[GRASS5] Re: [GRASSLIST:8031] Re: Oblique Mercator Projection in GRASS

Morten Hulden morten at untamo.net
Wed Aug 24 08:50:00 EDT 2005

I just tested an omerc_michigan PROJ_INFO file with the alternative 
projection parameters and it does not work - the alpha: key-value pair 
mysteriously disappers somewhere in the subroutines that construct the 
proj command.

As a matter of fact /any/ key-value pair in the PROJ_INFO file starting 
with either "a" or "f" will fail to show up in the constructed proj 
commmand, except "a:" and "f: of course". Could this have something to 
do with the routines extracting ellipse parameters? Like discarding or 
not looking further into the projinfo stucture if first char of a key is 
"a" or "f"?

Before staring to trace through the routines I would like to ask if 
someone might have an idea where this happens.


PS. The PROJ_INFO file I tested:

name: Oblique Mercator
datum: nad83
towgs84: 0.000,0.000,0.000
proj: omerc
ellps: grs80
a: 6378137.0000000000
es: 0.0066943800
f: 298.2572221010
lat_0: 45.3091666667
lonc: -86.0000000000
alpha: 337.25556
k_0: 0.9996000000
x_0: 2546731.496
y_0: -4354009.816
no_rot: defined
no_uoff: defined
rot_conv: defined

Morten Hulden wrote:
> Dave Kindem wrote:
>>  From what I can tell, the Proj4 OMERC program accepts parameter  
>> information in a two ways:
>> --the two point method and
>> --the central point and azimuth method.
>> Currently, the GRASS seems to accept only two point information, and  
>> the State of Michigan publishes central point and azimuth parameters.
>> Is there a way to set up an OMERC-based location within GRASS using  
>> the data I have?  Or, alternatively, can the OMERC interface be  
>> modified to accept central point and azimuth parameters in a future  
>> release?
> Not quite sure, but I think this restriction only concerns the g.setproj 
> program, normally used only when the location is created. After creating 
> an omerc-location you should be able to edit the PROJ_INFO file to 
> conform to the other method.
> Remove the lines starting with lon_1, lon_2, lat_1 and lat_2 and instead 
> add two lines with your central point and azimuth parameters:
> lonc: <central point longitude>
> alpha: <azimuth value>
> Don't do this if you already have imported data into your location - 
> create another location for testing.
> After this change [rv].proj tools /should/ work as normal.
> Please let us know if this worked or what error messages you get.
> rgds
> Morten

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