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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


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Hal Mueller                hal@mobilegeographics.com
Mobile Geographics LLC     http://www.mobilegeographics.com/
Seattle, Washington        (206) 297-9575
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