Oblique Mercator vs. Hotine Oblique Mercator

[crschmidt]

From ​http://lists.osgeo.org/pipermail/metacrs/2008-December/000185.html:

• EPSG:2056, "CH1903+ / LV95"
• EPSG:2057, "Rassadiran / Nakhl e Taqi"
• EPSG:21780, "Bern 1898 (Bern) / LV03C"
• EPSG:21781, "CH1903 / LV03"
• EPSG:23700, "HD72 / EOV"
• EPSG:29700, "Tananarive (Paris) / Laborde Grid",
• EPSG:29702, "Tananarive (Paris) / Laborde Grid approximation"
• EPSG:29871, "Timbalai 1948 / RSO Borneo (ch)"
• EPSG:29872, "Timbalai 1948 / RSO Borneo (ft)"
• EPSG:29873, "Timbalai 1948 / RSO Borneo (m)"

According to EPSG, all these use the projection method "Oblique Mercator", coord op method 9815. But on SpatialReference.org, these ten CRSs are given the projection method "Hotine_Oblique_Mercator", which is a different method (coord op method 9812). It is true that each instance of "Oblique Mercator" can be reformulated as an instance of "Hotine Oblique Mercator", but this requires that you adjust the False Easting and False Northing, which has not been done. EPSG Guidance Note 7.2 gives the example of "Timbalai 1948 / R.S.O. Borneo (m)", which has

• Easting at projection centre (EC) = 590476.87 metres
• Northing at projection centre (NC) = 442857.65 metres

This can be reformulated as an instance of "Hotine Oblique Mercator", but only if you adjust these parameters to

• False easting (FE) = 0.0 metres
• False northing (FN) = 0.0 metres

Note also that some CRSs in the ESPG database do use "Hotine Oblique Mercator", so it is no solution to just replace "Hotine_Oblique_Mercator" by "Oblique_Mercator" everywhere. The two coord op methods must be distinguished.

[Even Rouault]

Other (possibly) related tickets : #3038 and #3311