# [OSRS-PROJ] Oblique Mercator, and Swiss Oblique Cylindrical

Frank Warmerdam warmerdam at pobox.com
Wed Dec 11 15:49:51 EST 2002

```Folks,

I am working on getting Swiss Oblique Cylindrical working properly in a
higher level tool build on PROJ.4, and am running into a number of problems.

1) The omerc projection in PROJ.4 does not let me select an azimuth of 90
degrees.  My understanding is that Swiss Oblique Cylindrical is essentially
Oblique Mercator with an azimuth of 90 but that is clearly considered a
degenerate case in the PJ_omerc code (and it reports an error).  Is this
just due to the mathematical formulation of the omerc code?

2) I have tried setting the azimuth quite close to 90 (ie. 89.99) and
PROJ.4 seems to work but the results do not match what I am expecting.
Off by a few thousand meters in my test case.  My test point was derived
by using FME to reproject some swiss data into lat/long on the same datum.
Does anyone have a sample point I could use in swiss oblique cylindrical?
That is, the location in lat/long and soc?

3) Just to ensure I had a known starting point, I tried reprojecting the
example point from the EPSG note on oblique mercator.  This is described
in section 1.4.7 of their "Guidance Note # 7".  The section is available at:

http://www.remotesensing.org/geotiff/proj_list/epsg_om.html

However, I could get my results to agree.  Basically I tried:

proj +proj=omerc +lat_0=4 +lonc=115 +alpha=53.31582047222222 +k=0.99984
+x_0=590476.87 +y_0=442857.65 +ellps=evrstSS +units=m < rso_eg.dat

where rso_eg.dat contains:

115d48'19.8196"E 5d23'14.1129"N
# 679245.73 596562.78 expected

But instead of getting the expected results I get:
679743.48       596274.24

It is in the right ballpark, but still on the order of 600 meters off.
Can anyone suggest where things are going wrong?

Actually, looking over things, there doesn't seem to be a
"rectified to skewed grid" argument in my proj.4 formulation.  Is that
an issue?  I don't really follow the purpose of this parameter as opposed
to the azimuth.

Best regards,

--
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
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