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Calculate area between contour lines on a DEM with Qgis 1.8.0-Lisboa

Based on a DEM of a whole country I have reclassified altitudinal bands with steps of 100 metres (100-200, 200-300...). Then I polygonized the bands in order to get shapefiles and summarized them with the tool „Singleparts to Multiparts“

Now I have one entry in the attribute table for each altitudinal band.

I would like to calculate the area of each band but no matter what I write or select at the field calculator settings, the result is alwasy the same (Even if I change the CRS). And when I save it (finishing the edit mode) the number turns to 0. What am I doing wrong?

I am using qgis 1.8.0.

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marked as duplicate by whuber Jan 3 '13 at 18:00

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

What value returns typing $area in the raster calculator ? –  Curlew Jan 3 '13 at 17:52
For the new field / column being calculated, what type is it? Integer, Double Precision, String? –  Willy Jan 3 '13 at 23:27
The value in the attribut table is integer. The area I would like to calculate in real. I always get numbers like 0.0484618034274718 no matter if I change the CRS for the project or the layer. I dont know what type this is cause I always get the same and cannot compare anything. –  Sebastian Jan 4 '13 at 9:23

1 Answer 1

In the function list on the left hand side of the field calculator, click the plus sign on the Geometry function. In the expanded list of geometry functions, select $area.

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