Spatial measurement

Before the advent of GIS, spatial measurement was quite a tedious task. On printed (or drawn) maps, this usually meant aligning a piece of string or wire on a map carefully and then measuring it with a ruler. Another possibility was to use a specialized tool, called an opisometer (curvimeter). The whole process was error prone and subject to a number of factors, including the stretching or shrinking of the map material. For surveyed coordinates, measurement meant doing lots of math. Things were even worse for geodesic (latitude-longitude) coordinates, where complicated formulas had to be used.

Now, with spatial databases, it's possible to get spatial measurements in milliseconds. This doesn't mean that one can blindly push the button, though. The details will be explained in this section.

General warning - mind the SRID!

Measurement functions come in two variants: planimetric and geodesic. The former operate on a flat surface, while the latter operate on a curved surface...