Activity 12: Pix4D Mapper

Introduction

In order for the Pix4Dmapper software to process imagery it needs matched keypoints. Matched keypoints are two points from two different images that are in the same location. The more matched keypoints there are, the more accurately 3D points can be computed, so a high level of overlap is required for an accurate output. Recommended overlap is 75% frontal and 60% side. For area with little visual content; such as snow and sand, frontal overlap should be at least 85% and side overlap should be at least 70%. The exposure should also be set to get as much contrast as possible. Rapid check is a processing image that can be used to quickly produce low-resolution images to assess quality and completeness of the image while still onsite. Pix4Dmapper can process multiple flights however the pilot needs to maintain a similar altitude for the flights.   Oblique images can be processed, this data will need to be collected with the camera at a 45^o angle and succeeding images should be higher with a decreased angle. To combine oblique imagery with nadir images it is highly recommended to have GCPs and/or tie points, however GCPs are not necessary for Pix4D. After each step of processing a quality report will be displayed. The quality report is very a comprehensive report of how the process went, it includes a quality check, a preview of the images produced, number of images overlapped throughout the output, and much more.  This lab is designed to introduce these basic functions of the program using an image taken of the Litchfield mine, southwest of Eau Claire (figure 1). 

Figure 1. The study area

Methods

For this lab Pix4Dmapper Pro was used to create a 3D image of the Litchfield mine from a series of images collected using a Phantom UAV; to do this a new project was started in Pix4D and the images from the flight uploaded into the project. A quality report was automatically generated after the process was finished. The image was then used to experiment with a few of the various tools available in Pix4D mapper. A line measurement, a surface area, a volume calculation, and a video tour of the study area were all made. The image was also brought into Arc Scene to create another 3D rendering of the imagery.

Results/Conclusions

Pix4D mapper is a relatively user friendly program. The import and manipulation of imagery is rather straightforward, however the import process does provide a lot of information in the quality report that is a bit overwhelming. Measurements taken provided a reference of the overall size of the mine. Figure 2 displays the location of the measurements taken; the line length is 19.5 m, the surface area measured is 66.23 m², and the volume measured is 32.93 m ± 1.44 m³. Figure 3 is the 3D rendering of the study area made in Arc Scene, set with the image itself as the floating surface to improve the appearance. Finally a video was made to give a virtual tour of the image produced in PiX4D, attached below. 

Figure 2. imagery with location of measurements taken

Figure 3. Arc Scene rendering of imagery