[GRASS5] Michigan Georef parameters?

jaggett jaggett at vt.edu
Wed Sep 15 12:33:56 EDT 2004


Thanks very much for your suggestions. I tried what you suggested regarding 
the dummy values, but this didn't work. As soon as I tried projecting a map 
into the Georef location, from another location, I received an error message 
regarding the projection parameters.

You are quite right...there are 2 methods for specifying the projection. 
Apparently, the more commonly used method is the "one point method", for which 
I have the necessary projection parameters. GRASS, however, seems to be set up 
only to allow for the "two point method".

I would appreciate any further advice...



>===== Original Message From Paul Kelly <paul-grass at stjohnspoint.co.uk> =====
>On Tue, 14 Sep 2004, jaggett wrote:
>> Hi There,
>> I'm trying to set up a Georef coordinate location for Michigan (Oblique
>> Mercator Projection), and could use some help. I am not sure what GRASS is
>> wanting when it requests certain parameter values (seems like a "Two Point
>> Method" for parameter specification). Amongst others, GRASS asks for the
>> following parameters:
>> Enter Central Parallel [lat_0] (0) :
>> Enter First Standard Parallel [lat_1] (45S) :
>> Enter Second Standard Parallel [lat_2] (45N) :
>> Enter Central Meridian [lon_0] (20E) :
>> Enter First Standard Meridian [lon_1] (40W) :
>> Enter Second Standard Meridian [lon_2] (40E) :
>> I believe the Central Parallel (lat_0) and the Central Meridian (lon_0) 
>> the following values, respectively: 45.30917 deg N & 86 deg W.
>> However, I do not have answers for the following: lat_1, lat_2, lon_1 and
>> lon_2. Does anyone know the values for these parameters?
>This is a wild guess, put perhaps there are two ways of specifying the
>projection, either a "one point method" using the central values, or a
>"two point method" using the 1st and 2nd. The code in GRASS that
>interactively prompts for projection parameters is a little illogical in
>places, but there is currently nothing better available.
>If my guess is correct then you could put in dummy values for the
>parameters you don't know, and then after the process has completed and
>the location created manually edit the PROJ_INFO file in the PERMANENT mapset
>directory of your location and delete the lines for lat_1, lat_2,lon_1 and
>lon_2 (leaving those for lat_0 and lon_0 and the other parameters intact).
>I don't know about the omerc projection though and this might be totally
>> My other question is: Is there any way of using a "Central Point and 
>> Method" for specifying the projection parameters for an Oblique Mercator
>> projection in GRASS?
>You'd be better asking that on the PROJ.4 mailing list (or simply looking at
>the PROJ.4 documentation)---all GRASS' projection capabilities are built
>on top of PROJ. See http://www.remotesensing.org/proj/
>Paul K
>grass5 mailing list
>grass5 at grass.itc.it

More information about the grass-dev mailing list