Choice of Datum
Take data for the polygon of the City of London from https://geoportal.statistics.gov.uk.
show complete Wolfram Language input
In[1]:=
coords = {{532140.3, 182191.9}, {532141.5, 182188.7}, {532142.4,
182189.0}, {532146.6, 182190.3}, {532158.3, 182151.1}, {532189.3,
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180647.1}, {532228.8, 180651.9}, {532170.6, 180663.1}, {532109.1,
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180683.3}, {531230.5, 180680.7}, {531214.2, 180679.9}, {531182.6,
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181598.1}, {531473.7, 181616.4}, {531496.4, 181627.9}, {531521.2,
181642.1}, {531554.6, 181659.1}, {531555.2, 181659.5}, {531595.2,
181683.2}, {531697.9, 181736.9}, {531778.2, 181780.0}, {531796.2,
181790.5}, {531813.0, 181800.3}, {531847.4, 181818.9}, {531865.9,
181828.1}, {531891.4, 181842.3}, {531900.3, 181844.7}, {531903.3,
181845.5}, {531913.5, 181846.5}, {531944.7, 181863.1}, {531965.7,
181873.1}, {531996.3, 181888.2}, {532027.3, 181899.0}, {532081.1,
181915.5}, {532083.4, 181950.0}, {532086.5, 181980.3}, {532089.2,
182013.7}, {532097.1, 182012.8}, {532097.7, 182016.7}, {532098.7,
182027.8}, {532097.7, 182041.9}, {532087.7, 182083.6}, {532078.1,
182111.2}, {532073.6, 182121.4}, {532067.3, 182133.7}, {532059.0,
182147.3}, {532056.7, 182150.8}, {532053.9, 182154.6}, {532134.9,
182206.1}, {532140.3, 182191.9}};Those points are specified using the Ordnance Survey National Grid system of coordinates, corresponding to the following projection specification in the Wolfram Language.
In[2]:=
proj = {"TransverseMercator", "Centering" -> {49., -2.},
"GridOrigin" -> {400000, -100000},
"CentralScaleFactor" -> 0.9996012717,
"ReferenceModel" -> "Airy1830"};This changes the coordinates to latitude, longitude pairs.
In[3]:=
cityoflondon = GeoPosition[GeoGridPosition[coords, proj]];A change of datum also needs to be specified to obtain fully precise results.
In[4]:=
cityoflondonWGS84 =
GeoPosition[GeoGridPosition[coords, proj, "OGB7"], "WGS84Original"];The blue line is the correct one. Note, for example, how well it fits Blackfriars Bridge.
In[5]:=
GeoGraphics[{Thick, Red, Line[cityoflondon], Blue,
Line[cityoflondonWGS84]}, GeoGridLines -> True, ImageSize -> Large]Out[5]=
