[Proj] RSO, +gamma and Hotine Oblique Mercator Variant A

Hilmy Hashim hilmyh at gmail.com
Sat Apr 9 06:54:40 EST 2011


This issue has been discussed many times before, so I'm trying to assess the
current situation.

I'm a new user of Quantum GIS and Postgis which I understand uses Proj.4 for
projections. I'm working with the RSO projection for Peninsular Malaysia
both old and new definitions (EPSG:3168 and EPSG:3375). From my readings
on various lists, RSO for Malaysia requires the +gamma parameter and the
projection method is now classified by epsg-registry.org as EPSG:9812 Hotine
Oblique Mercator (Variant A). This means that the final rotation applied is
at the natural origin as opposed to the center of projection which is
Variant B (EPSG:9815)

In QGis, versions 1.6 and up in OSGeo4W, the default proj.4 string for
EPSG:3168 and EPSG:3375 is without the +gamma parameter, so the projection
is not correct. Looking at the OSGeo4W\share\proj\epsg file, gamma is indeed
defined in there. So I'm confused where QGis srs.db is getting it's
information from.

Creating a custom CRS in QGis with +gamma seems to have an effect, so the
latest omerc.c does take into account the gamma parameter. However, the
projection is still not quite correct. It looks like it is
correctly aligned to north, but the x and y translation is off and I
surmised that this has to do with the final center of rotation. I did an
empirical correction with x_0 = false_easting + Uc sin(gc) and y_0 =
false_northing + Uc cos(gc) approximately I think. Attached is a diagram of
a comparison I did with different parameters. C is approximately the correct
projection with a cadaster layer in EPSG:3384 overlayed at the bottom.

As proj.4 is used by many applications, my question is how and where can I
get the projection parameters for RSO Malaysia corrected and how can the
omerc projection be made to use variant A. I am advising a state government
to standardize on QGis and PostGIS so there will be many users.

Thank you for any help  and my apologies for bringing up this Malayan
Monster again.

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