Introduction
Back in November, the Geospatial Field Methods class went to the Litchfield Mine in Eau Claire, Wisconsin to collected GCPs and observe various UAS Platforms launch and take imagery of the mine. Now the data collected from that day will be processed in Pix4D, the most current and easy-to-use software for constructing point clouds. Basic operations are easy to learn in Pix4D, but the software offers much more advanced processing options. The Pix4D Manual gives the user the proper knowledge to prepare projects suitable for the software to render accurate output and how to process the data in the software. Certain questions were asked in order to better understand Pix4D and how it works.
o What is the overlap needed for Pix4D to process imagery?
High overlap is recommended by Pix4D to process imagery - at least 75% frontal overlap and 60% side overlap (Figure 1). In addition, a grid pattern and uniform camera height above the terrain is recommended to ensure accuracy. The high percentage of overlap will allow the software to find thousands of matched key points, which are characteristic points that are found within two images that will generate a 3D point. Therefore, more matched key points will create a more accurate 3D image.
High overlap is recommended by Pix4D to process imagery - at least 75% frontal overlap and 60% side overlap (Figure 1). In addition, a grid pattern and uniform camera height above the terrain is recommended to ensure accuracy. The high percentage of overlap will allow the software to find thousands of matched key points, which are characteristic points that are found within two images that will generate a 3D point. Therefore, more matched key points will create a more accurate 3D image.
Figure 1: Ideal Image Acquisition Plan that will result in the recommended overlap percentages.
o What if the user is flying over sand/snow, or uniform fields?
The general case previously mentioned won't generate accurate results in Pix4D if the terrain in sand, snow, or uniform in general. Such fields have little visual content, making the collection of key points difficult for the software. The frontal and the side overlap should be increased to 85% and 70% respectively. Adjusting the camera exposure settings to increase the contrast of the field will making mapping such difficult fields easier.
o What is Rapid Check?
Rapid Check is a program in Pix4D that checks the requirements for images quickly, sacrificing accuracy by reducing the resolution of the image. It's used preferably in the field.
o Can Pix4D process multiple flights? What does the pilot need to maintain if so?
Pix4D can process multiple flights, but the pilot needs to have a proper flight plan if the software is too process them correctly. The different flights need to have enough overlap between them along with the recommended overlap within the individual flight plans (Figure 2). The flight plans also need to be taken in relatively similar conditions. The flight height should be very similar too, as different flight heights will lead to different spatial resolutions.
o Can Pix4D process oblique images? What type of data do you need if so?
Pix4D can process oblique images, in fact, it's often used for building reconstruction (Figure 3). It's recommended to fly around a building at a 45 degree angle and then increase the platform's height while decreasing the angle of the camera for a second flight, taking images every 5-10 degrees.
o Are GCPs necessary for Pix4D? When are they highly recommended?
GCPs are not necessary for Pix4D but are highly recommended as they will increase the accuracy of the project. They strongly recommended if a project has no geolocation for its images. The final results of a project that has no GCPs or geolocation will have no scale, orientation, or absolute position information and measurements, overlay, and comparisons will be impossible. They may produce an inverted 3D model in the rayCloud. The 3D reconstruction may not preserve the shape of the surveyed area. When using GCPs, 5 is the minimum recommended amount for a project (Figure 4).
o What is the quality report?
A quality report is automatically displayed after initial processing which checks the quality of the data. It's the user's responsibility to verify that the data is accurate and of good quality.
The general case previously mentioned won't generate accurate results in Pix4D if the terrain in sand, snow, or uniform in general. Such fields have little visual content, making the collection of key points difficult for the software. The frontal and the side overlap should be increased to 85% and 70% respectively. Adjusting the camera exposure settings to increase the contrast of the field will making mapping such difficult fields easier.
o What is Rapid Check?
Rapid Check is a program in Pix4D that checks the requirements for images quickly, sacrificing accuracy by reducing the resolution of the image. It's used preferably in the field.
o Can Pix4D process multiple flights? What does the pilot need to maintain if so?
Pix4D can process multiple flights, but the pilot needs to have a proper flight plan if the software is too process them correctly. The different flights need to have enough overlap between them along with the recommended overlap within the individual flight plans (Figure 2). The flight plans also need to be taken in relatively similar conditions. The flight height should be very similar too, as different flight heights will lead to different spatial resolutions.
Figure 2: Proper and improper multiple flight plans.
o Can Pix4D process oblique images? What type of data do you need if so?
Pix4D can process oblique images, in fact, it's often used for building reconstruction (Figure 3). It's recommended to fly around a building at a 45 degree angle and then increase the platform's height while decreasing the angle of the camera for a second flight, taking images every 5-10 degrees.
Figure 3: Ideal flight plane for building reconstruction, utilizing oblique images.
o Are GCPs necessary for Pix4D? When are they highly recommended?
GCPs are not necessary for Pix4D but are highly recommended as they will increase the accuracy of the project. They strongly recommended if a project has no geolocation for its images. The final results of a project that has no GCPs or geolocation will have no scale, orientation, or absolute position information and measurements, overlay, and comparisons will be impossible. They may produce an inverted 3D model in the rayCloud. The 3D reconstruction may not preserve the shape of the surveyed area. When using GCPs, 5 is the minimum recommended amount for a project (Figure 4).
Figure 4: Recommended distribution of GCP placement.
o What is the quality report?
