Structures for Storing Geographic Data - Introducing Geographic Information Systems with ArcGIS - page 232

Just to exhaust the fundamental types of GIS storage methods, although it doesn't help solve this
particular problem: Partition a surface that is above (or below, or both) the area of interest into
irregular triangles. Except for the periphery, each triangle shares sides and vertices with an adjacent
triangle. The triangles approximate the height of the surface (e.g., elevation), the slope, and the
direction (e.g., aspect). This sort of dataset is known as a triangulated irregular network, or TIN,
which you met briefly in Chapter 2.

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The Raster Data Model

One way of systematically dividing up an area of interest is shown in Figure 4-3. Here, regularly spaced
horizontal and vertical lines, like those that generate the squares of a chess board, make a grid that creates
relatively small areas called cells . In the past, and sometimes currently, the practice was to index each
cell by a row number and a column number. Generally, the top (north-most) row was numbered one
(1) and the left (west-most) column was numbered one (1). More recently the indexing has shifted to
strictly geographic coordinates. In this case, the coordinates of the center of the southwest-most (lower-
left) cell are specified by the easting and northing of (usually) the center of that cell. The horizontal and
vertical lines are parallel to the x- and y-axes of the coordinate system. Since the cell size is known, the
coordinates of the center of any cell may be easily calculated.

Column number
3 1 2 4

1

2

3

4

Cell
size

FIGURE 4-3 A basic raster that allows
storage of categorical data

2 A vector is a mathematical or physical entity that has magnitude (in this case length) and direction (it has a starting
point and an ending point, and is, therefore, an arrow pointing in a geographic direction).