Lab 11- GCP's

Overview:
    In this weeks lab we were introduced again to the software Pix4D, however this week we would use GCP's while using the software to tie down our pictures to create true orthomosaics. Pix4D can process projects with or without GCP's. GCP's make higher global accuracy of the project when processing the images. When adding GCP's, there are several different ways to go about this depending on: if the initial images are geolocated, the coordinate system of the original images, and the coordinate system of the GCP's. I will go into greater depth about these three different methods.
    The first method I want to discuss is used when the image geolocation and the GCP's have a known coordinate system that is already in the Pix4D database. Although they may be in different coordinate systems, the software is able to do a conversion between the two. This is the most common case, and it allows to mark the GCP's on the images with little manual intervention. Since this method does require manual intervention, it is not recommended for overnight processing because you do have to mark the GCP's in the images. This is the method we used in lab 11.

 figure 1

    The second method that can be used is when the initial images are without geolocation, the initial images are geolocate in a local coordinate system, or the GCP's are in a local coordinate system. Once again with this method you have to manually mark the GCP's on the images allowing for better picture clarification. Since it does require a manual step, it is advised not to let this process run overnight since only the first initial processing will be done up to the part of manually marking the GCP's. 

 figure 2

    The third and final method of adding GCP's works for any case, no matter the coordinate system of the images or GCP's but it does require more time to mark the GCP's on the images. After importing the images and GCP's the processing can be done without any intervention by the user. This is the best choice for over night processing. 

figure 3

    The next process of creating a true orthomosaic and dealing with your GCP's is choosing a coordinate system. When creating a new project, the select output coordinate system window is displayed. The output coordinate system does not need to be the same as the images or GCP's coordinate system, however it is recommended that the output coordinate system is the same as the GCP's coordinate system. When choosing a coordinate system the default for images is WGS84. It is easier to display your GCP's on your images when the coordinate systems match, allowing for better accuracy in your pictures. 

Methods:

    Now that you have a basic overview of the different methods of adding GCP's and choosing a proper coordinate system for your images, I will now talk about the processing that we did in this lab. There was two parts to this lab and creating images, one with GCP's and one without GCP's. I will first talk about the part of adding GCP's and manually tying them down to your images along with the quality reports associated with them. 

    The very first thing that I needed to do was create a new project in Pix4D. This allowed me to save my project into my folder so I could easily access the reports and images. I now needed to add the images that I wanted to use. We used 342 images from the flight mission over the south middle school pond and surrounding area. The camera we used was the Canon Power Show SX260, with a default coordinate system of WGS84. Like I talked about earlier, this is the default coordinate system that is in the camera, however we want to change it. Since we are looking at a relatively small area we want to use the North American Datum (NAD83) Zone 15 North. This singles the area out to a coordinate system that fits the Wisconsin area very well. After I added the images and changed to coordinate system I was ready to go on through to adding the GCP's. This part was tricky at first because there were a few steps I had to jump through but was able to accomplish the task of adding the GCP's. In figure 4 on the right hand side menu in the lower portion you can see 5 small boxes. I check the GCP box then right clicked on it and went into the GCP/ manual tie point manager. 

 figure 4

The new window that opened allowed me to import our GCP's that we took while in the field for the area of interest. One small change when importing the GCP's was that I had to change he 'coordinates order' box to Y,X,Z. I then was able to go into my flight folder and import our six GCP's from the mission flight. This showed exactly where my GCP's were located on the Pix4D window. (figure 5)

 figure 5

After having imported all my images and the GCP's, I was now ready to run the initial processing. On the lower bar in figure 5 you can see three boxes, while one of them is the initial processing box. I had to uncheck the other two boxes so only the initial processing would run. Then I was ready to hit start located just below that. The initial processing took close to 45 minutes to run, and also gave us a quality report. The quality report (figure 6) basically gives an overview of the project after the intial processing was run. It gives all kinds of information about the area covered, images, flight path, and importantly about the GCP's and their errors. 

figure 6

After I looked over the quality report, and the initial processing was completed, I was ready to go back into the GCP/ manual tie manager under the GCP box on the lower right hand side. With the new window popped up I then clicked on the rayCloud Editor button in the lower left. This now brought me to another window that had all my GCP's referenced in it. I was able to go in and manually adjust exactly where the center was on every GCP. The more images I did the more the software adjusted to putting my cursor closer to the center of the GCP each time. A safe number of times to adjust each GCP was around 10. I went a little wild with a few getting into the 20 times adjustment range, but this just lead to better accuracy of my tie points. After I was happy with the number of adjustments with my GCp's I clicked okay which brought me back to the map view screen. I checked box two: point cloud and mesh, and box three: DSM, orthomosaic and index boxes in the lower tab. I was now ready to finish running my project. Again after each step was completed it would give me another quality report, this time though it would include GCP information such as errors and locations. (figure 7,8)

 figure 7

 

figure 8

    When I was finished running the last two boxes, my final orthomosaic came through. (figure 9) The final orthomosaic showed all the GCP's on it as well as the flight path along with the exact tie points to each image. At first it came across as being very jumbled but that was just because all the lines showing the tie points. Once I unchecked the tie point box and had the triangle mesh created, it was easy to see exactly what I was looking at. 

figure 9

In figure 9 you can see exactly the area that we planned to have made into an orthomosaic. The triangle area with the pond in the middle has the GCP's on the path along the outside. You can faintly see some of them. You can also see our cars parked in the lower right hand corner of the image as well, with the first GCP just in front of them. After completing the orthomosaic and the GCP processing, along with looking over the quality reports, I wanted to create a flight animation of the orthomosaic to give it justice of exactly the area we are looking at. Figure 10 shows the flight animation. 

 figure 10

    After completing everything with this orthomosaic and GCP's, it was now time to continue on with the lab and create another orthomosaic. This orthomosaic was of the same area, but this time I didn't use any GCP's when creating it. The reason I didn't use any of the GCP's was to see just how off the images and flight path is without having the GCP's to tie the images down. One way to see if there is any GCP's was by looking at the quality report on the first page under 'Quality check' 'georeferencing'. This will tell you exactly if you have nay GCP's in your orthomosaic. (figure 11)

 figure 11

You can see that it says that it is georeferenced, but does not contain any GCP's. This is one of many ways to tell if you have GCP's in your orthomosaic. Also, since I did not contain GCP's in the ortho, the flight path is way off and doesn't follow our area that we wanted to take pictures off. It has it going way out to the west. This is because we did not use GCP's. The GCP's allows for those images from the flight path to be pulled back into the designated area of interest. (figure 12)

 figure 12

    The only reason we ran two different orthomosaics was to see exactly how much GCP's can contribute to the accuracy of the ortho image. After completing this lab I realize that it is vital almost always to use GCP's when conducting flight mission. This allows for better accuracy along with making sure your showing exactly the area you want. Without Pix4D we wouldn't be able to show the difference between the two orthoimages, as they appear very similiar, but with a quality report, Pix4D, and the flight mission, I can see why it is vital to use GCP's. Now we have used Pix4d twice in lab and still have only covered the basics with it, GCP's and creating orthomosaics and using the measurement tools. There is still lots to learn from this software as it can shed light onto stuff that we never thought imaginable.