Geodesy, Coordinate Systems, Geographic Projections, and Scale

First, a disclaimer: This text does not pretend to cover in detail such issues as geodetic datums, projections, coordinate systems, and other terms from the fields of geodesy and surveying. Nor will the text rigorously define most of these terms. Simply knowing the definitions would mean little without a lot of study. Many textbooks and Web pages are available for your perusal. These fields, concepts, and principles may or may not be important in your use of GIS, depending on your projects. However, the datum, projection, coordinate system designations, and measurement units must be identical when you combine GIS or map information. If not, your GIS project may well produce inaccurate results.

How we apply the mathematically perfect latitude-longitude graticule to points on the ground depends partly on human’s understanding of the shape of the Earth. This understanding changes the more we learn. Geodesy is the study of the shape of the Earth and the validity of the measurements human beings make on it. It deals with such issues as spheroid and datum. You don’t have to know much about geodesy to use a GIS effectively, provided your data are all based on the same spheroid and datum (and projection and units, as you will see later). It is the application of geodesic knowledge that caused the differences in the coordinates of that hypothetical object I discussed earlier that was put into the ground six or seven decades ...

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