A quality report is automatically displayed after initial processing which checks the quality of the data. It's the user's responsibility to verify that the data is accurate and of good quality.
Methods
When starting a project in Pix4D, select New Project, which the software offers when it's opened (Figure 5).
Figure 5: The first page displayed in Pix4D where a user can start a new project.
The user is then prompted to name the project (Figure 6). It's recommended to include information on the date, site, platform, sensor, and altitude in the project name. Save the project to an appropriate folder.
Figure 6: Name the project that represents it's specific characteristics.
Next, the user needs to select the images that will by processed by Pix4D (Figure 7). The user can add pictures individually or by directory. If the flight images have been cleared of any useless photos, like images taken during takeoff, uploading images by directory is much easier.
Figure 7: Pix4D page where the images are uploaded.
Once the images are added, the software reviews the exif file to read the image's metadata. It will note if the images are geotagged and the coordinate system, though most UAS platforms default to WGS 1984 in Decimal Degrees. The camera information it provided are often not accurate and need to be edited. Open the Camera Editor window and click edit under the Camera Model Name (Figure 8). Change the Shutter Model from a global shutter to a linear rolling shutter, which is what a Phantom 4 Pro has.
Figure 8: Camera Model Editor window where the shutter model needs to be changed for Phantom 4 Pro.
Next, leave the Output Coordinate System as its default (Figure 9). It can be reprojected, if need be, in GIS software.
Figure 9: Pix4D page where the user can change the output coordinate system.
In the Processing Options Template, chose 3D Maps, which will generate a DSM and an orthomosaic (Figure 10). The Processing Template come with preset processing parameters and outputs, but 3D Maps is the most commonly used template.
Figure 10: Processing Potions Template page where the processing parameters and output is selected.
Afterwards, the flight plan will be shown in map view. Uncheck steps 2 and 3 and then process the data. The Processing Options window will open where a Full, Rapid, or Custom Keypoint Image Scale can be selected. In the DSM, Orthomosaic, and Index tab, chose triangulation for a raster DSM method. Select start to officially start the initial processing. A quality report will display in the screen when Step 1 is finished (Figure 11). It's important to thoroughly review the quality report to assess the quality and accuracy of the data. Some of the problems can be addressed before processing the data completely. According to this the quality report in Figure 11, the median number of key points per image is acceptable, but only 197 out of 222 images were processed (88%). Most, if not, all of the images should be processed. This is probably do to complex data in the images from the trees. This can be prevented by increasing the overlap to above 80% and flying at a higher altitude. The percentage of difference between initial and optimized focal length is between 5% and 20%. It should be less than 5%. The median number of matched key points calibrated per image is acceptable. The report also notes that no GCPs were added to the data.
Figure 11: First page in the quality report.
The quality report provides an examination of many aspects of the data, including the initial image positions, which resembles the flight plan of the Phantom 4 Pro (Figure 12).
Figure 12: Figure given by the quality report of the Initial Image Positions.
Image overlap is critical to the accuracy of the data. The quality report generates a figure of the number of overlapping images per pixel for the orthomosaic (Figure 13). Five images per pixel is the recommended number. Pixels represented in red and yellow represent pixels with low overall and poor results will be generated for these pixels. The gap in the AOI is evident, indicated earlier when the report noted that only 88% of the images were processed. The area omitted covered trees, difficult terrain for a general flight plan in Pix4D. Poor overlap seems to be concentrated to the edges of the AOI, probably from not having images surrounding those pixels. The omitted area also is surrounded by poor overlap since a good portion of nearby images and their associated overlap where cut from the processing.
Figure 13: The quality report figure showing the image overlap for the orthomosaic pixels.
The quality report also provides a preview of the DSM and Orthomosaic that the final processing operations will generate (Figure 13).
Figure 13: A preview of the DSM and Orthomosaic that Pix4D will generate.
After reviewing the quality report, uncheck the initial processing option and check Steps 2 and 3 to finish processing. All the steps will turn green when processing is complete. The data can be viewed in rayCloud or the camera can be turned off and the triangle mesh turned on to view the model.
Results
The orthomosaic of the data allows the viewer to identify certain features in the mine - different rock piles, location of machinery, and roads (Figure 14). The gap in the data in right in the center of a cluster of trees, which had a higher probability to cause issues in the processing. The Hillshade tool was used to give the rock piles more relief, giving an idea of their height relative to other features (Figure 15). However, since GCPs were not used, the elevation of features are highly inaccurate and the entire study area is potentially off by 30 meters, illustrated by the misalignment of the large water body in the DSM and Hillshade (Figure 16 and 17). Figure 16 and 17 also illustrated the processing errors in the forested areas south of the mine. Another issue with the data is that it was processed as if the images were taken using an ellipsoid height but it was really using a geoid height.
Figure 14: Orthomosaic map of Litchfield Mine.
Figure 15: Transparent orthomosaic with Hillshade effect.
Figure 16: DSM map of Litchfield Mine.
Figure 17: Hillshade of Litchfield Mine.
Conclusion
Pix4D proved to be an easy to understand and easy to learn software for generating 3D images that can be used to locate features and measure distances, elevation, and volume. Concerning the mine, knowing the volume of their materials would be one of their main goals when processing this data (though, not THIS data, because it's bad data). The software made it clear that developing a proper flight plan for its purposes and knowing the parameters of the platform and sensor are critical steps for generating accurate data. Next time, GCPs will be brought into the equation and will hopefully fix the problems with this data.
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