# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**Re: Coordinates**

**From:**Hal Mueller

**Date:**2001 Sep 25, 11:07 PM

Peter Dana has a very nice exposition of datums, ellipsoids, and coordinate systems. I just took another look at his stuff while thinking about this question--it's at http://www.colorado.edu/geography/gcraft/notes/datum/datum.html (the drawing of coordinate shifts for a particular point at http://www.colorado.edu/geography/gcraft/notes/datum/gif/shift.gif is worth a look also) My thoughts (I am not a real geodesist, someone please correct me): 7 seconds difference is not a terribly gross error. It's about 230 yards at the equator, which is in the same ballpark as the difference between NAD27 and WGS84 (pre- and post-GPS coordinate systems; the error you get when you pick coordinates off an old chart and enter them into your GPS as modern coordinates). Your source says La Canoa's earth radius is 6378388 meters. WGS84's earth radius is 6378137.0. These give equatorial circumferences of 40076594 meters (La Canoa) and 40075016 meters (WGS84), a difference of 1577 meters. At 10 degrees latitude the difference would of course be a bit less, but I'll skip over that. Let me push a little further--those radii mean, if I understand correctly, that WGS84 says the distance from 0 deg N 71.5 W to 0 N 0 W is (71.5/360)*40075016 meters, or 7959344 meters, while La Canoa says that distance is (71.5/360)*40076594 meters, or 7959657 meters, a difference of 313 meters. Longitude by WGS84 is 71.473250; by La Canoa it's 71.471256. WGS84 long * WGS84 circumf = 7956365.8 m La Canoa long * La Canoa circum = 7956456.9 m so now we only have a 91 meter discrepancy. This is without accounting for the differences in latitude due to the difference in the assumed shape of the earth, and assuming also that the position of that cathedral was accurately determined in the first place. So I think those two coordinate representations are at least reasonable. Hal ---- Hal Mueller hal{at}mobilegeographics.com Mobile Geographics LLC http://www.mobilegeographics.com/ Seattle, Washington (206) 297-9575 Get CelestNav 2.2.2! http://www.mobilegeographics.com/celestnav/