Volumetrics

Introduction

In this week's lab the task was to learn about volumetrics and the different ways to create volumetric data from imagery. Volumetrics is a widely used application for using GIS systems. Companies, like sand mines use GIS companies to measure the volume of their product when in piles. This information is critical to their inventories and financial statements' validity.

In this lab we used volumetrics to estimate the volume of sand piles. Multiple methods were used to measure the volumes of three separate piles within Pix4D and ArcMAP. Imagery from the Litchfield Mine in Eau Claire, Wisconsin was once again processed in Pix4D software. Once done, volumetric measurements were taken from Pix4D and then ArcMAP functionality in various ways.  

Methods

- Pix4D - Pix4D's tool was the easiest to use. Once the imagery was processed, Pix4D has an easy and conspicous tool that creates new objects. the user simply makes a polygon shape around the area intended to be measured. The user is then able to press the compute button and Pix4D measures the volume.

  Pix4D then displays the volume on the left hand side of the screen (Figure 1). The user can measure and display multiple area volumes at once (Figure 2), which is very convenient. The sheer ease of the tool within Pix4D, combined with the known robust functionality of ESRI products like ArcMAP and ArcPRO add skepticism to the accuracy of the volumes measured in Pix4D.

Figure 1: Pix4D highlight of pile 2

Figure 2: All 3 piles in Pix4D with calculated volumes

- ArcMap - Multiple methods were used in ArcMAP. ArcMAP is a robust software sweet, that has increased functionality when compared to Pix4D. Within ArcMAP, there are many ways for users to derive volumetric data. Some tools that were used include raster clip, raster to TIN, add surface information, surface volume, and polygon volume.

  Once the imagery was processed in Pix4D and a geodatabase (Figure 3) was created, the imagery was brought into ArcMAP. Within ArcMAP, the various ArcMAP tools were then utilized. For this project, a digital surface model (DSM) and othomosaic were also created and imported via Pix4D. The DSM layer was especially important to the measurements. 

Figure 3: Geodatabase for volumetrics project

Figure 4: DSM layer properties of pile 1

Figure 5: DSM clip of pile 2

    - Raster Clip - A raster clip is a tool that allows the user to take a clip out of imagery (Figure 6), in this case, a DSM. Raster Clip was used to clip out a portion of the hillshade DSM in the processed Litchfield Mine imagery. Like Pix4D, the shape used to take the clip was a polygon. However in this case, the user doesn't want to simply go around the base of the area being measured, like in Pix4D. The user needs to include some area around base. This helps the imagery better process the starting elevation of the volume that's being measured, when measured. 

Figure 6: Raster clip around pile 1 in hillshade

    - Raster to TIN - A Triangulated Irregularity Network (TIN) is a digital data structure used in GIS to represent a surface. Raster to TIN allows a user to manipulate imagery to create TINs out of surface area. The result is a polygonal representation of surface area that looks like a playing surface or boarder in an old video game (Figures 7, 8, and 9). TINs make estimating volume easier for the software because it simplifies the surface.

Figure 7: TIN of pile 1

Figure 8: Close up of pile 3 TIN

Figure 9: All 3 TINs in hillshade

    - Add Surface Information, Surface Volume, and Polygon Volume - Within ArcMap is the ability to calculate this vital information. Adding surface information utilizes the area clipped but not included within the piles to get a good starting elevation. Once this critical starting point is established, measuring the surface volume (Figure 10) and polygon volume give the user the rest of the information needed to complete a volumetric analysis.

Figure 10: Adding surface volume information to pile 1

Maps - The Pile Surface map shows hillshade elevation as well as a portion of the raster clips at the base of the areas measured. These clips were taken from the DSM and then derived from the polygonal areas around the bases of the piles.

The TIN map show those same 3 piles taken from rasters and made into TINs. The TINs are what the volumetric data was taken from and this map highlights there own varying levels of elevation within the piles.  

Pix4D Volumes -

Figure 11: Pix4D pile volumes

ArcMAP Volumes -

Figure 12: ArcMAP pile volumes

Conclusion - Though the increased functionality that comes ArcMAP makes it perhaps the more accurate tool, Pix4D's method was much easier to use. Accuracy is key to volumetrics as it justifies the value of the data presented. Accurate volumes of inventories are critical to businesses and GIS users will undoubtedly choose the best tool available to them.

This was an informative lab, as it displayed a common and marketable application of UAS technology, in volumetrics. Knowing various ways to harvest volumetric data is valuable as well, in that the user can produce comparable data, validate data, and troubleshoot methods if needed.