Geographic Information Systems

Spatial data types

Field Vs. Object

(Geographic) objects populate the study area, and are usually well distinguishable, discrete, bounded entities. The space between them is potentially empty.

A (geographic) field is a geographic phenomenon for which, for every point in the study area, a value can be determined.

Field View Vs. Object View

Examples:

Object View: Trees, Houses, Streets.

Field View: Elevation, Temperature, Rain Intensity.

General rule-of-thumb is that natural geographic phenomena are more often fields, and man-made phenomena are more often objects.

Geographic Objects

For example roads are objects, they are

characterized by:

• location (where does it begin and end)

• shape (how many lanes does it have)

• size (how far can one travel on it)

• orientation (in which direction can one travel on it)

Computer representations of geographic information

In GIS, fields are usually implemented with a

tessellation/raster approach, and objects with a

(topological) vector approach.

Regular tessellations

A tessellation (or tiling) is a partition of space into

mutually exclusive cells that together make up the

complete study space.

The simplest example is a rectangular raster of unit

squares, represented in a computer in the 2D case

as an array of n × m elements

Raster Example

Regular tessellations

• Square, regular tessellations are known under

various names in different GIS packages: raster

or raster map.

• The size of the area that a raster cell represents

is called the raster’s resolution.

Point representations

• Points are defined as single coordinate pairs (x, y)

when we work in 2D or coordinate triplets (x, y, z) when

we work in 3D.

• Points are used to represent objects that are best

described as shape- and sizeless, single-locality

features.

Line representations

• Line data are used to represent one-dimensional

objects such as roads, railroads, canals, rivers and

power lines.

• The two end nodes and zero or more internal nodes

define a line.

• Another word for internal node is vertex (plural:

vertices);

Line representations

• Another phrase for line that is used in some GISs is

polyline, arc or edge.

• A node or vertex is like a point (as discussed above)

but it only serves to define the line

Start Point End Point

VertexVertex Vertex

Area representations

• Employed when area objects are stored using a vector

approach

Spatial Data Models

Raster

exhaustive regular or irregular partitioning of space

associated with the field view

location-based

Vector

points, lines, polygons

associated with the object view

object-based

Spatial Data Models

ESRI Shapefile

Designed by ESRI for ArcView

Implementation of the vector model

An individual layer stores a single type of geometry (i.e. point, line, polygon)

ESRI Shapefile

Four primary files in a shapefile: .shp, .shx, .dbf and.sbn

All files must share the same prefix for one shapefile,

e.g. road.shp, road.shx, and road.dbf

.shp : stores the feature geometry (binary)

.shx : index for .shp file

.dbf : attribute data stored in dBASE format

.sbn: for indexing

Quick Tour