Lab 8: Calculating and Comparing Volumetric Analysis Methods in Pix4D and ESRI Software.

Introduction 

Within the growing industry of geospatial technology, volumetric analyses is becoming a highly used application as companies start seeing the value such operations provide them in both accuracy and cost effectiveness.  Of the various industries that are beginning to employ its uses, the mining, construction, and materials industries are the the industries in Wisconsin which are using the most.  The technology allows for companies to know how much material is being extracted, transferred, and applied, with a higher degree of accuracy and efficiency than traditional methods.  The data that will be used in this lab was imagery collected from a UAS mounted with a high resolution camera.  Once the individual images have all been tied together into an geo-accurate orthomosaic, volumetric analyses can be conducted.  In previous labs, students conducted all volumetric analyses in PIX4D, and were essentially given the freedom to conduct volumetrics on anything in the AOI, which was a soccer field with a track around it.  In this lab, volumetrics will be conducted using both Pix4D and ESRI Software, and the volumetric results of three separate piles will ultimately be compared.  

Study Area

Figure 1: Litchfeild Mine and Piles for volumetrics

The area where this volumetric analyses will be conducted is the Litchfeild Mine, an active materials mine that is owned by the Kreamer Construction Company.  Relative to the city of Eau Claire, the site is located just south of town and is roughly a 10 minute drive from the campus.  Figure 1 to the right shows the totality of the sight as well as the three piles which will have the volumetrics calculated for.    The imagery for this map was captured using a hexicopter mounted with kinematic (RTK) GPS system and a Sony A6000 Camera.  The flight mission took pictures at 200 feet off of the ground at 24 megapixels.  The orthomosaic that was created as a result of this flight used 301 images, almost all were able to be calibrated to be apart of the orthomosaic.    

The Litchfeild mine has a number of piles apart from the ones selected for this  analyses.  between the piles, the land is uniformly level, allowing for the heavy machinery to pass through unopposed of difficult terrain or obstacles.  If students were at the sight when machines were operating on materials, they class would have to be equipped with MSHA certified clothing and hardhats, while also being guided by an employee of the Kreamer Company who was certified MSHA.  Safety is a very important aspect to consider when on a mining sight, as they can be very dangerous. 

When highly accurate data is desired, like in mining operations, the common practice is to use a dual frequency survey grade GPS unit to place ground control points to increase the accuracy of the orthomosaic during post processing.  However, on the day this data was collected, there TopCon GPS unit was not working.  However, there is a way to still provide further accuracy in this situation and this technique, known as using tie-points, will be discussed further in the methods.  

Methods

Before volumetrics can be conducted, the images must be processed and properly geolocated.  On the day that the data was collected, the TopCon Survey GPS failed and thus gcp points could not be obtained.  Luckily, there was dataset from October of 2015 that had been optimized with GCP points, and from that data set a table of tie point locations could be created.  Tie-points are land/structure attributes that are in the same location in both sets of imagery.  By importing the table of tie-points coordinates from the October dataset into Pix4d's GCP and Tie-Point Manager, the user can effectively use those objects which are in the same location in both sets of imagery as make shift GCP points. It should be noted, however, that in an ideal situation, the TopCon unit would have worked and we could have laid out the vast spread of GCP markers that the class showed up intending to do.  Had the class been able to do this, the output orthomosaic and point cloud would have been even more accurate. Figure 2 below shows the RMS error produced from this Pix4D projects from using these tie-points.

figure 2:  The RMS reports associated with the 24MP imagery at 200 ft using 4 tie points

figure 4: Volumetrics in Pix4D

Processing Volumetrics in Pix4D

Conducting volumetrics within Pix4D, the software that of course processed all of the imagery to this point, is a very user-friendly and straight forward task.  Like digitizing a polygon any other GIS software, creating a volume measurement in Pix4D requires one to click around the edges of a given pile.  Once the polygon is complete, the user can add, adjust, or omit vertices that have created the plane that surround the pile.  Once the volume plane has been created, the user can 'update measurements' with a simple click, and list of values are produced pertaining to the specific volume object selected.  A new feature of the updated Pix4D is a 'Base Surface Setting' menu, which enables the user to adjust certain aspects that ultimately affect the measuring parameters of  a pile.  Figure 3, to the right, shows the pix4D interface that is shown when a surface is created. At the top of the image are the measurement figures, which provide geometric properties of the pile created, as well as projected error and number of vertices used in the pile.  Below that, is the Base Surface Settings menu.  Here the user can change certain aspects that mainly pertain to the elevation and plane created from the surface polygon which encircles the pile.  A very hand tool that comes from this menu is the 'Align with  with lowest point' which levels the plane of the polygon created to of the surface to the lowest point, which could be helpful when calculating volumes that are placed on non flat plane.  The default parameter for this selection of settings is 'Triangulated' which connects all the vertices and triangulates the volume based on relative heights above/below the base surface. At the very bottom of the surface interface is the 'Images' pane.  Here, a user can scroll through and see all the images that have surface either completely or partially in that images field of view. Using the scroll bar on the right, one can scroll up and down and select a desired image, and from there zoom in and out / pan left and right within that specific image.  Not pictured in this figure, but no less useful, is the ray cloud viewer which allows the user to scroll through and pan around the 3D map created from the initial processing. Overall, this interface is very well organized and designed in such a way that it using the software requires little, if any, instruction at all.  It is very intuitive and sleek in its design, especially with the features that were introduced with the new update.

