I tested an omerc file last night also. Here's the info on my mine:
PROJ_INFO
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
rot_conv: defined
I was able to import and display a shape file, but when I try to re-project the vector information into another location, I get the following error:
v.proj input=ottawa location=OttawaOMERC output=county
Unable to initialise PROJ.4 with the following parameter list:
+proj=omerc +lat_0=45.3091666667 +lonc=-86.0000000000 +k_0=0.9996000000 +rot_conv +a=6378137 +rf=298.257222101 +no_defs +towgs84=0.000,0.000,0.000
The error message was 'lat_1=lat_2 or lat_1=0 or lat_2=90'
As you noticed, the alpha value is missing. I'm also curious about the f= value, which is evidently derived, since I did not enter it.
It also seems that the displayed image is basically an x-y plot, with no connection to a particular location. Running d.where will return the northing and easting of any location on the image, but the program returns errors when asked for the lat/long location. In order to make the connection, wouldn't GRASS need to use the false northing/easting (or perhaps the lat/long location of the point of origin) information supplied in the documentation?
Thanks for your help sorting through this.
Dave
On Aug 24, 2005, at 8:50 AM, Morten Hulden wrote:
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.
rgds
MortenPS. 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: definedMorten 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