[GRASS-user] v.proj pj_transform() failed for points but not for a vector "grid"

[Markus Metz]

On Sat, Jan 31, 2015 at 5:41 PM, Patrice Dumas <pertusus at free.fr> wrote:
> Hello,
>
> grass 6.4.4 on Centos 6 (from EPEL).
>
> Before I try to turn this into a bug I'd like to be sure that I am not
> doing something stupid with projections (happened in the past...).
>
> I have a Lambert equal area location and a lon-lat location, the lon-lat
> location covers the whole earth, the Lambert does not.  I would like to
> project some points from the lon-lat location to the Lambert one,
> selecting only those points in a specific region on the Lambert map
> which is much smaller than the whole Lambert map.
>
> To that aim, I first do a "grid" vector over the region of interest in
> the Lambert location with "v.mkgrid map=region_countries_whole_grid
> grid=40,40" (a rather loose grid such that it is easier to project on
> different locations).  I import that grid in the lon-lat location, use
> it to select points and then, back in the Lambert location use v.proj to
> import the points.  Except that it gives me
>
>
> v.proj --verbose input=lon_lat_demand_relocated location=whole_region_demand_map_asiamed_cities

Is lon_lat_demand_relocated really the point selection?

> Input Projection Parameters:  +proj=longlat +a=6378137 +rf=298.257223563 +no_defs +towgs84=0.000,0.000,0.000
> Input Unit Factor: 1
> Output Projection Parameters:  +proj=laea +lat_0=55 +lon_0=20 +x_0=0 +y_0=0 +no_defs +a=6370997 +b=6370997
> Output Unit Factor: 1
> Reprojecting primitives: WARNING: pj_transform() failed: latitude or longitude exceeded limits
> ERROR: Error in pj_do_transform
>
>
> Visually it seems that all the points are on the "grid" vector (which is
> obviously not a regular grid once projected on the lon-lat location).
> What is very strange is that projecting the "grid" vector back to the
> Lambert location works, but not projecting the points back??

>From your description, the workflow is

# create grid in LAEA
v.mkgrid map=region_countries_whole_grid grid=40,40"

# reproject grid to latlong
v.proj in=region_countries_whole_grid location= mapset=

# select points with grid in latlong
v.select ainput=points binput=region_countries_whole_grid atype=point
btype=area operator=overlap output=points_selected

# reproject selected points to LAEA
v.proj in=points_selected
location=whole_region_demand_map_asiamed_cities mapset=

# test: project grid back to LAEA
v.proj in=region_countries_whole_grid
location=whole_region_demand_map_asiamed_cities mapset=

>
> Am I doing something silly, or does it looks like a bug?
>
If you are sure that v.proj fails with the selected points, it is a bug.

v.proj will likely fail with all points if the points are distributed
all over the world, that would be correct.

>
> The Lambert location map comes from HYDRO1K (the eu basins):
>   http://edcftp.cr.usgs.gov/pub/data/gtopo30hydro/
>   https://lta.cr.usgs.gov/HYDRO1K
>   https://lta.cr.usgs.gov/HYDRO1KReadMe
> I used the grass helper to setup the region and it didn't set a datum
> (but it set an ellipsoid).  In the .prj file that comes with vectors,
> there is
>   PROJCS["Sphere_ARC_INFO_Lambert_Azimuthal_Equal_Area",GEOGCS["GCS_Sphere_ARC_INFO",DATUM["D_Sphere_ARC_INFO",SPHEROID["Sphere_ARC_INFO",6370997.0,0.0]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],PROJECTION["Lambert_Azimuthal_Equal_Area"],PARAMETER["False_Easting",0.0],PARAMETER["False_Northing",0.0],PARAMETER["Central_Meridian",20.0],PARAMETER["Latitude_Of_Origin",55.0],UNIT["Meter",1.0]]
> but I couldn't find the "D_Sphere_ARC_INFO" datum in the datum proposed
> by grass.

It's a simple sphere with radius 6370997.0, without datum.

Markus M

> The other information from the .prj file were correctly
> extracted by the grass helper, as far as I can tell.
>
> --
> Pat
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