Calculating Volumes in ESRI Software - Raster Mask

The first of the two methods that were employed using ESRI software involved clipping a raster around the specific pile profile in the DSM produced in Pix4D.  Within ArcMap the specific extension that was used to conduct this analyses was 3D analyst.  Similar to the process of creating the volumes in Pix4D, the first step was to create polygon clippings around the piles.  Unlike in Pix4D however, the polygon created in ESRI need not be as tight to the edges of the pile.  Once the three polygons have been created around the pile, the Extract by Mask Tool is used to cut out the area of the DSM contained within the polygons created prior.  What is left from this operation are three chunks of the previous complete raster that contain only the piles that of interest.  Once the piles have been isolated, the user needs to use the identify tool to measure get the lands mean sea level height around the piles.  This value will be needed for the process of calculating the volumes of the piles, which is the next step.  Also, it is important to note that this is why the the polygon created around the pile should be loosely around the pile, not directly up to the edges, so that there is room to identify the height of the ground surrounding the pile.

Now that clippings of the DSM have been created, the volumes can be calculated using the Surface Volume Tool. Unlike with the last tool, which produced raster features of the isolated piles, this tool will produce a table with the volume and various 2D and 3D characteristics of each pile. Using the identify tool, input the flat height of the area around the pile and int the Plane Height input of the tool.  Figure 5 below shows what the inputs of this tool look like for the Surface Volume Tool.

Figure 5: Surface Volume Tool

Calculating Volumes in ESRI Software - TIN 

The final method for calculating the volumes of the three piles was by creating a Triangulated Irregular Network (TIN) out of the clipped raster piles. A tin takes the points produced in the digital surface model and coagulates them with an algorithm that turns the point cloud into a grouping of triangle planes.  Creating a TIN(s) is done using the tool Raster to TIN Tool, and calculated using the Add Surface Information Tool. These tools, in coordination with one another, ultimately create a field within the original shape file that surrounds each individual pile that contains that volume produced by the data contained within the TIN.  Below in figure 6, is an example of the attribute  table of one of the piles once both these tools were ran.  The attribute table for each pile looks the same, with different values, since the same process that were used to create the values viewed in the figure were conducted on each pile.

Figure 6: Geometric attributes added after data was processed through Raster to Tin and Add Surface Information, Tools.

Now that all three processes have been completed, the results will be discussed and concluded upon with speculative comments on what potentially could have caused the discrepancies between the pile volumes for each pile that was produced using the different software packages and methods.

Results and Discussion

Below, in figure 7, is a table that shows pile sizes of each pile that was created using each of the three methods. As one can see, there is a large difference between each method.  Based off of these results, the most accurate measurements are likely those that were produced by Pix4D.  In fairness, the software package apart of the Pix4D is more tailored for applications like measuring volumes of piles produced on mining sights.  Establishing the plane from which to conduct the volume measurements allows for allot more precise/customizable input, where as in ESRI, you have to more or less establish the entire plane based off of identifying the cell values around the base of the pile, which allows for inconsistent and very much random results, to be produced.  This is likely why the values for ESRI Raster Volume method are sometimes higher and at other times lower than the value produced in Pix4d. Similarly, the TIN values produced are all higher than the ESRI Raster Volume and ESRI TIN Volume for the same reason, the input parameters available are much less inclusive than that of Pix4d.  The options available for establishing the measuring plane are either the max z, min z, or mean z, which is not very realistic, because in the real word these piles could be laying on on slants, or be connected to another pile of different material that will throw off the mean z value.

figure 7: Comparative volumes of each pile using the ESRI and Pix4D.

For reference of what these piles actually look like, below in figure 8 is a map that displays what these piles look like relative to one another and the surrounding landscape. Within the map are images of the piles as they are presented in both Pix4D and ESRI packages.

Figure 8: ESRI and Pix4D views of piles and subsequent recorded volumes. 

Conclusion 

This lab focus's on the using three different methods sing both Pix4D and ESRI products to do volumetrics for comparative results has shown a contrast how these two software packages compare in this specific application of volumetrics. Pix4D, as their superior results indicate, is designed with this being a corner stone application within its software.  ArcMap, on the other hand, has a much broader spectrum of applications within GIS and remote sensing application, and so a drop off in accuracy is somewhat expected.  The same conclusion also applies for working with the data prior too producing any sort of volumetric results.  With Pix4D, working with the data is seamless and very self explanatory as you can process and conduct the volumetric analyses all in the same interface.  With ESRI methods, the process of creating the volumetrics takes several more steps and produce the values.  Thus, in the regards of ease and efficiency, using UAS remote sensing data analyses to produce volumetric measurements, for the very purpose of it being all inclusive and highly accurate.  ArcMap does have Drone2Map software that allows one to process UAS data, but that is another coast for an already expensive platform.