[OSRS-PROJ] Rectified Skew Orthomorphic...

[Carl Godkin]

Greetings,

The national grid for East Malaysia uses a projection called
Rectified Skew Orthomorphic a/k/a Hotine Oblique Mercator
with an apparently well-known set of parameters.

It is very similar to PROJ.4's Oblique Mercator (omerc)
projection, but not exactly.

Given in the nad/esri file is this setup for conversion
to RSO Borneo in Imperial feet:

# Timbalai 1948 / R.S.O. Borneo (ft)
<29872> +proj=omerc +lat_0=4 +lonc=115 +alpha=53.31582047222222 
+k=0.99984 +x_0=590476.8727431979 +y_0=442857.6545573985 +ellps=evrstSS 
+to_meter=0.3047994715386762 +no_defs  no_defs <>

This doesn't quite do it, however.  I've gotten three sets
of test points from various sources and the conversion
using this setup from lon/lat to x/y is off by ~2000 Imperial
feet on each.

The difference is a parameter (known as gamma) that is the
azimuth from skew to rectified.  There's no such parameter
for "omerc," but it appears to be needed.  For Timbalai,
this is 53d07'48.3685".

Frédéric Trastour asked this same question back in April 2002
but the list archives don't show a reply and he hasn't
answered a private follow-up from me.  So I made some minor
changes to the code in PJ_omerc.c (from PROJ 4.4.7) to make
my own PJ_rso.c.

Now I get exactly the right eastings and northings out of
PROJ.4.  My question is, what should I do next?

Did I miss some trick that would enable me to enter this
gamma value for PROJ.4's omerc?   If so, please enlighten
me.

If not, I would be happy to contribute either my PJ_rso.c
or at least explain on this list what I had to change.
(I based my changes on the mathematics described by the
EPSG.)

Thanks for maintaining such a great package,

carl

Carl Godkin                             carl at dgi.com
Manager, Software Engineering           http://www.dgi.com
Dynamic Graphics, Inc.
1015 Atlantic Ave.                      tel: 510/522-0700 x3106
Alameda, CA 94501                       fax: 510/522-5670

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