[Qgis-user] calculating area

[John C. Tull]

Hi Nik,

On Nov 20, 2009, at 12:04 AM, Nik Go wrote:

> 
> Hello. I'm trying to calculate area of my shapefiles but I'm not really sure if I'm doing it right.  I expect results to be in meters (sq meters or sq. kilometers) but read out is in sq. degrees.  I'm told that I have shapefiles in geographic coordinate system WGS 84 but I don't know to convert this to a projected coordinate system from qgis.
> 
> So, how should I go about doing
> converting from geographic coordinated to projected coordinates so I could
> determine/calculate area
> determine distance between points
> convert sq. degrees to sq. meters?

You can change your project CRS from the default geographic coordinate system of WGS84 to a projected coordinate system based on meters, such as a UTM zone. Turn on the "Enable 'on the fly' CRS transformation, and you will be set for vector data. If you have raster data, Qgis does not handle on the fly transforms on them. For those, I recommend looking at gdalwarp, part of the gdal tools. In particular, look at the vrt output format so you do not create numerous large file. A vrt invokes gdal to, effectively, perform an 'on the fly' reprojection of the original raster data.

Regards,
John
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