[Qgis-user] Google Mercator

[Nick Hopton]

I avoid using the Google Mercator CRS (EPSG:900913, in QGIS Master) in QGIS
projects because I know that it has problems. For example, users get into
trouble trying to measuring distances using the tape measure. But people do
use Google Mercator for projects and a recent post here suggest that they
also do things like calculating the areas of polygons from shapefiles
projected to EPSG:900913.

This looks like a dangerous thing to do. Recently I created a test shapefile
containg one object, a polygon, square and having sides 100 km in length.
This corresponded with the extents of the British Ordnance Survey's 100 km
grid square 'SU'.

The shapefile was loaded into QGIS and the layer CRS and the project CRS
were set to EPSG:27700. I then used the field calculator to calculate the
area of the polygon, which it did correctly (10,000 sq km).

I then reprojected the shapefile to EPSG:900913 and used the field
calculator to calculate the area of the polygon. The area of the 100 km x
100 km square was now reported as being *25502 sq km*.

One other oddity came to light too. As mentioned, I loaded the shapefile
projected to EPSG:27700 into QGIS, set the layer CRS and the project CRS to
EPSG:27700. The 'Identify Features' tool then correctly reported the area of
the polygon correctly (10,000 sq km). Enabling OTF and checking again with
the 'Identify Features' tool then produced an area of 9,980 sq km. But why?

Any comments?

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