Lab 10- Construction of a point cloud data set, true orthomosaic, and digital surface model using Pix4D software.
Introduction:
Pix4D is powerful mapping program capable of converting images taken by drone to highly precise georeferenced maps, mosaics and 3D models. This program is unique compared to other processing software (such as the GEMS software) used in previous labs because it has the power to orthorectify data using digital elevation models. Pix4D also comes equipped with editing tools such as the rayCloud Editor, which combines the 3D point cloud and images to allow the user to assess the quality of their results (Photo 1). Other tools such as the Mosaic Editor and the Index Calculator allows the user to improve the quality orthomosaics and generate NDVI vegetation maps, respectively.
Other aspects that separate Pix4D from other software packages is the its ability to process multiple flights and oblique images. However, it is important to note that if the pilot is intending to process data from multiple flights, that he/she must ensure enough overlap between images, and that the two flights are conducted under as similar conditions as possible (sun direction, weather, no new objects, etc.). Similarly, when constructing an image acquisition plan for reconstruction 3D buildings, it is important for the pilot to take one image every 5-10 degrees to ensure enough overlap for processing in Pix4D. Additionally, it is important to note that Pix4dmapper only generates a point cloud for oblique images, and does not produce an orthomosaic.
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| Photo 1: Showing the densified point cloud from the RGB GEMS data set. Green dots indicated where the drone is, and blue dots indicate where the drone should be at the time of the image capture. Red dots indicate data that was discarded. |
Other aspects that separate Pix4D from other software packages is the its ability to process multiple flights and oblique images. However, it is important to note that if the pilot is intending to process data from multiple flights, that he/she must ensure enough overlap between images, and that the two flights are conducted under as similar conditions as possible (sun direction, weather, no new objects, etc.). Similarly, when constructing an image acquisition plan for reconstruction 3D buildings, it is important for the pilot to take one image every 5-10 degrees to ensure enough overlap for processing in Pix4D. Additionally, it is important to note that Pix4dmapper only generates a point cloud for oblique images, and does not produce an orthomosaic.
Study Area:
Data were obtained at the Eau Claire Soccer fields southeast of the intersection of Craig Rd and Hamilton Avenue in Eau Claire Wisconsin on September 23rd, 2015. Weather conditions during data collection were Mostly Cloudy-Overcast,72°F, and the wind speed 5-8 mph from the southeast.
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| Photo 2: Base map of study area obtained from ArcMap GIS 10.3.1 |
Methods:
Pix4D integrates an extremely user friendly interface and performs most functions at a click of a button. For this Lab, photos obtained from the SX260 camera were uploaded to the software using the new project wizard on the welcome page (Photo 3). Because this specific camera contains a GPS unit, the geographic locations of each photo are stored with the image and the program is able to automatically detect the coordinate system and camera information. Therefore, no additional settings were altered in the "new project" wizard. Comparatively, the GEMS camera stores the geographic coordinate information separately from the image data as a text file (Photo 4). However, before loading the txt file, it is important for the user to input the camera specs first (Photo 5). After the camera information is loaded, load the txt file found within the original data collection folder so that the images may be georeferenced.
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| Photo 3: Displaying the uploaded photos in the new project wizard. |
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| Photo 4: Txt. file showing the GEMS images and their associated Latitude and Longitudes |
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| Photo 5: Displaying the edit camera menu within the new project wizard. To add information for the GEMS camera, RGB was selected under the "Bands" drop down bar, focal length was set as 7.7, and the sensor width was set at 4.8. |
After all necessary information is entered, it is crucial to double check that the latitude and longitude of the images are in the ball park of what is expected. If all information is accurate, finish the new project wizard and run the data analysis of the images to create a digital surface model, point cloud data set, and orthomosaic.
Pix4D generates a data quality report after analysis to provide the user with information regarding the number photos calibrated and georeferenced, the amount of overlap, and geolocation error. Links to the quality reports from this lab are posted directly below:
SX260 Quality Report (Link)
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| Photo 6: Calculating volume of shelter from SX260 camera data set. Edges of the shelter are highly pixelated, making it difficult to capture the full shelter volume. |
GEMS Quality Report (Link)
Pix4D processed 215 of the 220 images for the GEMS data and geolocated 219 of the 220. As represented under the computed Image/GCPs/Manual Tie Points Positions heading, the only images discarded were those taken directly after the roto-copter took off from the ground. The remaining photos were properly processed, and had a high average number of overlapping images (5+). The quality report shows that areas that only have three overlapping images or less are located predominately at the edges of the map area, as expected. Overall, the quality of this data set is extremely good. Video 2 shows the GEMS data as it is represented in Pix4D
Post-processing geometric calcuations
Calculating length of polylines, areas, and volumes using Pix4D is relatively simple due to the tools integrated into the software. After drawing the line or polygon in the rayCloud editing tab of Pix4D, the software will automatically determine the length, area, or volume (Photo 7).
