Why Google Maps is often wrong about your exact location


I love these moments–combining high tech, grandfathered bureaucratic standards and science. Especially the bits about the grandfathered standards–reminds me of the joke regarding the origin of road and rail widths.


Things like earth tides and such are just spooky when you see how much (from an earth-centered) frame everything around you moves up and down twice a day.

This is just one reason why differential GPS is the best method for really (centimeter–ish-level) detailed work. You assume all the errors are the same between the nearby reference point and your target point and just take the relative position. Otherwise, you get planes trying to land on runways +/- 10 meters above/below where they really are…

This is a bit silly. GPS has +/- 30 METERS precision. Everything else mentioned here is a rounding error in comparison to that.

If you take many GPS readings over a period of time in the same place you can get much more accurate positioning.
With the right equipment you can get an error of a few cm (in the horizontal dimensions) in an hour or two.

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There are multiple GPS frequencies transmitted, which have theoretical precision ranging from hundreds of meters to a few centimeters. You can get sub-cm precision by correlating multiple frequencies (though atmospheric dispersion may get in the way) and taking multiple measurements (like knowing where you were along the previous portion of your route) and using other available data (like combining GPS data with cell tower information and your spedometer and odometer readings).

I think military GPS is a lot more accurate, but it’s dumbed down for the enemy, such as civilians.


Altough amusingly the military can’t afford/source sufficient quantities of the receivers capable of decrypting the noise-free military signal, so they just turn off the noise on the civilian channel during active operations

Sometimes they do the reverse, as anyone can buy a GPS. During the NATO air strikes in Ex-Yougoslavia you’d be sitting at your desk in, say Munich, and your GPS would tell you that you were moving.
To the best of my knowledge full decrypted GPS is accurate to the tune of 1 foot / ca. 30 cm.

If you visit the Prime Meridian in Greenwich with a GPS, you will note that the positions of the official meridian and the GPS-calculated meridian vary by several hundred feet. The caretakers are used to hearing this from visitors.

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