Found the gorgeous proj application on freshmeat, which maps coordinates into any system you care to name. Cue lots of trial-and-erroring, graphing differences in gnuplot and other fun stuff.

We have a match.

I think they're using Mercator projection with an elliptical distortion known as Clarke 1866. Everything lines up to within 3 Mercator meters. This is quite a relief.

Just for comparison, this is the full list of elliptical forms supported by proj:

MERIT a=6378137.0 rf=298.257 MERIT 1983
SGS85 a=6378136.0 rf=298.257 Soviet Geodetic System 85
GRS80 a=6378137.0 rf=298.257222101 GRS 1980(IUGG, 1980)
IAU76 a=6378140.0 rf=298.257 IAU 1976
airy a=6377563.396 b=6356256.910 Airy 1830
APL4.9 a=6378137.0. rf=298.25 Appl. Physics. 1965
NWL9D a=6378145.0. rf=298.25 Naval Weapons Lab., 1965
mod_airy a=6377340.189 b=6356034.446 Modified Airy
andrae a=6377104.43 rf=300.0 Andrae 1876 (Den., Iclnd.)
aust_SA a=6378160.0 rf=298.25 Australian Natl & S. Amer. 1969
GRS67 a=6378160.0 rf=298.2471674270 GRS 67(IUGG 1967)
bessel a=6377397.155 rf=299.1528128 Bessel 1841
bess_nam a=6377483.865 rf=299.1528128 Bessel 1841 (Namibia)
clrk66 a=6378206.4 b=6356583.8 Clarke 1866
clrk80 a=6378249.145 rf=293.4663 Clarke 1880 mod.
CPM a=6375738.7 rf=334.29 Comm. des Poids et Mesures 1799
delmbr a=6376428. rf=311.5 Delambre 1810 (Belgium)
engelis a=6378136.05 rf=298.2566 Engelis 1985
evrst30 a=6377276.345 rf=300.8017 Everest 1830
evrst48 a=6377304.063 rf=300.8017 Everest 1948
evrst56 a=6377301.243 rf=300.8017 Everest 1956
evrst69 a=6377295.664 rf=300.8017 Everest 1969
evrstSS a=6377298.556 rf=300.8017 Everest (Sabah & Sarawak)
fschr60 a=6378166. rf=298.3 Fischer (Mercury Datum) 1960
fschr60m a=6378155. rf=298.3 Modified Fischer 1960
fschr68 a=6378150. rf=298.3 Fischer 1968
helmert a=6378200. rf=298.3 Helmert 1906
hough a=6378270.0 rf=297. Hough
intl a=6378388.0 rf=297. International 1909 (Hayford)
krass a=6378245.0 rf=298.3 Krassovsky, 1942
kaula a=6378163. rf=298.24 Kaula 1961
lerch a=6378139. rf=298.257 Lerch 1979
mprts a=6397300. rf=191. Maupertius 1738
new_intl a=6378157.5 b=6356772.2 New International 1967
plessis a=6376523. b=6355863. Plessis 1817 (France)
SEasia a=6378155.0 b=6356773.3205 Southeast Asia
walbeck a=6376896.0 b=6355834.8467 Walbeck
WGS60 a=6378165.0 rf=298.3 WGS 60
WGS66 a=6378145.0 rf=298.25 WGS 66
WGS72 a=6378135.0 rf=298.26 WGS 72
WGS84 a=6378137.0 rf=298.257223563 WGS 84
sphere a=6370997.0 b=6370997.0 Normal Sphere (r=6370997)

Next thing is to implement that datum in our HCProjection class. Two days to go until I'm on holiday for the new house. Woohoo!

T
- posted by Tim Bruce @ 5:51 PM