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Introducing Geographic Information Systems with ArcGIS: A Workbook Approach to Learning GIS, 3rd Edition by Michael D. Kennedy

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Determining Where Something Is: Latitude and Longitude

A fundamental principle underlies all geography and GIS: Most things on Earth don’t move (or move very slowly) with respect to each other. Therefore, we can talk about the position of something embedded in or attached to the ground and know that its position won’t change (much). It seems like a straightforward idea, but position confuses a lot of people when it is described as a set of numbers.8

Let’s suppose that in 1955 somewhere in the United States you (or your parents, or their parents) drove a substantial metal stake or pin vertically into solid ground. Now consider that the object, unless disturbed by human beings or natural forces such as erosion or an earthquake, would not have moved with respect to the planet since then.9 In other words, it is where it was, and it will stay there. Three numbers—latitude, longitude, and altitude—could identify the location of the object in1955. However, over the last half century, teams of mathematicians and scientists (skilled in geodesy) developed other sets of numbers to describe exactly the same spot where your object now resides. The actual position of the object didn’t change, but additional descriptions of the where of the object have been created.

Ignoring the matter of altitude for the moment, suppose that the object was driven into the ground at latitude 38.0000000° (North) and longitude 84.5000000° (West), according to calculations done before 1955 that indicated the location ...

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