[Qgis-user] Calculating area and length from DTM

[Lene Fischer]

Hi Again,
I have found tree solutions for my problems- Can anyone tell me if they are correct.

I clip the DTM with the polygon and the use the SAGA Real surface area. It gives me a new raster where the pixelvalues has changed from Z to a calculated area. Finally I use the Zonal statistic and add the SUM value to the polygon.

The second is a Grass function r.surf.area It gives me an area in the log.

The functions has a slightly different result, but it´s only 3/1000 :-)
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For calculating the fence I had to be a bit more old school. Distance= SQRT((x2-x1)^2 + (y2-y1)^2 + (z2-z1)^2) I took the polygon and converted it to points with a ordered pointnumber. 
Used the SAGA function Add Grid Values to Point It takes the Z Value from the DTM and add an attribute to a new pointlayer. 
The pointlayer is saved as CSV. It gives me a table with x,y and z.
And in Spreadsheet I can easily calculate the length af each vertice and make a sum. :-)


Regards
Lene Fischer
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Sendt: 31. januar 2016 17:52
Til: qgis-user
Emne: [Qgis-user] Calculating area and length from DTM

Hi,
Today I have been out collecting data with my GPS in an area with huge differences in heights.
My length and area will be from the "flat" area and not accurate/correct.

If a contractor should order a fence - it would be too short.... :-O

How do I calculate correct when I have the polygon and a terrain model.

Regards
Lene Fischer
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