Assignment 3: Processing Imagery with Pix4D

Introduction

The goal of this assignment was, as the title implies, to get familiar with processing imagery in the Pix4D. Before this was done, however, it was important to understand a little bit about the software and what requirements the software needed to process imagery from a UAS flight.

One of the most important aspects of processing quality imagery in Pix4D is the amount of overlap between the images. According to the Pix4D user guide, the recommended overlap for image processing is 75 percent frontal overlap (distance between pictures taken in flight path) and 60 percent side overlap (distance between photos taken along flight columns).

Figure 1: Sample flight plan that ensures enough overlap

When it comes to snow, sand, and other flat or uniform surfaces, the amount of overlap required increases to at least 85% frontal overlap and 70% side overlap. Once enough overlap is established, initial processing is the next step. Initial processing is a way for the user to ensure that the images will process correctly and accurately. Rapid check is an alternative form of initial processing. This option doesn’t produce as good of a quality initial image as the standard initial process, but can process imagery much faster and is used to ensure that the images have enough overlap to fully process later.

In the instance of this assignment, as well as other projects, Pix4D does have the capability to process multiple flights at once. This actually enhances the accuracy as long as the pilot ensures that there is enough overlap between the two flights and enough overlap within each flight plan, that the conditions are relatively the same, and that the altitude of each flight doesn’t vary too much.

As far as camera orientation goes, if the pilot captures oblique images, Pix4D is able to process them. The Pix4D user guide recommends conducting multiple flights at different altitudes to ensure accuracy. The user would need the distance covered on the ground per image and its direction, the image width, and the desired ground sampling distance.

Figure 2: Flight plan for best processing oblique imagery

Geographic Coordinate Systems are not required for processing imagery in Pix4D, because the images taken from the UAS have coordinates in their metadata. It is recommended that the user inputs a coordinate system when processing imagery due to the fact that this further enhances the accuracy of the image when placing it into a spatial reference.

After the imagery has been processed (either after initial processing or after fully processing), Pix4D will provide the user with a quality report. This report contains information that allows the user to determine the quality and the accuracy of the image processing. This will help the user in determining whether the output images are good enough to use or not.

Methods

In this assignment, images were taken from two separate flights completed at the same site, the Litchfield mine in Eau Claire, WI. The first step was to add the images from both flights into the Pix4D image processing wizard.

Figure 3: Selecting images from both flights

Once the images were uploaded to the wizard, the image properties were examined. In this stage, the camera shutter model was set to "global shutter or fast readout" by default. However, for this assignment, the shutter model needed to be set to "linear rolling shutter".

Figure 4: Editing the camera shutter model to linear rolling shutter

Next, initial processing was ran on its own, before the "Point Cloud and Mesh" and "DSM, Orthomosaic, and Index" processing was done. The reason for that was the initial quality report could find any errors before the second and third steps were ran. This could save the user a lot of time and help ensure that the images were processed accurately.

Figure 5: Initial processing settings

Once the wizard was completed and the settings for processing were established, the initial processing could begin. Upon completion, Pix4D generated the quality report. This report notifies the user of any errors with the imagery and can help the user determine whether or not to follow through with completing the image processing.

Figure 6: Quality report summary

Figure 6 shows the summary portion of the quality report after initial processing was complete. This summary gave details about the file name of the project, date processed, camera model, ground sampling distance, area covered, and duration of the initial processing.

Figure 7: Quality check and preview

Figure 7 shows the quality check and preview portions of the initial processing. 100 percent or all 155 images were deemed usable. A green circle with a check mark in the quality check portion indicates that the parameters for optimal processing quality have been met. The yellow triangle with an exclamation point indicates that there may be a problem with the resolution or quality of the image processing. In this case the precautionary symbols were overlooked as the camera optimization was barely over the minimum optimal limit (<5%) and the georeferencing wasn't vital to the integrity of the image processing (see Introduction > GCS section).

Figure 8: Overlapping images computed for the orthomosaic. 

Figure 8 shows that red areas can result in poor quality processing of those areas and green areas are deemed sufficient for quality processing by having five or more overlapping images. The more overlap the better. 

Figure 9: Map view showing where the images for both flights were taken.  

Figure 9 shows what Pix4D calls a "map view" of the site. This displays the flight path and photograph locations of the flight. These points were also used in the "ray cloud" which is a 3-D interactive model created upon processing.

After fully processing the imagery, a digital surface model (DSM), an orthomosaic rendering, a 3-D interactive model, and a "fly" with me animation were created in Pix4D. The DSM and orthomosaic were used to create maps in ArcScene and ArcMap.

Results

Figure 10: Screen grab of DSM Vertical Exaggeration 

Figure 11: Screen grab of orthomosaic vertical exaggeration

Figure 12: DSM Hillshade

Figure 13: Orthomosaic Hillshade

Figure 14: "Fly" with me animation

Looking at the results, Pix4D truly is an amazing software and can produce incredibly high-quality imagery. A few areas of relatively poor-quality were noticeable near the southwestern and northeastern parts of the renderings. This was most-likely due to minimal overlap in those areas. The DSM turned out great and did a good job of showing the relief of the site. The orthomosaic turned out alright though as well, minus a few problematic areas, and did a good job of showing the surface. The "fly" with me animation didn't turn out as great as expected and, due to the size-limit for uploading videos on Blogger, a lower quality rendering was uploaded instead of a longer and slower flight that would better allow the viewer to overlook the site. The animation is really something though and testifies to the power and capability of Pix 4D.