Selecting a Projection for Spatial Analysis

More map making! When you’re first starting a new project and have data files in different projections, you can still view all your data together at once, using something called on-the-fly projection (a common feature of most GIS software). On-the-fly projection displays all your files using your “project CRS” and lets you have map layers with different projections. But, you can’t use geoprocessing features or point sampling tools unless your files are actually in the same coordinate reference system (CRS). It’s very easy to switch the projection or CRS within common GIS software, but switching creates more copies of each file and is annoying. You can save some time by picking a CRS and sticking with it.

When I’m working with spatial data, the first choice I make is whether I should build my map in a projected system or an unprojected system.  An unprojected system like WGS 84 is great if you’re planning on using common mobile apps like Google Maps or Orux Maps, since it’s really easy to collect latitude and longitude information on handheld GPS devices. The downside of a latitude and longitude system is that the units are wonky. If you’re trying to calculate how many sampling points fall within 10 meters of a certain river, that’s hard to query in an unprojected system.

Latitude and longitude is also just hard to wrap your head around. Degrees longitude are closer together at the poles and far apart near the equator. Degrees latitude are all the same size, but I bet you still can’t picture how far you’d have to travel to move 1 degree North. Say you’re using a G-Function to study spatial patterns in tree disease. G-Functions let you know at what scale there is evidence that the arraignment of points varies from a spatially random distribution. If you use a lat/long system, the scale information is hard to read and unintuitive.

If you decide that you should use a projected system, the two biggest decisions you need to make are which datum you’re using and what units you want your CRS to be in. If you’re based in the US, I highly recommend using the national datum recommended by that National Geodetic Survey. Right now it’s NAD83, which was updated to the NAD83(2011) realization in, you guessed it, 2011. A lot of files I use are still in NAD83(NSRS2007) realizations, and you shouldn’t notice much difference between the two. The units question is purely personal, but I recommend using units that make sense for the scale of the analysis you’re doing. The foot based NAD83(NSRS2007) CRS is commonly used for OGRIP data sources, so I do a lot of my analysis there.

To summarize, I first decide if I want to use a projected or unprojected system. Then if I’m selecting a projection I select the datum and units I want. Next week I’ll go into some more detail about projections of NAD83 and the way we communicate CRS information, EPSG numbers!

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