Here is sort of an interesting puzzle. This map is archived in the Libary of Congress as "Plat of Unidentified City" and contains some clues that I'd think a surveyor could use to identify the place. Note that the scale is given in English feet, but the notation is in Spanish. Note also the West variation of magnetic North.
http://www.loc.gov/resource/g3934p.lh000891/&apos ;">[Plat of unidentified city].
The folks at NOAA have obligingly provided an interactive map of magnetic declinations. If the date of the plat is about 1845, this will generate the isogonic lines of 4 degrees West variation.
I guess there are mysteries and then there are mysteries. I just noticed that the metadata for the map in the Library of Congress catalogue lists it as probably a map of Pensacola, Florida. Generally, it does appear to be consistent with the above example, aside from the declination noted on the "unknown" map.
There can't be too many places where a particular isogonal line crosses a shore line. It looks like a pretty substantial fort there, that might still be standing today.
rfc, post: 342530, member: 8882 wrote: There can't be too many places where a particular isogonal line crosses a shore line. It looks like a pretty substantial fort there, that might still be standing today.
As it turns out, the magnetic declination represented by the North arrow wasn't a very good clue if the arrow with the half fleur-de-lis head represented magnetic North and the other with the smaller, double-sided head was true North. Here are the estimates of the actual angle of variation of magnetic North for Pensacola, Florida.
Even as an East declination, it isn't terribly close to the estimated declination for around 1810, the supposed year of the map if these estimates from the National Geophysical Data Center models are reasonably accurate:
Latitude: 30å¡ 29' 16" N
Longitude: 87å¡ 11' 22" W
Year Declination
1810 7å¡ 10.0' E
1820 7å¡ 16.0' E
1830 7å¡ 16.0' E