How to Choose a Coordinate System

For better or worse, ArcGIS makes it easy for us to avoid the onerous task of truly understanding coordinate systems. This is unfortunate because it works directly against our ability to produce meaningful and defendable results. So much of the math associated with coordinate systems goes on behind the scenes that it is often difficult or impossible to know how our analysis results are being calculated, let alone how to verify the accuracy or precision of the resulting values.

But intelligent and deliberate GIS work must account for the variations in accuracy that result from the coordinate systems assigned to our data and our maps. You should no more perform GIS analysis with a poor understanding of coordinate systems than attempt to build a fire with a hand full of jelly and an empty shampoo bottle.

Often our data comes to us with an appropriately assigned coordinate system. We don’t have to hand select coordinate systems and account for all these details. But when collecting data, creating data, or managing projects, sometimes we must choose the coordinate system ourselves. The following three things should be taken in consideration when choosing a coordinate system:

  1. Where is the area of interest (the extent of data),
  2. What general shape is the area of interest (trending north/south or east/west),
  3. The purpose of the map or analysis.

Map or data extent: If you are mapping or analyzing a relatively small area, a county or state, often the UTM or State Plane coordinate systems are appropriate and effective at reducing all types of distortion. Larger areas of interest require coordinate systems that deliberately concentrate distortion away from the main areas of focus. Remember, very generally speaking, world maps tend to be mapped with cylindrical projections, continents with conic projections, and azimuthal projections are used for small areas located in only one hemisphere.

Shape of area of interest: Very generally, Transverse Mercator projections are used for regions that are oriented more north-south and Lambert Conformal Conic projections are applied to those that run more east-west.

Map purpose: What will you be doing with your map project? Is it a visualization tool? Will it display analysis results? It is thematic, like your standard Election Results map? Will it be used for navigation? What scale and extent will be most appropriate and effective for displaying your results?

Above all, don’t panic. In the big picture, it isn’t that hard to pick the right coordinate system. You have a lot of help because ArcGIS has so many predefined coordinate systems and has the math in hand to do accurate transformations and re-projections when necessary and distortion is relatively small overall if you are working in typically small regions.

So after all this, what is the best way to flatten a round world onto a flat map? I hope you realize now that this really isn’t the right question to ask. What you want to ask yourself is: What is the best coordinate system for the analysis you want to do in your particular area of interest? And hopefully you understand the individual components of coordinate systems so that you can make informed and wise decisions.