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 Vertex Vertex Vertex Start Point End Point

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

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