Re: inverse mapproject -C confusion

OK, so your x,y points were obtained that way. Given that, one thing is 
certain they are not Mercator coordinates so you should not try to 
inverse project them with -Jm (though I don't know why the -R 
experiments you made resulted in what you showed us).

I think what I would do would be to assume that those are well 
approximate by a transverse Mercator with center at -122,38. E.g

echo 0 0 | mapproject -I -Fk -C -Jt-122/38/1:1 -R-125/-120/0/40
-122    38.0000000001



> Hi Joaquim,
>
> I'm probably still confused about the projections, but I don't 
> understand the different behavior (-C and -R) of the inverse 
> projection between -Je and -Jm. The inverse projection appeared to 
> work as I expected using the Azimuth Equidistance (-Je) projection 
> with "-C", but, with the same set Cartesian (km) points, the Mercator 
> (-Jm) inverse projection only works without "-C" and with a modified 
> range.
>
> I'm trying to create a geographic grid covering a 100-km radius circle 
> centered at -122,38 with 1-km spacing. Is there a better way to 
> generate a geographic grid (data table) with arbitrary grid spacing 
> specified in km?
>
> I started by creating x-y points for a 150x150 (km) grid using perl 
> (example uses 10-km spacing).
>
> perl -e 'for $x (-15..15) {for $y (-15..15) {print $x*10," 
> ",$y*10,"\n";}}' > grid_pts_km.xy
>
> I expected the -C option to put 0,0 km at the projection center 
> (-122,38). This seems to have worked with -Je, range set to encompass 
> expected range of inverse projected coordinates:
>
> mapproject grid_pts_km.xy -Fk -I -C --Je-122/38/1:1 
> --R-123.75/-120.25/36.5/39.5 > grid_pts_ll_Je.xy;
>
> To get this to work with the -Jm inverse projection, I had to omit the 
> -C option and change the range so that the West and South values were 
> the projection center coordinates (i.e. I wanted 0,0 km to be -122,38 
> degrees):
>
> mapproject grid_pts_km.xy -Fk -I    -Jm-122/38/1:1 
> --R_-122.00_/-120.25/_38.0_/39.5 > grid_pts_ll_Jm.xy;
>
> The attached map compares the two grids.
>
> Thank you,
>
> Jason
>
> *From:*Joaquim Luis [mailto:[email protected]]
> *Sent:* Tuesday, January 14, 2014 7:00 PM
> *To:* World-Wide GMT Usage and Help Mailing List
> *Cc:* Altekruse, Jason [FWLA]
> *Subject:* Re: [GMT-HELP] 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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