[gdal-dev] Mixed coordinate systems in VRT?

[Jim Lacy]

I have an area of interest that is split across two UTM zones.  My desired end result is a single stitched, clipped, and re-projected image in a local coordinate system.

My usual workflow is to build a VRT that contain only the input tiles that touch the area of interest.  Normally that works great, but not when split across zones. I know it's possible to create a VRT containing two or more coordinate systems with the gdalbuildvrt  -allow_projection_difference option.

There are at least two ways I could proceed:


A)     Two steps: reproject all input tiles to a common base with gdalwarp, then stitch/clip with gdalwarp -t_srs "EPSG:7645"  -cutline clip.shp -crop_to_cutline aoi.vrt output.tif  (where aoi.vrt contains a list of tiles that touch the AOI)

B)      One step:  gdalwarp -t_srs "EPSG:7645" mixed_utm_zones.vrt output.tif

"B" is  nice option in theory. However, in my experiments, it appears that only half the AOI is processed when I try.  Option "A" works fine, but it seems like the image would be resampled twice... once for each gdalwarp command?

Can anyone confirm if B is theoretically possible?  Better approaches?  This must be a common issue.

Thanks for advice!

----------------------------------------------
Jim Lacy
Wisconsin State Cartographer's Office
University of Wisconsin-Madison
(608) 262-6850
www.sco.wisc.edu<http://www.sco.wisc.edu/>
My calendar:  http://go.wisc.edu/xziys4

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