Calculating volume is one of main tasks when working with terrain or
topographic data. In other posts I already discussed some topics related to
terrain data processing like creating DEM from point dataset, creating profile
and
visualize DEM data in 3D. In this tutorial I would like to explain how to calculate volume in QGIS. I
will discuss some topics such as raster volume algorithm calculation, how to
calculate raster volume in QGIS and cut/fill volume.
Raster Volume Calculation Algorithm
Calculating volume using raster data has a simple algorithm which is area of a
pixel(p) multiply with the value(h) of respective pixel that
contains the height information. While a raster which the volume will be
calculated comprises of many pixel, then the total volume is summation of each
pixel area multiply with respective pixel value as shown in the following
formula \[V={\sum_{i=1}^n p_i*h_i} \] Area of a pixel depends on pixel
dimension which is called pixel resolution. A raster with a high resolution
like Digital Elevation Model(DEM) from drone survey with 0.1 m resolution
compare with DEM SRTM with 30 m resolution will give very different
result, because a pixel of DEM from drone survey represents an area
0.1 m x 0.1 m, while a pixel of SRTM DEM represents area
30 m x 30 m. Which one is more accurate? Of course DEM from drone survey will give more
accurate result compare to the SRTM. That's why in a project that is
required volume calculation, a raster with higher resolution is preferred.
Figure 1 shows 2D profile of an object which related to different raster
resolution. In the figure, can be seen that a raster with higher resolution
represents the actual topographic surface closely compare to the other one
with lower resolution.
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Figure 1. Raster resolution and volume accuracy |
Volume Base Level
Next, we will discuss about base level, what is it actually? Base level is a
reference level from where the height will be calculated. In the previous
formula we can see that a volume of a pixel is the product of pixel area with
the pixel value. The pixel value contains the height information. Next
question is, from where the height is measured? On the other way, where is
zero value of height that a pixel is referenced from?
In technical term the zero point of height value is called vertical datum.
There are several vertical datums can be used as height reference such as Mean
Sea Level (MSL), Geoid, Ellipsoid model and local vertical datum. Then what it
has to do with volume calculation? In a real application, it is really rare
that a volume is calculated from vertical datum because the zero height is not
presents in a DEM data, and usually we only calculate a volume of object from
it's foot.
To get a volume of specific object area or object in a raster of DEM data, we
need to determine a base level to where the volume is referenced from. Then
how this base level is applied in the volume calculation? After a base level
is specified, it will be used to get the height of a pixel from the base
level. The pixel value will be subtracted with the base level value. If a base
level has symbol \( b_0\) then the formula for raster volume calculation will
be \[V={\sum_{i=1}^n p_i*(h_i-b_0)} \]
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Figure 2. Volume and base level reference |
Figure 2 shows relation among Mean Sea Level (MSL) as vertical datum, base
level and volume. If a volume that to be calculated without a base level is
determined then the volume is referenced from the datum. When a base level is
determined then the volume can be calculated from the base level both above or
below base level.
How to Calculate Raster Surface Volume in QGIS
After knowing the algorithm how to calculate a surface volume, next let's
compute it. In this tutorial we will calculate the volume of a pile as shown
in figure 3 using GIS open source software QGIS. For this tutorial I'm using
QGIS 3.14 Pi.
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Figure 3. DEM data for volume calculation |
To make sure only the volume of the pile will be calculated, it's recommended
to clip only the pile DEM data. This can be achieved with two steps.
1. Digitize a polygon area for the pile.
2. Clip the DEM based on the polygon using
Clip Raster by Mask Layer tool, that can be found in Raster >>
Extraction.
After finishing those two steps, the result of clipped raster data can be seen
as in figure 4.
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Figure 4. Clipped DEM |
To calcualte raster surface volume in QGIS can be used
Raster surface volume
tool with the following steps.
1. Open Processing Toolbox, then select Raster analysis and
choose Raster surface volume as in figure 5.
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Figure 5. Raster surface volume tool |
2. The Raster surface volume window will appear as in figure 6. In
Input layer parameter, select a DEM to be calculated, for my case
pile_dem. Then select Base level value. I gave 670, because the minimum
raster value (height value) of the DEM data is 670 m. Next in
Method parameter, select a method to be used. There area several
methods available such as: Count Only Above Base Level (A), Count Only Below Base Level (B), Subtract Volumes Below Base Level (A-B) and
Add Volumes Below Base Level
(A+B). Since I want to calculate the volume of the pile from the bottom, then
I chose Count Only Above Base Level. Lastly for output parameter it can
be saved to a file or in temporary file.
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Figure 6. Raster surface volume window |
3. Click Run button. The process will running. When it's done, the result can
be seen in the Log tab as in Figure 7.
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FIgure 7. Volume result |
In the result can be seen some records like area of raster surface, pixel
count and volume. What is area and volume unit? It follows the unit of DEM
data. If it is in meter both for pixel size and height, then the area is in
square meter and volume in cubic meter. Therefore it's really important to
check the coordinate system of the data first, before calculating the volume.
If the coordinate system has an unprojected coordinate system like Geographic
coordinate system which has degree unit, it's better to reproject it into a
projected coordinate system like Universal Transverse Mercator(UTM).
How to Calculate Cut and Fill Volume
In this section we will discuss about cut and fill volume calculation. What is
cut or fill volume? Figure 8 shows the difference between cut and fill volume.
Cut volume is a volume of a mass object. On the other hand, fill volume is a
volume of empty object. In the previous step we already calculated a pile
volume, which is a cut volume. What about fill volume, how to calculate it?
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Figure 8. Cut and Fill Volume |
In this tutorial we will calculate fill volume of a small polygon area
from the previous DEM data as shown in figure 9. The figure also shows the
2D profile
along the red line. From the profile we will calculate fill volume below
679.024 m base level.
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Figure 9. Fill volume and profile line |
To calculate the fill volume below a base level can be done with following
steps:
1. Clip area of interest using a polygon where a fill volume to be calculated.
2. Open the Raster Surface Volume tool. Select the input layer and
determined the base level. In this case I want to calculate the fill volume
below 679.024 m. In the Method parameter choose
Count Only Below Base Level.
3. Click the Run button. The result can be seen in the Log tab as in
figure 10.
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Figure 10. Fill volume result |
The fill volume is 260.432 cubic meter. The minus sign can be indicated as
fill volume. This is because the height of DEM (\(h_i\)) is below the base
level (\(b_0\)).
That's all tutorial how to calculate raster surface volume in QGIS using
Raster surface volume tool. In this tutorial we had learnt the
algorithm how to compute volume in raster data, base level, calculate fill and
cut volume. Thanks for reading!
