Processing Pix4D Imagery

Introduction

Pix4D software is used for constructing point clouds that can be turned into orthomosaic images obtained from UAS platforms. This software has numerous applications including agriculture, mining, emergency response, and various others.

Part 1: Get familiar with the product

What is the overlap needed for Pix4D to process imagery?

The recommended overlap for most cases is at least 75% frontal overlap (with respect to the flight direction) and at least 60% side overlap (between flying tracks). It is recommended to take the images with a regular grid pattern shown in figure 1. The camera should be maintained as much possible at a constant height over the object.

Figure 1. Grid pattern of UAS platform taking aerial pictures

What if the user is flying over sand/snow, or uniform fields?

If there is complex geometry or large uniform areas within the overlapping images, it is much more difficult to extract common characteristic points. In order to get good results the overlap between images should be increased to at least 85% frontal overlap and at least 70% side overlap. Also increasing the altitude will provide less perspective distortion and better visual properties.

What is Rapid Check?

Rapid Check is a processing system where accuracy is traded for speed since it is mostly used in the field while obtaining data. It processes faster in an effort to quickly determine whether sufficient coverage was obtained, but the result has low accuracy.

Can Pix4D process multiple flights? What does the pilot need to maintain if so?

Pix4D is able to process images from multiple flights but in order to maintain that the pilot needs to make sure each 1) plan captures the images with enough overlap, 2) that enough overlap between two image acquisition plans (figure 2), and 3) that different plans are taken as much as possible under the same conditions such as weather, time of day, altitude etc.

Figure 2. UAS grid pattern route with enough overlap to process images

Can Pix4D process oblique images? What type of data do you need if so?

Pix4D can process oblique images but it will not create a complete orthomosaic. For this type of data you would need multiple flights with images taken at angles between 10 and 35 degrees. The images should have plenty of overlap in each data set but it is strongly recommended to use GCP’s or Manual Tie Points to properly adjust the images.

Are GCPs necessary for Pix4D? When are they highly recommended?

Ground Control Points are not necessary for Pix4D but the accuracy of the data is dramatically increased when they are used. GCP’s are highly recommended for high precision data and using oblique and nadir images together such as city reconstruction.

What is the quality report?

The quality report is the final report that is automatically displayed after you have processed your data. It gives information on the images, dataset, camera optimization, matching, georeferencing, and other information about the data.

Part 2: Using the Software

In class professor Hupy went through the steps necessary to create a Pix4D project. A PowerPoint was also used to help explain the steps taken throughout.

After the flights have been made and all the data is downloaded onto your computer, open up Pix4D and select Project>New project from the menu bar to open up a New Project window (figure 3). Within this window you must create a name using the date, site, platform, and altitude. Also select a location to save the project in.

Figure 3. New Project window

Click Next and continue with selecting all of the images you want to process and create the point cloud with. Simply select the images out of the folder and import them (figure 4). Then edit the camera model by selecting the “Edit” button under “Selected Camera Model.” Here a new window is displayed and changing the camera properties to Linear Rolling Shutter is done so by clicking “Edit” under “Camera Model Name” and changing the “Shutter Model” to Linear Rolling Shutter (figure 5).

Figure 4. Select the images for processing

Figure 5. Edit Camera Model window and changing Shutter Model to "Linear Rolling Shutter"

Select Next and continue to the Processing Options Template. As shown in figure 6, you can select what type of project you want the software to develop. Here I will be creating a 3D map from the images so the 3D Map option is selected.

Figure 6. Selecting a Processing Options Template, in this case 3D Maps

The last step before processing the data is selecting the desired coordinate system for the project to be displayed (figure 7). In this case the desired coordinate system was already selected so I went ahead and clicked finish to create the project.

Figure 7. Select a desired Output Coordinate System

After clicking Finish, a map view screen appears showing the overall layout of the flight. Right away go to processing steps 2 and 3 and uncheck those, then click on processing options in the lower left corner (figure 8). Press start and go practice on the flight simulator to buy some time because it does take a while to process.

Figure 8. In the lower left corner of the screen, uncheck steps 2 and 3 then click start

Once it is done processing a quality report will provide information on the quality of data accuracy and collection that has been taken (figure 9). The report looks good if all the steps are green and no red errors appear.

Figure 9. Quality Report table 

Next go to RayCloud and turn off cameras and turn on the triangle mesh to view the model (figure 10).

Figure 10. In rayCloud, uncheck the cameras and turn on the triangle mesh located in the contents area

In the dataset 68 of the 68 images (100%) were calibrated and all of the images were enabled. Figure 11 displays the number of overlapping images computed for each pixel of the orthomosaic. The red and yellow areas indicate low overlap with poor results and the green areas indicate an overlap of over 5 images with good quality.

Figure 11. Overlapping images with green having high overlaps and red with low overlaps

Next I measured the volume of one of the piles within the area of interest. First, I clicked on Volumes, then I digitized around the base of the pile and when the last vertex was put in, Right-click and finish the sketch. Then click compute and it gives data for Terrain 3D area, Cut Volume, Fill Volume, and Total Volume as shown in figure 12.

Figure 12. Area of Interest where volume was calculated

The last feature I used in Pix4D was the “fly through animation video.” In order to create this video, click on rayCloud, and go to New Video Animation that is located towards the top. From there I drew out how many timestamps I wanted to create and what direction the video focused on. After the video was created select a folder to save to and select Render. A video has been added below to show the 3D area of Litchfield.

Part 3: Maps

Map 1. Orthomosaic of Litchfield underlain by World Imagery Basemap and with the "No Data" removed

Map 2. DSM of Litchfield with the elevations of the site, red is higher elevations while green is lower elevations