gdalwarp incorrectly re-projects data in EPSG:27700 to EPSG:3857

[millionpoundhat]

Expected behavior and actual behavior.

gdalwarp incorrectly re-projects data in EPSG:27700 to EPSG:3857.

There is 100m offset when re-projecting data from British National Grid to Web Mercator.

I believe the problem was introduced with Proj-8.

Steps to reproduce the problem.

Take the following WMS source xml as an example data source in 27700

  <Service name="TMS">
    <ServerUrl>https://api.os.uk/maps/raster/v1/wmts?key=yOMuONZcAGC54eFPLobKBAEmPGCIq3Gm&amp;service=WMTS&amp;request=GetTile&amp;version=2.0.0&amp;layer=Leisure_27700&amp;style=default&amp;outputFormat=image/png&amp;tileMatrixSet=EPSG:27700&amp;tileMatrix=9&amp;tileRow=${y}&amp;tileCol=${x}</ServerUrl>

  </Service>
  <DataWindow>
    <UpperLeftX>-238375</UpperLeftX>
    <UpperLeftY>1376256</UpperLeftY>
    <LowerRightX>700185.000,</LowerRightX>
    <LowerRightY>0</LowerRightY>
    <TileLevel>0</TileLevel>
    <TileX>0</TileX>
    <TileY>0</TileY>
    <SizeX>536320</SizeX>
    <SizeY>786432</SizeY>
    <YOrigin>top</YOrigin>
  </DataWindow>
  <Projection>EPSG:27700</Projection>
  <BlockSizeX>256</BlockSizeX>
  <BlockSizeY>256</BlockSizeY>
  <BandsCount>4</BandsCount>
  <MaxConnections>100</MaxConnections>
  <ZeroBlockHttpCodes>204,404</ZeroBlockHttpCodes>
</GDAL_WMS>

To test with

• GDAL 3.2.2, released 2021/03/05
• Proj Rel. 8.0.0, March 1st, 2021

I run the following

docker run -it -v "$(pwd)":/app osgeo/gdal:alpine-small-3.2.2 /bin/sh 

Then inside the container /app

gdalwarp -te 360443.864631 174074.392422 360828.386103 174457.931515 -te_srs epsg:27700 -t_srs epsg:3857 gdal-osgb-tms.xml gdal-3857-from-tms-3.2.2-gdal.tif

reprojects the tile incorrectly

---

To test with

• GDAL 3.2.1, released 2020/12/29
• Proj Rel. 7.2.1, January 1st, 2021

I ran the following

docker run -it -v "$(pwd)":/app osgeo/gdal:alpine-small-3.2.1 /bin/sh 

Then inside the container /app

gdalwarp -te 360443.864631 174074.392422 360828.386103 174457.931515 -te_srs epsg:27700 -t_srs epsg:3857 gdal-osgb-tms.xml gdal-3857-from-tms-3.2.1-gdal.tif

reprojects the tile correctly

---

I also tested on my Debian system, with the ubuntugis-stable PPA

• Pop!_OS 20.10

• Release: 20.10

• Codename: groovy

• GDAL 3.1.3, released 2020/09/01

• Proj Rel. 7.1.0, August 1st, 2020

gdalwarp -te 360443.864631 174074.392422 360828.386103 174457.931515 -te_srs epsg:27700 -t_srs epsg:3857 gdal-osgb-tms.xml gdal-3857-from-tms-3.2.1-gdal.tif

---

I've also tested this with a geotiff in epsg:27700 incase there was an issue in the WMS/TMS driver and the error in reprojection remains.

---

Testing cs2cs

I also tested proj with cs2cs to see if the issue was there. Interestingly

Proj Rel. 7.1.0, August 1st, 2020
GDAL 3.1.3, released 2020/09/01

cs2cs EPSG:27700 EPSG:3857  <<EOF
360443.864631 174074.392422
EOF

Result = -286180.06 6703828.59 0.00

Proj Rel. 8.0.0, March 1st, 2021
GDAL 3.2.1, released 2020/12/29

cs2cs EPSG:27700 EPSG:3857  <<EOF
360443.864631 174074.392422
EOF

Result = -286180.20 6703828.38 0.00

There is a difference here but not 100m, I do note that the mercator projection was made more accurate in Proj 8 which could account for this.

[rouault]

Confirmed

[rouault]

This is indeed a PROJ 8.0.0 issue. Fix OSGeo/PROJ#2678 will be in PROJ 8.0.1

[millionpoundhat]

This is indeed a PROJ 8.0.0 issue. Fix OSGeo/PROJ#2678 will be in PROJ 8.0.1

Thanks!