Silly Geographic Precision

I tread water in oceans of latitude longitude coordinates.  Every vector data set I encounter is big fat sets of latitude (up-downiness) and longitude (left-rightiness) bits of info which combine (the "co" in coordinates) to pinpoint a spot on the surface of the Earth (and those combine to trace lines or areas).  The more precise the coordinate's numbers, the finer the pinpointing.  But sometimes I see data (usually the results of address geocoding) with a ridiculous number of digits past the decimal place, implying waaaaay too specific a location.  For example, today I saw the sample below as the result of an address geocode.  I'll use a large courier font to connote just how epic this is...

47.640120461583138
-122.12971039116383

That's a tight geocoding.  Not only does the level of precision pinpoint a building, it pinpoints a specific atom in the building.

Here's a breakout of coordinate precision by the actual cartographic scale they purport:

Decimal Places	Actual Distance	Say What?
6	10 centimeters	Your footprint, if you were standing on the toes of one foot.
7	1.0 centimeter	A watermelon seed.
8	1.0 millimeter	The width of paperclip wire.
9	0.1 millimeter	The width of a strand of hair.
10	10 microns	A speck of pollen.
11	1.0 micron	A piece of cigarette smoke.
12	0.1 micron	You're doing virus-level mapping at this point.
13	10 nanometers	Does it matter how big this is?
14	1.0 nanometer	Your fingernail grows about this far in one second.
15	0.1 nanometer	An atom. An atom! What are you mapping?

As a reference, six decimal places of precision is generally plenty-good-enough territory for cartography.  Unless you are collecting the cornerstone base survey coordinate for a mechanical engineer, let's call this good.

Not all Longitudes are the Same
A degree of Latitude is about 68.71 miles, and that's pretty* consistent as you go north or south (when you climb up or down the ladder of latitude, each rung is the same distance).  A degree of Longitude is widest at the equator (about 69.17 miles) but gets narrower and narrower until they all pinch together right down to nothing at the the poles.  The examples above are pretty much best case examples when it comes to Longitude; they get even sillier when you move away from the equator.

A Fine Mesh
Latitude and Longitude lines are the tics of an imaginary mesh that covers the world -like pixels on a screen.  Every time you add a number to the right of each decimal in a Lat Long coordinate, you subdivide the mesh of the Earth by ten each way (bumping your resolution 100 times finer each step).  Things get crazy in a hurry and it's common to encounter data with not just meaningless, but deceivingly precise coordinates.  Just because something is precise, that doesn't mean it's accurate.

P.S. I read a really good book by Dava Sobel a while back on the surprisingly epic history of Longitude.  It's called Longitude.  If you are nerdy enough to have read down this far then it's a safe bet you'll enjoy it.

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