lab_3
Understanding pix4D data processing & presentation of outputs
Introduction:
In previous lab, we have learned how to collect the data by UAS (Phantom 4 pro). Now, it's time to generate real outputs that people can actually see what is going on in those data. After we have done the mission without GCP(Ground Control Point), we moved on to pre-processing with pix4D software. This software will help us to generate useful outcomes such as orthomosaic, hillshade and DSM(Digital Surface Model) maps. These outcomes are significantly helpful to land analyst to interpret and identify potential needs of the land. In this lab, we are going to learn how does pix4D process the data and what are those outcomes representing.Methodology:
Make sure you have all the image data into your file, then you may open the Pix4D mapper software. As you can see Figure 1. , you can click new project to start your project. If you already have your saved project, you can import your project.
Figure 2. Importing images from previous mission flight.
You now have to import all your image data from previous lab. Count the number of images and make sure numbers are matched with mission parameters. You need to check your parameters and this should be coincidence with the information of your image file. In this case, we go for the WGS84, which is geographic coordinate system that based on world accuracy. Moreover, you need to select the camera model too.
After you sat all the parameters you need, you now need to choose which processing option you are going to apply. In this lab, because we are making 3D map and limited time for processing, we choose rapid 3D map option.
Figure 5. Start Processing Now
Before you actually start your project, make sure that leave the blank at "Start Processing Now" check button. This is because, we don't want our project to start processing automatically. In this lab, we demonstrate step by step.
Figure 6. is showing the map view of the mission flight. The parallel paths in the view represents the path of the Phantom 4 Pro while it was flying. The red points represents the spots that UAS has taken the photo imagery.
Make sure checking only initial processing and leave rest of option as blank. This is because, we want to see the first result of the processing that computes the key points on the image. After we have made the initial processing result, we now can proceed the rests of the options.
Figure 8. Summary of the quality report of initial processing .
Figure 8. is the quality report of your initial processing. It is showing you couple check points whether the processes had been done accurately or not. It is showing you warning sign at georeferencing check point. This is because, you have not applied the GCP(Ground Control Point).
After that, now you might have DSM and orthomosaic images ready. Therefore, to generate the map, you need to open the Arc Map and import the images you have made. The only thing that you need to do is, using your map design skill and make beautiful map.
Results:
Figure 9. Map of DSM Myrick Park, La Crosse, WI
Figure 10. Map of Hillshade Myrick Park, La Crosse, WI
Discussion:
1) Why is it necessary to have high amount of overlap in data collection using UAS for mapping application?
High amount of overlap in data collection using UAS for mapping application would be very crucial. This is because, the higher flight the less overlap required. The UAS we fly, has been legally limited by less than 400feet AGL(Above Ground Level). Therefore, to getting more accurate mosaic file, we need to have as many overlap as we can. However, make sure too much overlapping is not the best option because, it will be very time consuming. Additionally, because of not everyone can afford the expensive and good quality camera, overlapping can overcome this quality problem.
2) What is the difference between DSM and DTM?
DSM (Digital Surface Model) is the model that measures elevation on the surface. It accounts overall everything on the surface. For example, road, tree, building and every object that stands on the earth Figure9. DTM (Digital Terrain Model) is also elevation model but it doesn't count the tree, building and objects on the ground. Basically, it is naked earth.
3) What is the difference between TIN and DSM?
TIN (Triangulated Irregular Network) and DSM are coming from different type of data. TIN is a vetor file where as DSM is raster file. Because TIN is a vetor file, it is made of point, line and polygon. It triangulates a set of vertices to represent the elevation. DSM, as a raster file, it is created by each pixel that contains its own elevation value. TIN model is less likely to be used because, it is much more complex and more expensive to process.
4) What is the difference between a georeferenced mosaic and orthorectified mosaic?
Basically, the mainly difference would be georeference mosaic is based on the 2D feature whereas orthorectified mosaic is 3D. Georeference is more likely to define as flat surface whereas orthorectifed mosaic is surface with terrain. Therefore, georeferenced mosaic itself is less accurate than orthrectified mosaic.
5) What are the descriptive statistic of a DSM and how can we utilize this information?
like the word "Descriptive", descriptive statistic is statistical interpretation that tells overall view of the land. There would be mean, median,mode and IQR(Interquatile Range). Mean indicates the mean elevation based on the mean sea level. we are able to assume high mean means high elevation. The median value indicates steepness of the ground according to IQR. If the range between MIN or MAX and median, this tells us high steepness of the land.
High amount of overlap in data collection using UAS for mapping application would be very crucial. This is because, the higher flight the less overlap required. The UAS we fly, has been legally limited by less than 400feet AGL(Above Ground Level). Therefore, to getting more accurate mosaic file, we need to have as many overlap as we can. However, make sure too much overlapping is not the best option because, it will be very time consuming. Additionally, because of not everyone can afford the expensive and good quality camera, overlapping can overcome this quality problem.
2) What is the difference between DSM and DTM?
DSM (Digital Surface Model) is the model that measures elevation on the surface. It accounts overall everything on the surface. For example, road, tree, building and every object that stands on the earth Figure9. DTM (Digital Terrain Model) is also elevation model but it doesn't count the tree, building and objects on the ground. Basically, it is naked earth.
3) What is the difference between TIN and DSM?
TIN (Triangulated Irregular Network) and DSM are coming from different type of data. TIN is a vetor file where as DSM is raster file. Because TIN is a vetor file, it is made of point, line and polygon. It triangulates a set of vertices to represent the elevation. DSM, as a raster file, it is created by each pixel that contains its own elevation value. TIN model is less likely to be used because, it is much more complex and more expensive to process.
4) What is the difference between a georeferenced mosaic and orthorectified mosaic?
Basically, the mainly difference would be georeference mosaic is based on the 2D feature whereas orthorectified mosaic is 3D. Georeference is more likely to define as flat surface whereas orthorectifed mosaic is surface with terrain. Therefore, georeferenced mosaic itself is less accurate than orthrectified mosaic.
5) What are the descriptive statistic of a DSM and how can we utilize this information?
like the word "Descriptive", descriptive statistic is statistical interpretation that tells overall view of the land. There would be mean, median,mode and IQR(Interquatile Range). Mean indicates the mean elevation based on the mean sea level. we are able to assume high mean means high elevation. The median value indicates steepness of the ground according to IQR. If the range between MIN or MAX and median, this tells us high steepness of the land.
6) What are the spatial patterns visible in the DSM?
According to Figure 9, the gradation of the map represents the elevation. Therefore, dark color means high and light color means low. In the DSM Myrickpark, the dark to light is corresponding to south to north. This indicates elevation is decreasing toward the north.
7) What are the spatial pattern visible in orthomosaic?
According to Figure 11, objects on the surface are well distinguishable. The most striking factor on the map is the shadow. The shadows are facing toward the north. This means the son was located at the south.
8) What is a hillshade map and how can you produce one using the DSM?
Hillshade map (Figure 10.) is generated by the DSM which is raster format that contains the pixel value. You can convert to hillshade map from DSM by using Arc Map. You need to use "Hillshade" spatial analyst tool and set the parameters. In the parameter option, we need to adjust the altitude and azimuth which tells how high the sun is and where the sun is.
Conclusion:
learning how to process the data is very important as much as you fly the UAS. According to this lab, we have learned how to pre-process the data we acquired from the field. Using the well-made software is great advantage to analyst, however, we should know how to use it properly. Therefore, learning some more basic spatial terminologies is great benefit for you to better understand the how the software is actually working. For the next lab, we expect to have output with GCP and compare with output that we had in this lab.
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