Nelson's Weblog: tech / latitude-longitude-distance-approximations

Some Bits: Nelson's weblog

Rules of thumb for geocoordinates

I'm doing some geographic hacking. I get confused about degrees, seconds, fractions, and precision of coordinates. Here's some approximations for how far distances are for latitude degrees. Imperial measures, my apologies to my non-American friends who are not crippled by years of non-metric education.

1 degree	70 miles
1 minute	1.15 miles
1 second	100 feet

0.1 degree	7 miles
0.01 degree	3600 feet
0.001 degree	360 feet
0.0001 degree	36 feet
0.00001 degree	4 feet

Those numbers are for latitude. Longitudes get narrower at the poles. At San Francisco's latitude of 37 degrees, a degree of longitude is about 55 miles, or about 80% of a degree of latitude.

A modern consumer GPS with WAAS should be accurate to within about 15 feet. So for many contemporary geocoding applications, you want to record and publish numbers to about 0.1 seconds or 0.00001 degrees. That's 26 bits of precision, which fits comfortably in a fixed-width 32 bit number but is too precise for a 32 bit IEEE float.

Calculations thanks to this Javascript library

tech
2010-02-01 17:52 Z