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I am using Mapnik to create tiles on a tile server. I'm depicting sea level rise in North America using SRTM data, and it works fine in most of the continent. However, when zoomed out in my browser (zoom levels 5 and lower), I get a peculiar horizontal stripe across the entire continent:

horizontal stripe across north america

When I am zoomed farther in, this stripe does not appear, which makes me think it's not an issue with the data itself. The stripe is from 51 degrees N to 52N. I suspect it's some sort of projection issue, but I'm not sure how to troubleshoot.

Mapnik is being used with Tirex; it is reading from a .vrt file which in turn references ~2400 tifs.

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  • It looks like a pyramid issue if it's only at particular scale. What format is your SRTM in? You said VRT, but what are the tiles (TIFF, but GeoTiff, world Tiff, compression...)? Nov 25, 2014 at 2:31
  • They are GeoTiffs that I created with LZW compression using gdal_calc.py. I have not created any pyramids or raster overviews. Nov 25, 2014 at 2:34
  • The tile cache (if present) should have overviews, it could help to regenerate the tile cache or build the pyramids using GDALADDO -ro <name and path of vrt> 2 4 8 16 32 64 128 256 to avoid having to resample so much on-the-fly. Nov 25, 2014 at 2:41
  • It's a brand new tile cache (I deleted what was there previously to test). I will read about gdaladdo... Nov 25, 2014 at 2:48
  • If that doesn't help then perhaps uncompress some of the likely problem tiles. If you can track down the (likely) bad tile, or at least short list it to a half-dozen or so, then convert to GeoTiff uncompressed with GDAL_Translate then rename and replace the tiles keeping the same names you will not need to rebuild the VRT (names, projection and extent all being the same). This technique has helped me previously with a VRT with a bad tile (with very little data in it) LZW compressed... it seems the decompressor tripped up at certain zoom scales so by pre-uncompressing it fixed my problem. Nov 25, 2014 at 3:04

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For those who may be interested: the problem seems to have been that my .vrt had coordinates that spanned across the -180 longitude. Left was a pixel or two beyond the line (-180.00004) and right was ~0 degrees). Clipping it seems to have solved the problem.

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