[Qgis-user] QgsCsException - Error: acos/asin: |arg| >1.+1e-14

[Andre Joost]

Am 31.10.2016 um 12:03 schrieb Tom Chadwin:
> Someone using qgis2web has experienced the following error:
>
> QgsCsException: forward transform of
> (-18933215.197392, -9254444.351822)
> PROJ.4: +proj=moll +lon_0=0 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs
> +ellps=WGS84 +towgs84=0,0,0 +to +proj=longlat +datum=WGS84 +no_defs
> +ellps=WGS84 +towgs84=0,0,0
> Error: acos/asin: |arg| >1.+1e-14
>
> What causes the error? The QGIS project is in EPSG:54009, and either export
> to 3857 or to 54009 gives the error above.
>

The reason lies in the oddity of the Mollweide projection. At full size, 
large parts of the image can be reprojected to a point on the Earth. 
Using gdalwarp, this command line will work:

gdalwarp -overwrite -s_srs EPSG:53009 -t_srs EPSG:3857 infile outfile 
-wo SOURCE_EXTRA=1000

But with On-The-fly-reprojections you can not enter the SOURCE_EXTRA.

If you need a full Earth view, you have to stick to EPSG:4326 or 3857.

BTW Sphere Mollweide EPSG:53009 works far better than World Mollweide 
EPSG:54009 when it comes to reprojections.

HTH,
André Joost