Bug report #15686

Save as Image from WMS/WMTS has reprojection issues

Added by whatnick - about 6 years ago. Updated over 3 years ago.

Assignee:whatnick -
Affected QGIS version:2.16.3 Regression?:No
Operating System: Easy fix?:No
Pull Request or Patch supplied:No Resolution:end of life
Crashes QGIS or corrupts data:No Copied to github as #:23609


When saving images from WMS/WMTS to a projection not native to the source online dataset, reprojection is performed and artefacts are introduced. See attached sample images for a demonstration.

test_no_reprojection.tif - Image saved with reprojection EPSG:3857 to EPSG:28354 (1.81 MB) whatnick -, 2016-10-10 12:29 AM

test_reprojection.tif - Image saved without reprojection in EPSG:3857 (1.29 MB) whatnick -, 2016-10-10 12:29 AM


#1 Updated by Martin Dobias about 6 years ago

How did you do the export with and without reprojection? Do you have source WMS/WMTS to replicate it?

Anyway, it can be expected that some artifacts are introduced, especially if the attached reprojected image has slightly smaller resolution than the original layer.

#2 Updated by whatnick - about 6 years ago

Reprojection was performed by selecting the target SRS in the "Save As" dialog. The resolution in both case was set to 0.08cm . The WMS we are using is private. I can create an account for you to debug and email the auth details to you.

#3 Updated by Giovanni Manghi over 5 years ago

  • Easy fix? set to No
  • Regression? set to No

#4 Updated by Jürgen Fischer about 5 years ago

  • Category set to Unknown

#5 Updated by whatnick - about 5 years ago

We currently have a work around chaining the WMS Save-As with Raster tools (GDAL) warp - https://metromap.freshdesk.com/support/solutions/articles/22000000726--using . I have some time to do dev work and will submit pull requests over the next month, should be a relatively benign fix since reprojection is in place already just interpolation algorithms are not selectable.

#6 Updated by Giovanni Manghi about 5 years ago

  • Category changed from Unknown to Rasters

#7 Updated by whatnick - about 5 years ago

  • Assignee set to whatnick -

The issue is mainly due to "core\raster\qgsrasterprojector.cpp" performing fast/approximate raster reprojection meant for on-the-fly reprojection for saving high-quality files to disk. I propose the following enhancements:

1) Add check-box in Save As Dialog for WMS to trigger alternate resampling methods
2) Rewrite raster reprojector to respect this flag and implement at least bilinear resampling
Chain GdalWarp API at the end of the save process to reproject instead of using the simple reprojector.

#8 Updated by Giovanni Manghi over 3 years ago

  • Resolution set to end of life
  • Status changed from Open to Closed

End of life notice: QGIS 2.18 LTR


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