[gdal-dev] gdalwarp artifacts when using RPC_DEM

[David Shean]

Hi all,
I'm having some trouble with artifacts in the output from gdalwarp when using RPCs and a DEM for orthorectification.  The artifacts appear as rows or blocks of rows that repeat swaths of pixels from nearby rows, with variable horizontal extent (often spanning >1000 samples).  They are not coincident with the DEM pixel edges.  Using a different, lower-resolution DEM reduces the number of these artifacts, but they are still present.  I think it is safe to assume that issues with the input DEM are not responsible for the artifacts.  I'm running GDAL 1.8.1 (fink port) on OS X Lion.

Here is a sample command:
gdalwarp -of GTiff -ot UInt16 -t_srs EPSG:3413 -multi -r bilinear -rpc -to "RPC_DEM=dem.tif" infile.ntf outfile.tif

These artifacts are NOT present when mapping with RPCs to a constant height (using the RPC_HEIGHT transformer option).  I have eliminated potential causes related to output projection (both stereographic and sinusoidal are afflicted), interpolation method (tested nn, bilinear and cubic), and various other options (gtiff compression, gtiff tiling, multithreading, etc).  The problem is repeatable, and doesn't appear to be related to hardware limitations or network read/write issues.  Basically, I've spent a full day troubleshooting and I'm running out of ideas.  

I didn't see anything in the GDAL Trac about this - have these artifacts been documented in the past?  

I can provide a screenshot if desired.  On the whole, the image appears properly orthorectified, which is fantastic, but these artifacts are problematic for my application.  Thanks for any advice/assistance.
-David

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David Shean
Polar Science Center
Applied Physics Lab
University of Washington