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