From just playing around with GPSMapEdit I was able to figure out most of the shape file import options. Sometimes, however, I’d open a shape file and be confronted with a big red square, a mass of tangled lines at such a scale that it was impossible to view the entire mess at once.
I never did figure out what the “Coordinate system” and “datum” dropdowns on the 3rd step of the file import were for. I used to just choose different values at random until my maps seemed to line up. Turns out they are what determine not only whether your maps line up but also whether you get anything that resembles a map at all.
The Florida Department of Transportation GIS Data Directory has a wealth of data, and below each of the section headings their site says: Projection: UTM 17; Datum: NAD 83. Sure enough, there’s a UTM coordinate system defined in GPSMapEdit, and a Zone dropdown that lets you choose 17, and NAD 83 just so happens to be one of the available Datum’s.
This inspired me to find out what these things really mean.
“Projection is a fundamental component of map making. A projection is a mathematical means of transferring information from a model of the Earth, which represents a three-dimensional curved surface, to a two-dimensional medium–paper or a computer screen. Different projections are used for different types of maps because each projection particularly suits certain uses. For example, a projection that accurately represents the shapes of the continents will distort their relative sizes.”1 Wikipedia, not surprisingly, has an entire article devoted to different types of map projections.
The National Geodetic Glossary defines datum in general as “Any quantity or set of such quantities that may serve as a referent or basis for calculation of other quantities” and specifically relating to things geodetic as “A set of constants specifying the coordinate system used for geodetic control, i.e., for calculating coordinates of points on the Earth.”2
Specifically, “The North American Datum of 1983 (NAD 83) is ‘The horizontal control datum for the United States, Canada, Mexico, and Central America, based on a geocentric origin and the Geodetic Reference System 1980. This datum, designated as NAD 83, is the new geodetic reference system. … NAD 83 is based on the adjustment of 250,000 points including 600 satellite Doppler stations which constrain the system to a geocentric origin.’ (Geodetic Glossary, p 57)”3
WGS 84 is the World Geodetic System frame of reference.
So depending on what part of the world you’re mapping, and how you expect the map to be used, you may choose to encode the map using different projections and datums. When using maps made by other people, it helps to know what coordinate systems they used. To turn the red blob into a map, I just had to change coordinate systems. The map projection and coordinate system information should be in the map metadata file.
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