Over the last month or so I’ve had a little series about how we describe positions on the globe and how we visualize spherical space on a 2D map. I started with big picture information, like why we care about map projections. Last week began giving more concrete advice for picking a projected or unprojected system when you’re doing your own GIS work. Today is a continuation of last week, focusing on coordinate reference systems.
As a quick refresher, here are some words we’ve used so far:
- Datum– defines the not-quite-spherical shape of the earth using an ellipsoid shape, major axis, and minor axis (learn more here)
- Projection– a system to visualize 3D spatial relationships as 2D maps (learn more about what a map projection is here and more about choosing projected or unprojected data here)
I talked about coordinate reference systems in the last post, but here are some more details. A coordinate reference system (CRS) uses coordinates, just like X and Y on a scatter plot (see the Cartesian plane to the left as a visual reminder), to describe space on a 2D map. A CRS has an origin and units, and is defined for a specific region.
One example of a CRS that’s defined for northern Ohio is the datum NAD83(NSRS2007) projected using geocentric translations as the projection, centered over -9360811.02925174 4909282.25551509 using feet as the unit. That’s a lot of information to describe the coordinates for one map, and I didn’t even list all the relevant attributes.
Instead of trying to relay all that information to the people you’re sharing data sources with, you can just use the EPSG number. EPSG stands for European Petroleum Survey Group (source), which is relevant because the International Association of Oil and Gas Producers maintains a database of coordinate reference systems. Each CRS gets assigned a unique EPSG number, and you can learn more about the registry here.
Another place to learn about CRS and EPSG numbers is EPSG.io, which has wonderfully detailed attribute information for each CRS in the registry. It also has a tool to convert between common systems, although I usually convert things within QGIS or whatever software I’m using.
When you’re making maps frequently, it helps to use the EPSG number in the file name. My default file naming system is datatype_sitenumber_EPSG, and this has saved me a lot of trouble. Good luck on your mapping endeavors!