputs – one output To calculate data averages from different layers

Spatial Operations

Spatial Operations Depends on type of data model used Specific operations available in the GIS

software In many instances it’s more efficient to

convert data between data models and apply the desired operations and convert results back to the original data model

Spatial Operations Outputs

Non Spatial outputs Spatial operation produces scalar value, a

list, or a table with no explicit geometric data attached

Spatial New data layer is produced

Spatial ScopeSpatial operations may becharacterized by their spatial scope

Local Operations

Neighborhood Operations

Global Operations

Spatial Data Analysis

Selection Reclassification Dissolving Buffering Overlay

Selection

Features are identified based on given criteria

Example: in a political map of European countries, select all names and population of countries with more than 50M inhabitants

Above is an example of set algebra >, <, =, & <> Used either alone or in combination

Thematic map of countries of western Europe (and their population); selection of countries with more than 50M people

Selection

Selection: Set Algebra

Selection Boolean Algebra

Conditions: AND, OR, NOT

Combines set algebra and create compound spatial selection

Expression In Boolean Algebra

Selection by Location Adjacency

Containment

Reclassification

Classification of spatial objects based on spatial or non spatial data

Example: classification of Polygons based on size

Examples of Reclassification Equal Interval Equal Area etc.

Dissolve

Combines or dissolve similar features within a data layer

Adjacent polygons with identical values are merged into a single polygon

Proximity Modifies existing feature or creates new

features that depend on distance Available water represented by Points and

distance function applied to these points to create raster data layer containing the distance to nearest water feature.

Proximity: Distance Calculations Distance values are

calculated using Pythagorean formula

Calculated from cell center to cell center (when applied to raster data)

Buffers Commonly used proximity function Represents specific distance around a

feature May be determined

for point, line, or area features

Also for both types of data models

Overlay Overlapping different themes (multiple

layer operation) Areas where features in different layers

overlap Both spatial and attribute data is combined Clip, Intersect and union are special cases

of overlay

Vector Overlay Clip

Only the areas that overlap are contained Cookie cutter approach

Intersect Combines data from both layers but only for a

given region where both layers contain data Union

Both overlapping and non-overlapping areas are contained

Overlay - Examples

Raster Overlay Cell by cell combination of 2 or more data

layers Typically applied to nominal or ordinal data

Geoprocessing in ArcGIS

Set of software functions used to manipulate and transform spatial data (single layers or multiple sets of layers), to create new information

Map Overlay functions: combining layers to create single output Extraction Intersection Dissolve Buffer Append and Merge

Extraction Clip: cookie cutter function

Layers to be clipped: point, line, polygon Clipping layer:

When the Input Features are polygons, the Clip Features must also be polygons

When the Input Features are lines, the Clip Features can be lines or polygons.

When the Input Features are points, the Clip Features can be points, lines, or polygons.

Erase: Opposite of clip

Intersection & Union Intersect

Merges attributes between layers, points-lines-polygons

Union Combines two polygon layers, keeping all areas

and merging attributes

Intersection & Union

Dissolve Combines and dissolves similar features

within a data layer

Buffers Creates region at specific distance from

one or more features

Buffers

Undissolved Buffer Separate Polygons

Dissolved Buffers Merged Polygons

Append and Merge Combine 2 layers into 1 Merge “edges” of adjoining layers with

identically matching polygons and attributes Append data while maintaining the attributes

contained in the selected layer

ARcGIS Tool for Geoprocessing To be covered in Lab

a. Clip toolArcTool Box- Analysis Tools – Extract - Clip

b. Erase toolArcTool Box – Analysis Tools – Overlay - Erase

c. Intersect toolArcTool Box – Analysis Tools – Overlay – Intersect

d. Union toolArcTool Box - Analysis Tools – Overlay - Union

e. Dissolve toolArc Tool Box – Data Management Tools – Generalization - Dissolve

f. Buffer toolArcTool Box - Analysis Tools – Proximity - Buffer

g. Append toolArc Tool Box – Data Management Tools-General – Append

h. Merge toolArc Tool Box – Data Management Tools- General – Merge

Using Model Builder Models are built to automate geoprocessing

workflow

References webhelp.esri.com/.../spatial_operations.htm http://www.wou.edu/las/physci/taylor/es341/ge

oprocessing_ArcGIS.pdf www.ianko.com/.../images2/buffer_dissolve.jpg

NCRG Training Courses, “Introduction to GIS”

Prepared by Training, R&D Division GIS Fundamental by Paul Bolstad

Union Query

Union operation from relational algebra Applied ONLY to thematic maps with same

attribute schema

Example: union of the map containing European countries with more than 10M inhabitants and the map containing European countries with less than 10M inhabitants. The two maps both have schema (name, population)

Thematic map of countries (and population) of western Europe with less than 10M inhabitants

Thematic map of countries (and population) of western Europe with more than 10M inhabitants

Union Query