QGIS – How to Perform Volume Calculation

errorgrasspythonqgissextante-qgis-plugin

EDIT 07/12/12

Ok, meanwhile I came to another problem I want to discuss.
I just tried around with the volume calculation with some layers and came to a point where I receive the following error.
Screenshot:

error

I did some research and discovered this explanation:
http://blog.webforefront.com/archives/2011/02/python_ascii_co.html
The problem is, that I have absolutely no Idea about python programming.
So my question is: Does anyone know how to solve this described error problem?

Note: I left the older questions below in order to see the process I made.

I m currently trying to make a volume calculation in QGIS. I realised, that
using GRASS or Sextante is the right choice to do this. But since there are barely useful tutorials, that go into depht nor any other guides or hints that may help in this context, I want to seek help here.
So I want to ask these questions:

1)I want to calculate the volume of a specific map area. So I discovered the algorithm "r.volume."

1.1.)How does it work exactly?The Help text in that menue is not very useful for me as a Grass beginner.

1.2) Why do I need raster layers for this and no vector layers?

1.3) What certain criteria do these rasters have to fulfill?

1.4) I added a screenshot. What exactly are these "clumps" and what rasters have to be chosen in the drop down list?

1.5) Can I have an accurate example of how to perform such an r.volume algorithm?I d be thankful for any detailed help/ raster files to test this.

2)I learned, that GRASS (in QGIS) was integrated into the Sextante Plugin for QGIS.

2.1) Does this mean I can only use Sextante and still have all the functions of Grassgis?I d welcome that because I always had trouble with importing data into Grassgis. Sextante doesn t (obiously) require this.(?)

2.2) A strengh of gvSIG is Sextante, so I read; Is the Sextante Plugin for QGIS equal to gvSIG Sextante concerning performance/ possibilities?

thanks a lot,
Shepherdjo

EDIT:07/11/12 8.08am:

I know this is not easy to answer but it would be great too, if you can help me out with the other questions (2.1/2.2).

r.volume menue

Best Answer

Update; I tried the volume calculation again and came to the conclusion that it can be done with grass, since sextante produces this weird error (see above). Set the Mapset and convert your raster layer(it has to be a file that countains the elevation data(dem)) to a grass file. Make sure, all projections match, otherwise an error will occur during conversion. First run r.clump, that will produce another raster(clump-map).Convert this one into grass too. In r.volume you ll now use your both rasters and run the method. Worked good for me. Hope this helps. Jo

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[GIS] Cutting out depth information in raster. Calculating volume in Qgis

I believe you can use some map algebra (raster > raster calculator) before you can preform your volume measurements in grass.

Assuming that your bathymetric data use positive values to represent the sea depth, and using your example for the range as 50 the min_depth and 200 the max_depth. For each of the raster cells you need to "remove" anything below the 200 and above the 50. You have 3 possible situations:

your cell depth is higher than the max_depth

(@bathymetry > 200)
your cell depth in the min_depth - max_depth range

(@bathymetry <= 200) * (@bathymetry >= 50)
your cell depth is lower that your min_depth

(@bathymetry < 50)

In case 1 the size of the water column will be the entire size of our range:

(@bathymetry > 200.0) * (200.0 - 50.0)

In case 2 the "water column" is from your bathimetry depth until the minimum value of the range:

(@bathymetry <= 200.0) * (@bathymetry > 50.0)*(@bathymetry - 50.0)

In case 3 there is no useful "water column" for your specie therefore it would be:

(@bathymetry <= 50)*0.0

Since the 3 cases never occurs at the same time, all we need is to sum the expressions:

(@bathymetry > 200.0) * (200.0 - 50.0) + (@bathymetry <= 200.0) * (@bathymetry > 50.0)*(@bathymetry - 50.0) + (@bathymetry <= 50)*0.0

Since the last expression is always zero, the is no point in including it:

(@bathymetry > 200.0) * (200.0 - 50.0) + (@bathymetry <= 200.0) * (@bathymetry > 50.0)*(@bathymetry - 50.0)

And the generic version of the expression would be:

(@bathymetry > max_depth) * (max_depth - min_depth) + (@bathymetry <= max_depth) * (@bathymetry > min_depth)*(@bathymetry - min_depth)

After this, you can use the output raster in r.volume to sum all "water columns" values

[GIS] Calculate volume from raster (GRASS) with depth per cell

Are you aware that the output from r.clump does not have the depths? Each clump is given a unique category value, so you no longer have the cell depths. You can get the area of each clump, but not the total volume.

If I understand correctly your goal, you should convert the "clumps" to a polygon vector, then use v.rast.stats to get the total depth for each clump area. i.e.:

v.rast.stats map=clumps raster=depths column_pref=depth method=sum,minimum,maximum,range

This will add four new columns to the area vector of "clumps" with total volume, min and max depths, and the range of depths.

I'd also suggest you try to do this procedure directly in GRASS, rather than thru the processing toolbox in QGIS.

Best Answer

Related Solutions

[GIS] Cutting out depth information in raster. Calculating volume in Qgis

[GIS] Calculate volume from raster (GRASS) with depth per cell

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