[GRASS5] Michigan Georef parameters?

GRASS GIS GRASS GIS developer
1

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?

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?

I would really appreciate some assistance. Thanks!

Sincerely,

Jonathan Aggett

2

Hello

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 wrong.

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