[GRASS-user] r.proj: fixing a failure

[Daniel Victoria]

Hi Rich,

I'm no expert in projection or Grass for that matter but this is what
I understand about the way Grass projects rasters and vectors. Hope
not to be making things worse :)

In order to project a raster you have to "bring it in" your current
location from another location that has a different projection. That
is done using r.proj. The trick is that, when you bring your new
raster into the current location, it will respect you current region
settings, that is, limits and resolution. So, let's say you want to
project an Oregon map but you current region is set to Florida. This
will not work and your raster will not be projected.

So, in order to know where you projected raster will fall, you can use
the v.in.region trick. The command will create a bounding box in your
map and you can project the vector (using v.proj), and not wory about
the current region. Then you set your region to the recently projected
vector (g.region vect=box) and fix your resolution. Now you are ready
to project your raster and you know the current region matches the
region where your raster is coming from.

This is better explained in the r.proj manual page, in the notes section
http://www.ces.iisc.ernet.in/grass/grass62/manuals/html62_user/r.proj.html

Cheers
Daniel

On Tue, Dec 22, 2009 at 2:54 PM, Rich Shepard <rshepard at appl-ecosys.com> wrote:
> On Tue, 22 Dec 2009, Hamish wrote:
>
>> tip: v.proj'ing over a v.in.region box can help there. Then I usually try
>> to set the resolution to be a tiny bit better than the source rows x cols.
>> If there will bit a lot of rotation I try to set it even finer.
>
> Hamish,
>
>  I'm missing something important which is probably obvious to you and
> others. The project location contains vector maps. My reading of the
> v.in.region man page tells me its function is to "Create a new vector from
> the current region." That suggests that the resulting output map is the same
> as the displayed vector map but with a new name.
>
>  If I then run v.proj on the display of the new map what have I changed?
> More importantly, how does that help resolve my inability to run r.proj on a
> DEM map and reproject it to the project's location?
>
>  A clue stick would help.
>
> Thanks,
>
> Rich
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