GUS: 0262 Fundamentals of GIS Lecture Presentation 6: Raster Data Model Jeremy Mennis Department of Geography and Urban Studies Temple University

A raster representation is composed a series of layers, each with a theme Typically used to represent ‘field- like’ geographic phenomena Raster Data Model

Regular Tessellations SquaresTrianglesHexagons

Irregular Tessellations

Raster Grid –but most common raster is composed of squares, called grid cells –grid cells are analogous to pixels in remote sensing images and computer graphics

Raster Resolution The distance that one side of a grid cell represents on the ground = grid cell resolution The higher the resolution (smaller the grid cell), the higher the precision, but the greater the cost in data storage

Raster Data: Encoding

Raster and RDBMS Raster layer can be attached to a RDBMS Agricultural 2Road 3Residential 4Industrial IDLand Use

Raster Data: ArcGIS

Raster in ArcGIS – Floating Point

Raster in ArcGIS - Integer

Converting Polygons from Vector to Raster

Data Compression Common Methods: –run length encoding –value point encoding –chain codes –block codes –quadtrees

Run Length Encoding and Value Point Encoding

Raster Chain Codes directions around the boundary of a region Start North 1 East 1 North2 East2 South3 West 3 Value 4

Raster Bock Codes two dimensional run length encoding

Quadtree a partitioning of heterogeneous space into quarter sections

Quadtrees node is a quadrant that is heterogeneous leaf is a quadrant that is homogeneous quadrants are assigned an ID number according to their position and level