[GRASSLIST:8044]

[Dave Kindem]

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
> Morten
>
> PS. 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: defined
>
>
> 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  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
>>
>>
>>
>
>
>