[FWTools] gdalwarp issue?

[jkenneally at cartenav.com]

Hi Rengifo,

Thanks a lot for your reply.   I am aware that GDAL can't really perform an orthorectification, and I can easily see this getting to the point of requiring a 'proper' orthorectification through another 3rd party software if everything else works out. 

However, as it stands right now I'm working under assumed 'idea' conditions with my raw photo's, or that general errors due to lack of orthorectification are acceptable.  This is meant to provide a quick overview of an area being imaged (ie near real-time aboard an aircraft), with proper orthorectification, etc happening in a post-processing stage at a later time.

I'm more confused as to why GDAL doesn't want to warp my raw image into what I would consider a correct geographic shape when I specify that the bottom of the image isn't geographically as 'wide' as the top.   

I know Global Mapper doesn't perform orthorectification either, but does seem to display/save the raster data as I would expect, and I'm confused about what I'm missing to create the same result through GDAL.

Given the assumptions I am making above, I wonder if I should be specifying an 'orthographic' projection when doing the gdal_translate before the warp?  

Cheers!

-----Original Message-----
From: "Rengifo Ortega" <rengifoo at yahoo.de>
Sent: Wednesday, December 15, 2010 11:48am
To: jkenneally at cartenav.com
Cc: FWTools at lists.maptools.org
Subject: AW: [FWTools] gdalwarp issue?



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:///] http://forum.manifold.net/forum/t73326.13
[http:///] 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://]  http://live.osgeo.org/

The positive aspect of using OSSIM  is  RSM- approach. (Rigorous Sensor Modelling) 
[http://www.ossim.org/OSSIM/OSSIM.html] 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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