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

[Joaquim Luis]

Frederick,

I'm a geophysicist but never thought that an earthquake could have that 
side effect of image referencing.
Anyway the horizontal deformation caused by a big earthquake is of the 
order of meter scale. One idea I had is that you can use the shift grid 
to to relocate previous GCPs and than use those relocated ones in 
gdalwarp. You can compute 'old' GCPs by interpolating your images both 
for pixels and metric (UTM) units. The operations would be something 
like this

1. Get  "px py old_UTMx old_UTMy" by interpolating the referenced image 
at a certain well distributed positions
2. apply the shift grid to old GCPs and get  new GCPs "px py new_UTMx 
new_UTMy"
2. re-register with gdalwarp and new GCPs

Of course, this is in absence of a more straight solution where the 
shift grid is applied directly to the images.

Joaquim Luis

> Hi,
>
> 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?
> 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,
>
> Frederick
>
>
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