inverse mapproject -C confusion

Hi,

I'm trying to convert a table of x and y coordinates in km to geographic coordinates using mapproject -I. My grid of points is centered at W122 N38 (0,0). If I use an Azimuthal Equidistance projection with the -C option, (0,0) in km projects to (-122,38) as I expected. When I try a Mercator projection with the -C option, x projects to -122 but y is projected to 0 degrees latitude. I have to exclude the -C option and adjust the Range so that the lower left corner is where I want the center of the grid (-122,38).

Inverse projection (expect: 0,0 km --> -122,38 degrees)
$ echo 0 0 | mapproject -I -Fk -C -Je-122/38/1:1 -R-123.5/-120.5/36.5/39.5
-122   38

$ echo 0 0 | mapproject -I -Fk -C -Jm-122/38/1:1 -R-123.5/-120.5/36.5/39.5
-122   0

$ echo 0 0 | mapproject -I -Fk    -Jm-122/38/1:1 -R-123.5/-120.5/36.5/39.5
-123.5 36.5

$ echo 0 0 | mapproject -I -Fk    -Jm-122/38/1:1 -R-122/-120.5/38/39.5
-122   38

The forward projection behaves similarly (expect: -122,38 degrees --> 0,0 km):
$ echo -122 38 | mapproject    -Fk -C -Je-122/38/1:1 -R-123.5/-120.5/36.5/39.5
0      0
...
$ echo -122 38 | mapproject    -Fk    -Jm-122/38/1:1 -R-122/-120.5/38/39.5

0         0

Am I using the -C (or -Fk, -J, -R,) option incorrectly, or misunderstanding the projections (or making a poor choice)? Is there a better way to create a geographic grid with 1 km spacing?

I'm using GMT 5.1.1_r12604 on a virtual Ubuntu machine - I'm visiting another office and haven't managed to navigate svn through their proxy to update.

I'm ultimately trying to create a 100x100 km grid of points with 1 km spacing in geographic coordinates for use in a seismic hazard calculation, and will eventually calculate rate, beta, and cell area (in units of equatorial square degrees) at each point. I started by generating the x,y table with awk/perl.

Thanks for your help,

Jason


Jason Altekruse
Project Geologist

Fugro Consultants, Inc.
Lakewood, CO 80401


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