[GRASS-dev] Request to add a GCP file in vector directories

[Glynn Clements]

Michael Barton wrote:

> I'm integrating vector georectification (v.transform) into the GIS Manager
> georectifier. It would be nice if a GCP file could be saved in a vector
> folder so that it could be called up, edited, and reused and a user doesn't
> have to re-enter GCP's every time they start the georectifier. This is the
> case now for rasters, because a GCP file (POINTS) is saved inside the group
> folder for a raster group.
> 
> I'm proposing to have the georectifier (at the user's option) save a file
> named ³gcp² (points might be confusing in a vector folder) in the
> appropriate vector folder to be rectified. It would be a simple ascii text
> file and take the format of the standard file needed by v.transform, with
> the addition of a header indicating the target location and mapset it is
> aimed at. For example...
> 
> # target: /Users/shared/grassdata/spearfish60/newvector
> -584 585  598000 4920770
> 580  585  598020 4920770
> 580  -600 598020 4920750
> -584 -600 598000 4920750
> 
> Does anyone foresee a problem with this?

Unlike i.rectify, v.transform doesn't operate upon maps from a
different location. You have to get the source map into the current
location first.

[On the last point, g.copy should probably have an option to allow
data to be copied from different locations or even from different
database directories.]

Also, transformations are arguably a property of a location rather
than a specific map. If you had a number of maps, all in the same
(unspecified) coordinate system, you would probably want to use the
same transformation for all of them.

AFAICT, the POINTS file is just an artifact of the i.points/i.rectify
workflow. The data just needed to be stored somewhere, and not much
thought was really given to the details.

It's arguable that GCP files should be a separate entity, similar to
e.g. named regions.

Also, it might be useful to allow global GCP files for an X-Y location
which specify the transformations to/from lat-lon. Adding support for
arbitrary polynomial transformations to libproj would allow you to use
*.proj to project to/from X-Y locations.

-- 
Glynn Clements <glynn at gclements.plus.com>