[gdal-dev] How to apply a costum shift grid?

[Robert Coup]

Hi Frederick,

On Wed, Jun 29, 2011 at 1:25 AM, frederick keusch <
frederick.keusch at digimapas.cl> wrote:

>
> I am working here in Chile and we have a lot of earthquakes here, the last
> big one like a year ago. So the problem is that I have a lot of
> georeferenced images, but georeferenced before the earthquake. We thought
> the best way would be to calculate a shift grid and apply it with gdalwarp
> to all of the images. The only thing I would have to do is to apply the
> grid
> from one datum to the same datum instead of from one datum to another, this
> should be possible, shouldn´t it?
>

Yeah. I'm assuming you know that the grid shifts are applied as a transform
on the geocentric coordinates?


> Reading a lot in the mailing-list I found a link to an australian
> govermental site, where they provide a program to convert from NTv2_ASCII
> to
> NTv2_GSB. So I created an Ntv2_ASCII file with our shift values and
> converted it with this program to an .gsb-file and tried to apply that file
> with gdalwarp to our images. But no matter what I tried, nothing happened,
> the shift never was applied. I saw a lot of examples of possible commands
> searching in the mailing-list and tried everyone, also in nearly every
> possible combination but nothing lead to an applied shift. So I am a bit
> desperate right now, hoping someone can help me a bit or give an advice.
> Here in short what I have to do:
> Apply a shift grid from UTM 18s WGS84 to UTM 18s WGS84 in order to apply
> the
> coordinate changes of our images owing of the earthquake last year.
>
> I would apreciate any kind of help and thank a lot in advance,
>

First I'd get your grid working with proj4/cs2cs:

See how it's setup for the US/NZ/CA grids that are available as a companion
download
http://trac.osgeo.org/proj/wiki/GenParms describes the +nadgrids parameter
to tell proj4 to use it during datum shift.

Once that's all working...
In the gdal python bindings you need to add " +wktext" to the end of your
proj4 string to get it to recognise/use grids.

Hope that helps a bit,

Rob :)
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