Re: inverse mapproject -C confusion

Jason,

How were your x,y (cartesian) coordinates obtained? What you need to do 
is to inverse project using the same projection used to by x,y in first 
place.

mapproject -I -Jm... inverse project coordinates that are in Mercator 
projection, not any km(etric) units

Joaquim

> 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
>
> 00
>
> 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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