[Proj] specifying omerc parameters

Duffy, Garret gduffy at nrcan.gc.ca
Fri Sep 28 09:06:58 EDT 2007


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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