What the heck is a CRS?
The short answer to this question is: convert everything to WGS84 and pretend this question never came up.
The long answer to this question is: here we go!
Coordinates
You know what coordinates are, right? They look like this:
But hey, we’re all about maps, so we’re putting them around a globe!
Since a globe isn’t a 2-dimensional sheet of paper (hint: it’s a sphere), in that case coordinates might look more like this:
Latitude! Longitude! Degrees! We all know that.
Unfortunately, though it isn’t that simple. A CRS is a coordinate reference system, and it helps make sense of what the coordinates actually mean.
How did we get here?
Latitude and longitude (and optionally altitude) seems pretty simple. Turns out that instead, when you say something is at X degrees latitude and Y degrees longitude, you’re make a lot of assumptions about the Earth:
- How spherical it is (technically how ellipsoid-y it is)
- Where the center is
- What sea level is
- How wide it is at the middle
- How “tall” it is between the poles
- What exactly counts as 0 for latitude and longitude
- How tectonic plates are moving
- Where the center of Earth’s mass is
Doesn’t seem so hard to figure those out since we have Science Truths, right? But it is! It’s tough!
The biggest problem is that it turns out the Earth is a pretty weird shape, like a potato floating in space. A rather ellipsoid-y potato, but just to say: it ain’t perfect. And as we get better and better at analyzing exactly what shape it is and where points are in relation to each other, it just goes to show up how terrible we’ve been at measuring.
As a result, not everyone agrees on latitude and longitude! It seems like lat/lng always means exactly one specific point on the globe, but it just ain’t true. Depending on what assumptions you’re using, the same lat/lon pair could refer to points “tens of meters” apart!
That set of assumptions? That’s a coordinate reference system.
The two most popular ones (for Americans!) are WGS 84 and NAD 83
WGS 84is the World Geodetic System standard was last updated in 1984. It’s pretty good for the entire world.NAD 83is the North American Datum standard, which was last updated in 1983. It’s rather close toWGS 84, but is a slightly better fit for North America.
They both use latitude and longitude, and the difference to you is indistinguishable. If you’re a relativistic GPS clock or a millimeter-specific zoning fiend, it might matter, but they’re basically the same for everyone else.
For further reading, try out this or this or the Wikipedia pages I’ve linked above.
Moving beyond latitude and longitude
Now, let’s pretend you’re the US government. If there’s one thing you hate, it’s latitude and longitude. They use degrees, minutes and seconds! That’s so confusing you just might die.
I mean, sure, it makes sense to do that if you’re mapping the entire world, since it’s an ellipsoid (a.k.a. flattened sphere), but what if I’m like looking at Nebraska? Or New York? Can’t we just pretend it’s flat?
I don’t know if you can, but they do. Enter: The State Plane System.
Planes are flat, 2-dimensional things, right? Like sheets of paper? The State Plane System basically holds a sheet of graph paper up to every state, proclaims it to be flat, and says “let’s just use normal Cartesian coordinates like we learned in 7th grade.”
And so we do! Oddly, it works pretty well within the state.
The only issues crop up when you want to compare different states - the New York State Plane isn’t the same as the Connecticut State Plane, so you have to do all sorts of computing to convert between the two (although really your GIS program does that for you).
Or, of course, if you’re me or you and you want to do literally anything with that data. I don’t know a damn thing about the State Plane system, all I know is all my other data is in latitude and longitude and I need to make them play nice together.
Telling CRSes apart
You can tell a CRS apart from another CRS not by its crazy name like WGS 83 or New York State Plane, but because they each have wonderful little codes called EPSG codes.
Note: EPSG stands for European Petroleum Survey Group, which was an organization formed in 1986 that aimed to collaborate on mapping stuff for the oil industry.
| Normal name | EPSG code |
|---|---|
| WGS 84 | EPSG:4326 |
| NAD 83 | EPSG:4269 |
| New York State Plane (Long Island) | ESRI:102718 |
| New Hampshire State Plane | ESRI:102710 |
ESRI stands for Environmental Systems Research Institute, who are the people that make ArcGID.
Converting between CRSes
I like to make all of my content the same when I’m working on it in GIS software, and a lot of times it’s easier to just work with latitude and longitude. So I go ahead and open it up my shapefile in QGIS and export it with a different CRS