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| Photo 7: Displaying the surface area calculation of one of the soccer fields at the Eau Claire Soccer park. |
Results/Discussion:
The data obtained from this lab may be utilized/analyzed using a number of different methods. One objective of this lab was to learn how to calculate distances, surface areas, and volumes of aerial images. The calculated lengths, areas, and volumes of the objects measured (The shelter as shown in photo 6 and the soccer field in photo 7) are represented in table 1 and represented visually on the maps in photo 8.
| GEMS | |
|---|---|
| Bike Rack Polyline (length): | 2.77m |
| Soccer Field Area: | 327.29m2 |
| Shelter Volume: | 6.48m3 |
| SX260 | |
| Sidewalk Square Polyline (length): | 2.64m |
| Soccer Field Area: | 323.74m2 |
| Shelter Volume: | 8.13m3 |
Table 1: Lists the measured objects and their associated calculated
length, area or volume. The bike racks were not shown in the SX260
data within Pix4D, so the length of one sidewalk square was
measured instead.
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Photo 8: Comparing final maps between the GEMS and SX260 Cameras.
All images taken on September 23rd, 2015 at the Eau Claire Soccer Fields
with a GEMS and SX260 Camera for the purpose of learning how to capture
aerial field data and process it using Pix4D software. Cartographer Nick Matula
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The results from table 1 show that there are some differences between calculations. The soccer field areas differed by approximately 3.5 meters squared, and the volume of the shelter differed by 1.5 meters cubed. These differences may be accounted for by the differences in data quality. The GEMS data set had a substantially larger number of images, which allowed Pix4D to more accurately line up features within each image. It is important to note that the DSM images are also very different from each other. In the SX260 DSM, the main pavilion is not shown as having the highest elevation, which further indicates to a weaker quality of data. As a general rule, the recommended overlap for most cases is at least 75% frontal overlap (with respect to the flight direction) and at least 60% side overlap. However, when the study area is a uniform field or covered by snow, 85% frontal overlap and 70% side overlap is recommended. It is suggested therefore, that in the future that images are taken more frequently for fuller coverage. Although more data will result in longer processing times, it is necessary in order to obtain reliable data from volume calculations.
One aspect of the Pix4D that raises concern is that it is almost too intuitive. As a result, an unsuspecting user that doesn't know what is going on can easily end up with inaccurate results. For example, when georeferencing the GEMS images it was important to look at the order that the latitude and longitude were recorded and stored in the .txt file. Instead of listing the latitude first and the longitude second, the order of the values are switched. It is easy to assume that the program will correct the problem itself, but in reality it is necessary to tell the program the data from the .txt file is listed in the reverse order so that the images are referenced properly. Without having a general idea what the latitude and longitude should be in a target area, an unsuspecting user could easily georeference the images backwards and result in improperly located data.
The applications for aerial photography volumetrics is enormous in the mining industry. Specifically in sand mining, the size of storage piles are regulated by the DNR. Currently, surface area is being calculated using time consuming traditional survey techniques. With aerial photography and processing software such as Pix4D, storage pile volumes and surface areas could not only be calculated more efficiently, but also then used to show growth/decline of product reserves over time. Furthermore, aerial technology could also be used to assist reclamation by calculating excavated volumes and the area of lands environmentally disturbed by mining processes.
Conclusion:
Pix4D is a dynamic program of producing orthomosaics and calculating distances, surface areas, and volumes of objects. One strength of Pix4D is that it is extremely user friendly, and simplifies the process of georeferencing and inputting camera specifications if it is not stored with the images. If processing images without geolocation, ground control points should be used so that scale, orientation, and absolute position may obtained in the Pix4D software. Without geolocation, the data is unable to be used for mining, agriculture, and other applications.The data from the GEMS data averaged an image overlap of five images while the SX260 data averaged an average of three images. The lack of overlap in the SX260 data effected the ability of Pix4D to calculate geometric accurately, and demonstrates the need to maintain at least 75% overlap of images. When high quality data is combined with an expansive analyst tool set, Pix4D becomes a powerful tool to process aerial photography.
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