[Qgis-user] South Africa/Lesotho Projections in QGIS

[Andre Joost]

Am 03.01.2013 21:45, schrieb Claire McIntyre:
>
> Hello, I am creating data for Lesotho for a volunteer project. I am
> currently using OSM as a basemap which has a EPSG of 3857. I am
> creating new features stored in shapefile.I want to calculate area of
> polygons using the field calculator. When I use this projection&
> calculate area it is incorrect. I presume it is in decimal degrees.


Yes, field calculator does not look at the project CRS (as the 
measurement tools do), but the CRS of the shapefile. Most probably 
EPSG:4326.

> Is there a way to calculate metres/kilometres squared. I presume I
> need to change from a geographic coordinate system to a projected
> coordinate system.Ideally this would be Hartebeesthoek94/L027, but i
> cannot see any of the South African LO systems available to me. Any
> help would be very much appreciated.

Qgis Master (1.9.0) has all Hartebeesthoek94 CRS available. For LO27, it 
is EPSG:2052. For older QGIS versions, you can create a custom CRS with 
the following proj string:

+proj=tmerc +lat_0=0 +lon_0=27 +k=1 +x_0=0 +y_0=0 +axis=wsu +ellps=WGS84 
+towgs84=0,0,0,0,0,0,0 +units=m +no_defs

Or use the UTM projection for your part of the world.

You have to export your shapefile, and choose the custom CRS as target 
CRS to get the field calculator work right. On-the-fly-projection is not 
enough.

HTH,
André Joost