Daylon Graphics

About GIS

GIS is short for Geographic Information Systems. It’s used more as an adjective to indicate that a program or process involves geographic data. There’s a website here that goes into detail about it quite well. There’s also the NOAA’s Geodesy for the Layman.

 

Why Use Geography?

If you’re just using heightfields to make terrain that provides ground in a basic game or local landscape scene, then GIS doesn’t matter. But it can if you want to get more precise or work with other programs that use geographic measurement. An ambitious or planetary-style game could have enough terrain to cover most or all of an entire world, and at that point it makes sense to at least use latitude and longitude.

Most casual terrain users get into GIS when they want to make maps, or acquire actual scanned terrain files and/or merge them to model large areas. The files are georeferenced, and to fit them together properly requires understanding geography a little (or at least having a program that does). Programs which are specifically geared towards landscape design and rendering also use geographic measurement. For example, if you want to position the sun, it’s more natural to think in terms of inclination angle.

Geography basically formalizes the whole issue of terrain measurement. The main thing is that since planets are spherical, latitude/longitude coordinates can’t be directly used to compute distances — e.g., one degree of longitude at the equator is a lot wider than it is near the North pole. It gets even more interesting when you want to be super-precise because the Earth isn’t exactly a sphere. The distortion is tiny, but because the Earth is so large, positions can be off by a few kilometers if you assume a perfect sphere. Compounding the situation is that because people are relatively small, it’s preferable to use different measurement systems for local areas, which opens up a whole field of converting between local and global coordinate systems. Elevations can also be tricky because they depend on what is meant by sea level, but the Earth’s oceans don’t actually have a constant sea level.

Our software can use local or projected/geographic coordinate systems, or neither if you just want to use pixels. To keep things simple, rotated coordinate systems are not supported (i.e., DEM sides are always parallel to lines of longitude).

 

A Short Glossary

Here are short descriptions of some common GIS terms: