Spatial Analysis Based on Raster Data Processing - Introducing Geographic Information Systems with ArcGIS

Geography Reference

In-Depth Information

Each cell represents a specific location on the surface of the Earth. While each cell of a raster is square, the

surface area of the Earth the raster represents may not be. Such real-world areas should be almost square,

however. If the northing span of a raster is too great, the representation suffers. For example, take a UTM

zone whose “base” is at latitude 40 degrees and “top” is at latitude 48 degrees. Suppose the cells are 100

meters on a side. Let's say a cell at the base would represent 10,000 square meters. A cell at the top of the

zone, admittedly more than 500 miles away, would represent only about 8,800 square meters!

Rasters may be described as integer or floating-point. All the cells in an integer raster contain integers or,

if not, are designated NoData. All the cells in a floating-point raster contain floating-point numbers, or, if

not, are designated NoData.

Rasters with Integer Cell Values

Rasters with integer cell values are used to store what is variously called discrete data or

categorical data. Such data sets describe a phenomenon or object that exists at the location of

that cell, referenced by the cell Value.

Each cell in an integer raster belongs to a zone . The zone number—the Value—must be an integer—

positive, negative, or zero. Cells in a given raster that have the same value all belong to the same zone. So

a zone consists of all the cells with a given value. Each cell that has a Value belongs to some zone or other,

even if the zone is composed of only one cell. Cells in a zone may be adjacent, but may well not be. See

Figure 8-2.

An integer raster has associated with it a Value Attribute Table (VAT). 4 The table may have many fields

but the two primary fields are Value and Count.

There is a record in the VAT for each zone. The raster shown in Figure 8-2, consists of eight zones. The

zone numbers are 21, 22, 23, 24, 25, 26, 27, and 29. Obviously, sets of cells in the same zone may be

connected or not.

FIGURE 8-2 Basic_Raster with integer values indicating