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Locations in a Digital World, Position Measurements, Datums, Coordinate Systems, GCS, Map Projections, UTM, and SPCS
Think about this. You’re at the mall and your cell phone rings. It’s your friend Sara calling you with a really basic question: “Where are you?” For such a simple question, there’s really a wide variety of answers. You could say “I’m at the mall,” which gives Sara some basic location information—but only if she knows where the mall is. If she’s from out of town and unfamiliar with the location of the mall, you haven’t given her the information she needs to know where you are. If you tell her, “I’m in Columbus,” then she’ll have another type of information—but Columbus is a big city, and she still won’t know where in the city you are. You could give her the name of the cross streets by the mall, the address of the mall, or the name of the store you’re in at the mall, and even though all of the above contain different levels of information, your friend still won’t have the information she needs to know precisely where you are.
“I’m at the mall” effectively conveys your location only if your friend has some sort of knowledge of the mall’s location. If you give her a reference like “The mall’s on the north side of town,” or “The mall is just off exit 5 on the freeway,” then she’ll have some idea of your location. For the location information to make sense, she needs to have something (perhaps a map of the city you’re in) to reference the data to. If you’re asked to provide specific information about your exact location, it’ll have to be much more detailed than “I’m at the mall.” In a medical emergency, Emergency Medical Technicians (EMTs) need to know your exact location (they can’t afford to waste time searching the entire mall to find you). You need to provide precise location information so that the EMTs will be able to find you, no matter where you are.
Geospatial technology works the same way. First, since it deals with geospatial data (which is linked to non-spatial data), there has to be a way of assigning location information to the data being handled. There has to be some way of referencing every location on Earth. In other words, every location has to be clearly identifiable and measurable. Second, these measurements require some sort of reference system, so that a location at one point on Earth can be compared with locations at other points. Third, since we’re dealing with points on a three-dimensional (3D) Earth but only have two-dimensional (2D) surfaces to examine (i.e., maps of various kinds), there has to be a way of translating real-world data to an environment we can use. This chapter examines how these concepts are treated in all forms of geospatial technology. If you’re going to be able to pin down precisely the coordinates of a location, you must first have a datum and a coordinate system.
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