[Qgis-user] Google Mercator

[Albin Blaschka]

Hello!

There are some possible pitfalls with the so-called Google Mercator 
projection - look for example at

http://www.hydrometronics.com/downloads/Web%20Mercator%20-%20Non-Conformal,%20Non-Mercator%20%28notes%29.pdf

Specially at pages 5, 9 and 13 in the pdf linked above...might be the 
root of the problem... The summary brought there is: Use Google Mercator 
only for visualisation, but not for anything else...

And also interesting:

http://alastaira.wordpress.com/2011/01/23/the-google-maps-bing-maps-spherical-mercator-projection/


All this is not really a solution, but maybe an explanation...

regards,
Albin


Am 10.04.2012 11:51, schrieb 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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