Hi Frederick, On Wed, Jun 29, 2011 at 1:25 AM, frederick keusch < frederick.keu...@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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