Calculating Volumes

Introduction:

In this lab volumetric functions of two different pieces of software (Pix4D and ArcMap) were used to find volumes of piles of aggregate at a mine near Eau Claire, WI. Two different techniques of finding volume were used in ArcMap: the Surface Volume tool, supplied with a clipped DSM of each individual pile and a value for base height, and the Polygon Volume tool, supplied with a TIN created from the DSM.  DSMs from UAS, tied down with GCPs to ensure spatial accuracy are a great source to find volumes with. This data was already available to work with due to the past Pix4D processing that had been done in the class.

Methods:

Pix4D: This method was by far the easiest method to use. The Pix4D project file created earlier in the semester was opened and the Volumes tab was clicked on on the left of the screen. Next, the new object button was clicked, and the base of each pile traced. From there the compute button was clicked for each object, and the measurements recorded. The three piles that were measured are seen in Figure 1, the first pile measured is seen in Figure 2, the second in Figure 3, and the third in Figure 4.

Figure 1

Figure 2

Figure 3

Figure 4

Surface Volume: This technique was slightly trickier. First the DSM and Orthomosaic were brought into an ArcMap document, then three polygon feature classes were created in a new file geodatabase in the catalog with a right click on the geodatabase (Figure 5). With these three new feature classes care was taken to import the projected coordinate system of the DSM. The editor was then opened under the toolbar menu of the customize menu, and editing was started. After clicking on each feature class, the appropriate pile was traced with a polygon (Figure 6). Editing was now stopped. These three polygons were then utilized to extract their respective areas of the DSM by using the Extract by Mask tool (Figure 7). The three resulting DSMs were then used in the Surface Volume tool (Figure 8), along with a base elevation found by using the identify function, to find the volumes of the areas, which were saved in specified .TXT documents, along with the parameters used.

Figure 5

Figure 6

Figure 7

Figure 8

Polygon Volume: Using the individual DSM .TIF raster files earlier created for Surface Volume tool method and the Raster to TIN tool, TINs were made fore each aggregate pile (Figure 9). These were in turn referenced by the Add Surface Information tool for minimum elevations (Z_MIN), which
were added to feature classes' attribute tables (Figure 10). After this, both the base elevation in each piles feature class, and the TIN for each were referenced by the Polygon Volume tool to find a volume, which was then stored in a volume field for each feature class (Figure 11).

Figure 9

Figure 10

Figure 11

Results and Discussion:

The resulting volumes can be seen in Figure 12, and a map showing the three different piles can be seen in Figure 13.

Figure 12

Figure 13

A few points may be made about the comparability and the accuracy of the three methods' results, and then with this understanding the differences between each result will make sense. One first point is that although effort was made to draw the boarder of the pile in Pix4D the same as how the outline of each feature class was digitized in ArcMap, the results are going to be slightly different, resulting in different extents of area for each pile and slightly different volumes. Next, and most important to resulting volumes, each method calculates the base heights slightly differently. While Pix4D triangulates peripheral Z values to find an uneven base height (Figure 14), the Surface Volume tool, uses a user defined base height (the Plane Height parameter) and calculates the entire area above up to the supplied DSM raster. With the Polygon Volume tool, the same thing happens except the Z_MIN parameter in the feature class that was set from the lowest point on the TIN was used as the base. This made the volume found with this tool substantially larger than the volumes found with the other tools. To use the two ESRI tools for the most accurate results care must be taken in finding a base height or plane from which to measure from on uneven ground.

Figure 14
 https://support.pix4d.com/hc/en-us/articles/202559239-How-Pix4Dmapper-calculates-the-Volume-#gsc.tab=0

Conclusion:

UAS data can be used to find highly accurate volumes of features above the surface if tied down with GCPs so as to be spatially accurate, the different volume calculation options are understood and used skillfully, and the perimeter of the object is drawn carefully. The most automated, yet the most useful and accurate volume function may be the one contained in Pix4D due to its triangulation of an uneven surface below the measured object. Though only used for measurement of piles of mining aggregate in this project, these methods would also work for measurement of volumes of buildings and other above ground accurately sensed objects. Thinking farther, these methods could even be used to estimate volume of below ground objects sensed through GPR or other methods given compatible data is produced.