Lab 4: An introduction to Pix4D

Overview:

This exercise focused on using Pix4D, a premier software used to process UAS data. The software can perform a wide variety of functions, however for this assignment only a few basic functions were used including volume analysis and video fly-through. 

Before any project can be started, UAS data from the field is needed. This includes orthorectified imagery and ground control points. The higher the quality of the data, the better the project will be. Additionally, Pix4D requires a certain level of overlap between images. The recommended values are 75% front overlap and 60% side overlap. If flying over snow, sand or uniform fields these values should be increased to 85% frontal overlap and 60% side overlap.It is also recommended that the camera platform is kept at a constant height as much as possible.

When in the field, the Pix4D rapid check processing system can be used. This processes images more rapidly to determine if sufficient coverage was obtained, but sacrifices accuracy to do so.

The Pix4D software can process data in a variety of ways. For example, data from multiple flights may be processed together provided that both datasets were collected from the same height, in similar weather conditions and have enough overlap. The software can also process oblique imagery, but that is beyond the scope of this exercise.

When processing data in Pix4D, ground control points are not required, but can help with georeferencing and the accuracy of the final product. 

The final feature users should be aware of is the quality report. This is a report generated by the software after processing data to give an overview of what happened, including errors in processing and processing parameters. 

Using Pix4D software

Two techniques that can be performed with Pix4D include volume measurement and video capture. The volume measurement tool can be found under the volume tab. Once a project is loaded, control points can be placed around whatever object is being measured. In this exercise, we measured the volume of a large gravel pit.

The second function, video capture, can be accessed from the ray cloud tab. This function allows the user to create a video where the camera moves through the project scene. To do this, the camera can be adjusted and waypoints are created along a route for the different camera views. The camera then moves through this route. An example may be seen below in figure one. 

 

Figure 1

Video capture from Pix4D

Creating a Map from Pix4D Software

The final section of this exercise involved making two maps in ArcMap using data obtained by Dr. Joseph Hupy from the University of Wisconsin-Eau Claire. ArcScene was also used in creating these maps. 

Figure 2
Map of Ortho-Mosaic created from UAS imagery

The first map created was of an Ortho-Mosaic that was generated by Dr. Hupy. The file was edited in ArcScene to eliminate the black background and uploaded into ArcMap. One thing that was noted was that the image quality changed after uploading the file into ArcMap. The original file may be seen below.

Figure 3
Ortho-Mosaic image prior to being uploaded to ArcMap

The second map created is of a digital surface model, or DSM of the mine site. A DSM shows ground surface features as well as elevation, which is often obtained from Lidar data. A hillshade was taken from the DSM using the hillshade tool. This was then made partially transparent and overlaid on the DSM to better show surface features. To display elevation, a blue-red color ramp was chosen with blue representing low elevations and red representing high elevations. One thing to note is that many areas of "high elevation" displayed on the map are actually just trees.

Figure 4
DSM map of mine site

Summary and Conclusion

Pix4D is capable of performing numerous functions, and this introduction exercise just scratched the surface. The main objective of this lab was to learn how to enter data into a Pix4D project and perform basic processing operations. These basic functions are very important to have a good grasp on since they will be used again more in depth in future UAS labs.