To use a GIS, you need to understand coordinate systems. Here’s a short run through of how they work.
A GIS, or a Geographic Information System, is in simple terms a computerized map with tools to analyze, present, convert, and store the information contained within. To at all be able to use it, you need to have a rudimentary understanding of the coordinate systems the GIS relies on – which are surprisingly tricky.
The measurement framework
A coordinate system is always one of two types – geographic, or projected.
Geographic coordinate systems
The by far most common type of geographic coordinate system consists of lines of latitude and longitude, running north to south, and east to west, respectively. These are meant to represent the actual Earth, and are relatively straightforward.
Position is given by stating a point’s distance from the two 0° lines. These lines are the equator for lines of latitude, as it’s the mid point between the two poles; and the (more or less arbitrarily chosen) prime meridian for lines of longitude, which runs through Greenwich, England.
Projected coordinate systems
A projected coordinate system is what happens when you want to convert spatial information, which is three dimensional, into 2D. Since the Earth is curved, this is impossible to do without finding some way to “unwrap” it and lay it flat, which always causes some distortion.
There are several ways to do this. You could wrap a cylinder around the Earth and project the geographical data onto it, causing whatever parts that touch the cylinder to stay exact, but the parts far away to be less accurate. You could create a network of smaller “footprint” projections at a scale where the distortion is negligible, and fitting them together like puzzle pieces. However, this isn’t feasible for larger scale maps.
The most common solution is cutting the Earth into narrow slices that span all the way from pole to pole, wider in the middle, and then adding those together as if they had uniform width. This is called UTM (Universal Transverse Mercator).
Defining the ellipsoid
Further complicating things is the fact that the Earth isn’t even circular. The rotation causes it to slightly flatten around the poles, making it elliptical (and also causing the equator to not actually be the exact midpoint). Now you have to define the ellipsoid you’ll be using to approximate the Earth’s shape for your coordinate system.
You do this by defining the major and minor axes, in other words the circumference from east to west, and north to south. Here you have to pretend that gravity is equal all across the Earth, unaffected by topography, and then the ellipsoid is the resulting uniform shape. These are marginal differences, but for such a large area, marginal differences matter.
Choosing your datum
Now that you have your lines, and an approximate three dimensional shape to apply them to, you can start divvying the Earth up into benchmarks with known coordinates. A full network of these is called a datum, and was once physically measured from the 0° lines and hammered into stone.
Today we have more accurate, and less time consuming, ways to do this, but datums may vary depending on the methods used to assign them. Examples of datums are NAD83 and NAD27, both of which are in use today. Depending on the datum used, you may find the same point fluctuating by anything from 20 to 100 meters.
Why is all this so important?
Even though your GPS is incredibly accurate, it still needs to be able to translate its “knowledge” into the language of datums, to convey it back to you. This is why it’s so important to know what datum you’re using, or should be using.
The same thing goes for your GIS, and with many different layers operating at once, these need to be using the same datum and projection to overlay correctly. Especially when importing data, or inputting from a new source, this can easily lead to mistakes.
When mapping out distances it’s also important to know what system will lead to the least distortion for your use. Keeping all this in mind, however, you should be able to use your GIS with pinpoint accuracy.