Activity 7 - Pix4D

Overview:

     In past labs georeferenced mosaics have been mentioned but until now they have never been a true orthomosaic. Fortunately, Pix4D a UAS mapping software packages makes creating these extremely easy, and accurately. Although easy to navigate this software demands a lot of computer resources to process data quickly and has some guidelines such as in general your images should have 85% frontal overlap and at the very least 70% side overlap to be safe. Some terrains are more difficult than others such as snow and sand, to achieve the best results at least 85% frontal and 70% side overlap but also try and get adjust the exposure settings. With the right exposure you will be able to get the most contrast from each image.  Depending on the specs of your computer and how many images you are processing determines the time it will take to finish. Luckily, Pix4D includes a feature that allows you to view a preview, they refer to this feature as Rapid Check. This can save countless hours incase your data is not good and you wait for it to process sometimes for 12+ hours at a time. Rapid Check works by reducing the resolution of all your images to 1 mega pixel. This gives you not only a preview but gives you an idea of the quality level of your images. Whether or not the Rapid Check succeeds or fails is an indicator if you should move on with the project or collect data again achieving more overlap or a different method. Pix4D is also capable of handling multiple flights at a time. To do so the pilot has to make sure there is enough overlap and conditions remain as close as possible. For example, weather, sun direction, altitude, etc all need to be considered when handling multiple flights. While using Pix4D you are not just limited to using images shot in nadir, oblique images can also be used but there should be images taken from the ground as well and if you are capturing flat ground, nadir is recommended. Depending on your project you may want to use GCPs although these are generally optional. GCPs can be used to improve accuracy and is important to use if your sensor does not geolocate. During the various steps of processing a quality report is written. This gives the user an idea regarding the level of quality their project and if there are errors, what needs to be done in order to fix those errors.

Walkthrough:

     Although very user friendly Pix4D is a very powerful program with many uses. For this lab a very basic project was processed in order to introduce us to Pix4D and get us accustomed to the basic features. For this lab all of the images used were shot in nadir and it is important to note we did not use GCPs.

    When starting the program and a new project Pix4D prompts you to add images to begin processing. (Figure 1) Make sure to add all over your images in order to get a complete product.

Figure 1 - Adding Images to Pix4D

For this lab I processed two sets of images separately both featuring the Eau Claire Soccer Complex Pavilion shown in past labs.

     Once the images are added a screen will popup showing you the image properties. (Figure 2) Listed here are the coordinate system, if your images were geolocated or not and the camera/sensor used to take thos images. It is important to note, some sensors do not geolocate images and it is advised to supplement that through the use of GCPs or a georecording device that tracks points where images were taken.

Figure 2 - Image Properties Menu

     From there you are prompted with templates, this helps determine what direction you want to take the project. For this lab we went with the default 3D maps as you can see in (Figure 3)

Figure 3 - Project Template Selection

 

     Next, the processing begins, at this point its start and wait. Depending on the level of quality and number of images you have determines how long it will take for the entire process. The better of computer you have the less time it will take to process. Along the way, frequent process reports will pop up giving you quality updates and even previews of your final image/model.

     The quality report includes many useful things as mentioned earlier. One example is it shows you how many of your images were used in creating the finished product (Figure 4) and where your key areas of overlap were and were not. (Figure 5) In the case of the SX260 all of my images were used in creating the image.

Figure 4 - Beginning of Quality Report

Figure 5 - Overlap details

It makes sense my weakest areas of overlap were the outside fringes on the mission area. These areas were not the main focus of the mission therefor less images were meant to be taken.
     The first data I ran was from the Sony SX260, it was 32 images at 14 megapixels each. The SX260 geolocates images so I did not have to worry about that.  They were shot at 70m and in all it took around 30 minutes from start to finish to process. The other set of images I processed were taken by the GEMs sensor. There were 220 non-geolocated images so GPS data had to be added to maintain accuracy. Although there were many more images taken the GEMs only takes pictures at 1.3 mega pixels. This took slightly longer to process at around 40 minutes. The process for both the SX260 and the GEMs was nearly identical and consequently produced very similar results.

Post - Processed Data

     Once finished processing the data can be manipulated in a number of ways and almost "explored" much a like a virtual tour of the study area. A few of the basic features of Pix4D include, measuring features, find surface areas of objects and also some volumetrics. Because we are just getting introduced to Pix4D I used just those three tools. I measured the width of the sidewalk. (Figure 6), found the surface area of one of the soccer fields (Figure 7) and also the volume of the pavilion (Figure 8).

Figure 6 - Width of Sidewalk

Figure 7 - Surface Area of Soccer Field

Figure 8 - Volume of Pavilion

 You can also record an animation that "flys" you through the project. This is a relatively rough model given the fact we just used nadir images but gives a viewer a sense of the study area and can be made quickly. (Figure 9).

Figure 9 - Video Flyby Tour of GEMs 3D Model

With the processed data you are not restricted to just using the data in Pix4D. It can also be used and imported in to ArcMap. Here you can add metadata to your images, an important step in to ensuring data integrity. You can also create maps using the orthomosaic images  you created. (Figure 10, 11, 12)

Figure 10 - SX260 Orthomosaic made in ArcMap

Figure 11 - GEMs Orthomosaic made in ArcMap

Figure 12 - GEMs Orthomsaic with surface overlays from earlier calculations

Critique:
     Like Pix4D says, the program is very user friendly, accurate and efficient. With that being said what we did in this lab was barely get our feet wet. We did not do anything with GCPs or oblique imagery. With what we did use (nadir images) I was very impressed with the results and can begin to see the possibilities this software holds. This software was much easier to use than manipulating other forms of imagery such as Lidar and I can see why most believe this is the next "big thing" in 3d imaging. I look forward to exploring the software package more in depth and looking in to what others have done with it.