[FWTools] gdalwarp issue?

[Rengifo Ortega]

Dear jeff,
I don´t know if this helps you  that  much, but these threads seems to confirm, 
what I was about to answer you. I don´t think gdalwarp can do orthorectification 
( in a strict sense) but, transformation and georeferentiation among many other 
things.

http://forum.manifold.net/forum/t73326.13
http://www.osgeo.org/pipermail/gdal-dev/2004-November/004566.html

 I  recommend you to use OSSIM for this.

The good new is that the osgeo created a live cd with OSSIM among, other 
software. So, if you are familiar with linux. Here you will find all software 
you need.http://live.osgeo.org/

The positive aspect of using OSSIM  is  RSM- approach. (Rigorous Sensor 
Modelling) 

http://www.ossim.org/OSSIM/OSSIM.html


cheers 
Rengifo Ortega                 
                  






________________________________
Von: "jkenneally at cartenav.com" <jkenneally at cartenav.com>
An: FWTools at lists.maptools.org
Gesendet: Mittwoch, den 15. Dezember 2010, 15:04:59 Uhr
Betreff: [FWTools] gdalwarp issue?

Hello,

I'm having an issue using gdalwarp, and I'm not sure if I'm using the utility 
incorrectly, or if I'm just trying to make it do something it can't do.

I'm trying to perform a crude geo-rectification on a set of aerial photo's.  For 
the test case I am just assigning four gcps to the source image corners using 
gdal_translate.  This stage seems to work fine.  I am then intending to use 
gdal_warp to produce an output image that has been transformed based on the 
assigned gcps.  However, no matter what I assign as the image corner 
coordinates, gdal_warp still seems to produce a 'square' final image. 


The easiest way to demonstrate the issue is in cases where the camera was not 
pointed straight down when the aerial photo was taken.   This creates a source 
image where the bottom (near) corners are geographically closer to each other 
than the top (far) corners are.   In this case I am expecting the warped output 
image to contain a trapezoid-ish shape of aerial imagery with no data (ie white) 
pixels around the outside.   


However, while gdal warp does seem to rotate the source images to correctly 
align with lat/lon lines, it doesnt seem to distort the image into the trapezoid 
shape.  

If I load the image that was georeferenced through gdal_translate into Global 
Mapper it does appear properly (ie trapezoidal), and can be saved back to disk 
in that 'warped' form, but I'm really hoping to be able to do this through 
fw_tools/gdal.

Am I missing something obvious or is this not possible with gdal_warp?

Thanks!
Jeff

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