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:
- Arcsecond: 1/60th of an arcminute, or 1/3600th of a degree, equal to about 30 east-west meters at the Earth’s equator and 30 meters north-south. DEMs which have a three-arcsecond gridposting (or spacing), for example, are 90-meter DEMs, although this terminology is not entirely accurate for DEMs not near the equator.
- Dataset: Usually a collection of related DEM files. It is common practice, for example, for DEM suppliers to package DEMs which neighbour each other into a set.
- Datum: a model of the Earth used to determine geographic measurements. Popular datums use flattened spheres (ellipsoids) to more accurately model the true shape of the Earth, since its rotation and uneven mass density distort it slightly. Datums are useful to accurately determine elevations, since they can provide a better measure of the distance between the Earth’s surface (at a particular point) and its center. Because the Earth is actually an ellipsoid, its center is not a point but a line. Modern datums are based on precise satellite measurements of the Earth’s regional gravitational forces and are periodically updated. The WGS84 datum is widely considered to be accurate enough for most purposes.
- DEM: short for Digital Elevation Model, a file which is usually a georeferenced heightfield. Some DEMs are vector based, such as contour plots. Some DEMs are packaged as multiple files within a compressed archive file; the elevation data will be in one file, its descriptive metadata (header information) in another file, etc.
- DTM: short for Digital Terrain Model, which is a DEM.
- Latitude/longitude: how locations on planets are specified, usually used to indicate large areas and the locations of major items like countries, lakes, states, and cities. Lines of latitude and longitude correspond to the intersections of angles from the planet’s center with its surface, and are thus described in degrees. Longitude zero runs through Greenwich, England and latitude zero is the equator. The poles are at latitudes 90 degrees north and south and have undefined longitude. Since the Earth’s land masses are continually moving, the concept of longitude zero is somewhat relative, but the motion is so small compared to the Earth that it can be effectively ignored.Fractions of degrees are (in decreasing magnitude) minutes and seconds, or more formally, arcminutes and arcseconds.
- Georeference: to include geographical measurement data.
- Great circle: an imaginary curved line whose length is the planet’s circumference. Given any two surface points, the portion of the great circle intersecting them represents the shortest distance between those points. For this reason, long-range aircraft typically fly along great circles — e.g., to travel efficiently from Chicago to Moscow, one would fly over Greenland.
- Gridpost: analogous to a pixel in a heightfield. Gridposts represent fixed locations spaced equally distant from their neighbours and are arranged in a regular 2D array. The shorter the distance between gridposts, the tighter spacing a DEM has, and thus it has higher scan resolution since it shows more detail of the Earth’s surface. Modern survey satellites can achieve around 5m or 1m spacing, and finer resolution is obtained using reconnaissance aircraft or ground surveys.
- Projection: a way of mapping part or all of a planet’s surface onto
a flat surface. There are many ways to do this, each one being favored depending on the purpose.
The most popular is probably the global Mercator projection, which preserves distances accurately
at the equator but enlarges them greatly near the poles. For a temperate body such as the Earth,
this works fairly well since most people do not live near the poles.
- Shapefile: a file containing auxiliary data for a DEM, such as lot boundaries, road/street layouts, postal code regions and identifiers, etc. Shapefiles tend to be structured around vector geometry scenegraphs and database tables.
- USGS: short for United States Geological Survey, an American government organization responsible for mapping the Earth’s surface (with an obvious primary goal to map the United States in particular). Their expertise, large collection of data, and being a public body has resulted in their standards becoming common GIS industry standards.
- UTM: short for Universal Tranverse Mercator, a common measurement system
to make it easier to deal with local areas. The basic idea is to divide the Earth’s surface
into small patches and then use meters to measure distances and locations within each patch.
UTM is popular with GPS device users, hikers, mapmakers, etc. The Mercator aspect derives from
UTM being a distorted projection since distances and positions on UTM maps can be computed as if
the Earth was flat. Each UTM patch (or zone) is small enough that the difference between the
Mercator projection and the round Earth can be ignored.