[GRASS5] Michigan Georef parameters?

Paul Kelly paul-grass at stjohnspoint.co.uk
Tue Sep 14 16:58:11 EDT 2004


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) have
> 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 Azimuth
> 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

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