[FWTools] gdalwarp issue?

[jkenneally at cartenav.com]

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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