Coordinate reference systems and projections

I get confused as to the difference between coordinate reference systems (CRS) and projections. This overview clued me in some, but I still might be wrong. Here’s my current understanding.

 

CRS are all about spherical coordinates, naming points on the sphere. When I say “San Francisco is at 37.75, -122.45”, what I’m saying is it’s 37.75 degrees north of the equator of the sphere and 122.45 degrees west of the prime meridian through Greenwich. Points are defined in spherical geometric relation to absolute, fixed locations on the earth.

Only it’s not that simple. First, the earth isn’t a sphere, it’s squashed, and CRS are defined relative to a specific datum that has some oblate spheroid shape with complex geometry. Second, the prime meridian we most commonly use doesn’t go through Greenwich anymore, it goes about 100m east.

The most common CRS in use is WGS84, the basis of GPS navigation. WGS84 itself changes as measurements get more accurate, so the specific thing we all use is WGS84 (G1150). (I’m unclear on how measurement changes are managed; the earth itself is moving, too, making absolute location a slippery concept. See also ITRF2000) The other CRS I’m familiar with from NACO aviation charts is NAD83 (CORS96). It agrees with WGS84 to within two meters and I believe NACO advises treating them as interchangeable. In practice I don’t see any useful reason to do work in anything other than WGS84, unless importing data from some other source that used a different CRS.

 

Projections are all about planar geometry, warping a spherical map to a flat surface.  There’s a large variety of map projections in common use. Mercator is most popular for casual maps and shows up frequently online because it’s what Google Maps (and all slippy maps?) use. Lambert Conformal Conic shows up in NACO sectional charts and USGS sectionals; it’s sort of like a but the projection is parameterized to have the least distortion between two chosen latitudes. Also Lambert Conformal has the nice property that a straight line drawn on the map is very close to a great circle route (shortest distance); handy for airplane navigation.

In maps showing a relatively small area of the Earth, the differences between projections are relatively small. I think I’m inadvertently benefiting from this in my approach plate georeferencing experiments. I’m just treating the plate as if it were drawn in a mercator projection and it looks about right.