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In QGIS, I often find myself setting up projects with an ellipsoid coordinate system (e.g. WGS84) so that the resulting maps have lat lon scales, etc.

Often in these projects I also need to do some calculations in other units (e.g. calculate the sq km of a feature).

At present I do this my maintaining two versions of a shapefile, one projected for calculations and one ellipsoid for layouts.

This becomes complicated in large projects!

Is there an easier way? Am I overlooking a technique for doing these calculations on-the-fly from spheroid shapefiles?

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2 Answers 2

There is no need to have two shapefiles. If you calculate the area with $area function of field calculator, the layer CRS must be in a projected CRS like UTM.

For the measurement tool and scales , it depends on the project CRS whether you get metres or degrees.

So I think you will be better of with shapefiles in UTM and projects in lat/lon, if you need that. Setting up a copy of the project in another CRS would be a smaller effort than handling two shapefiles.

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thanks for this advice. as an example of a particularly complicated recent project: I needed to chop up a shapefile based on a 1degree grid, and also generate an x km buffer on the same shapefile. In this case would you go two shapefiles? Or two projects? –  Andy Harvey Apr 11 '13 at 14:45
    
For creating the buffer, you will need a projected CRS. I'm not sure how the degree grid comes in, but if you need the buffering only once, you can reproject it to degrees to work on with that. The project CRS can be different, and it is no great work to save the project to another name and CRS, without having to change the data layers. –  AndreJ Apr 11 '13 at 16:47

I had a similar requirement, and I solved it using PostGIS. I had a Table containing points in WebMercator and the Users wanted to see the position in Latitude and Longitude.

We solved it by creating a View where the two required fields were calculated on the fly using PostGIS's Spatial Functions.

This kind of functionality is not available in Shapefiles, and we were more than willing to use PostGIS.

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thanks Devdatta, that's an interesting idea. I'm not too familiar with PostGIS, but have been flirting with it recently for some web projects. I will look at this more closely. –  Andy Harvey Apr 11 '13 at 14:46

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