Coordinate Systems

Explaining where something is is ultimately an explanation of what the coordinate system is. There has to be an origin somewhere, and there has to be a distance from the origin is. There are several PDUs with position records, but the answer to where something is depends on explaining the semantics of the records.

A classic position record looks simple:

X	Y	Z
958506.1	455637.2	4344627.4

It's three double precision floating point numbers in the record. What on earth is the meaning of the numbers? Where is the origin, what are the units the numbers use, and is that a rectilinear coordinate system? The designers of DIS wanted to position items anywhere, including the air, below the ocean, submarines, land, and space.

DIS uses a rectilinear coordinate system with its origin at the center of the earth. The units are meters. This seems odd if all you want to do is place an aircraft carrier in the right location, but a better idea when you realize the simulations often use curvature of the earth. Mathematicians are sophisticated enough to supply conversion formulas between DIS and latitude/longitude/altitude, or Military Grid Reference System (MGRS), or other grid systems. The DIS position system solved problems most computer entertainment games didn't bother to solve--the games stuck to a perfectly flat game terrain instead of replicating a horizon. DIS can also, with some more math, place of flat service at a point on the globe. This can introduce some errors at longer ranges, but at the same time make programming easier.