Introducing Geographic Information Systems with ArcGIS: A Workbook Approach to Learning GIS, 3rd Edition

Zero-Dimensional Entities in a Two-Dimensional Field: Points

While vital to the functioning of a GIS, a zero-dimensional thing (generically a point) is pretty dull from a geometric view. It is basically a pair of numbers (x- and y-coordinates, or perhaps, latitude and longitude coordinates) stored as single- or double-precision numbers.6

The concept of a point is used in a variety of ways in GIS to represent features, as end points of lines, as vertices in sequences of line segments, vertices of triangles, as reference points tying to the feature set to the real world, as locations to hang labels on, as centroids of areas, as junctions and nodes in geometric networks, as centers or corners of raster cells, and others.

We work mainly with points in a two-dimensional arena, but of course they exist truly in three-dimensional space. ArcGIS will let us add information about this third dimension, sometimes as what amounts to an attribute (a “z” value) and sometimes (for example, in a TIN) as a measure in the true third dimension. Even when a true 3-D point is represented, the units of measurement of the vertical may not be the same as those used in the horizontal plane. Some of the uses of points are described below.

Points representing features—In these cases, a point has associated with it a row in a relational database table that identifies the point and allows the user to add other (attribute) information about the feature the point represents. You became acquainted with points ...