[Proj] Re: specifying omerc parameters (Duffy, Garret)

Melita Kennedy mkennedy2 at earthlink.net
Fri Sep 28 17:58:53 EDT 2007

Hi Garret,

Try omitting +lon_0 and making sure that lat_0 is on the central line of 
the projection. I was able to initialize proj with:

proj +proj=omerc +lat_0=0 +lat_1=-20 +lat_2=23 +lon_1=-120 +lon_2=45 +no_uoff +rot_conv +a=6378137 +rf=298.257223563 +no_defs +towgs84=0.000,0.0,0.0

+45.1875 might be just above the central line. 

I don't have an answer about the alpha range, but depending on how it's set 
up, you should be able to use 0 to 360, or possibly -180 to +180. Not in 
proj, but in an ESRI projection package we have a math problem when alpha=90 
that we had to fix.


>Date: Fri, 28 Sep 2007 09:06:58 -0400
>From: "Duffy, Garret" <gduffy at nrcan.gc.ca>
>Subject: [Proj] specifying omerc parameters
>To: <proj at lists.maptools.org>
>I am trying to specify an oblique Mercator projection using proj.4 in
>I am looking for some advice about the valid range of the parameters
>lat_1, lat_2, etc.
>I am trying to specify an "omerc" as follows:
>>+proj=omerc +lat_0=45.1875000000 +lat_1=-20.0000000000
>> +lat_2=20.0000000000 +lon_0=0.0000000000 +lon_1=-120.0000000000 
>> +lon_2=-60.0000000000 +k_0=1.0000000000 +no_uoff +rot_conv +a=6378137
>> +rf=298.257223563 +no_defs +towgs84=0.000,0.000,0.000
>GRASS support said that absolute value of lat_0 must be less than 36.
>Sure enough when I put lat_0 equal 35, proj worked.  BUT then I wanted
>to change the other parameter lat_1, Lat_2, etc. to achieve the required
>rotation.  I would like:
>Lat_1 = -20
>Lat_2 = 23
>Lon_1 = -120
>Lon_2 = 45
>(I estimated these parameters by doing a reprojection of my dataset in
>Global Mapper)
>But proj fails.
>On a related note, what is the accepted range of the +alpha parameter in
>the alternative specification of omerc?
>Any advice would be appreciated,
>Garret Duffy.

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