Processing Pix4D imagery with GCPs

Introduction:
This project introduces the user to manually tying Ground Control Points (GCP's) to UAS imagery in Pix4D.  It is virtually the same as the previous 'Building maps with Pix4D' lab with the exception of manually adding ground control points.  The ground control points in this lab were physically places at the site of the imagery by the previous class before they took the flight data.

Methods:
The method to add pictures and process them is the same process as the 'Building maps with Pix4D' lab except for one step before the processing is actually started.  When the user gets to the main Pix4D screen, they want to go to Project - GCP/MTP Manager- and import their GCP coordinates from a text file.

 The next step is to run the initial processing.  After that is complete the it time to manually edit the GCP's to tie them to each individual image.  This can be done by clicking basic editor.  The user will then click on a GCP from the list and a list of images that are near that point will pop up.  The user will then click on a image and select the center of the point.  This is done for 5-10 GCP's in the data set and the program will automatically tie the rest down.

When the user finishes tying GCP's to images, it is time to reoptimize.  This ties the images down to the GCP's in the system.  If this is done correctly it is time to run the rest of the processing to get a point cloud and a DSM.

GCP's and Data Quality:
In the previous lab, the same imagery was processed without using GCP's.  This resulted in a good looking point cloud and DSM.  Pix4D automatically generates GCP's from the image data to tie the images to a location on earth.  To the untrained professional this may be enough.  Manually placing GCP's at a project site and taking their location with a highly accurate GPS allows for the user to input these into Pix4D and tie them to the imagery.  This creates maps that are highly accurate to the real world.  Highly accurate imagery and maps is highly valuable in many situations.

Maps:

The resulting map is the exact same as when processing it without adding GCP's.  The real benefit is with the spatial accuracy of the files.  This can be seen in the report that Pix4d creates when it finishes processing.

 No GCP

GCP's

The X variance when using GCP's is almost 7% higher than when done without GCP's.  This is saying that each point is within one meter of the actual location on earth.  The reason the Y variance is higher with the GCP's is because two flights were used and the data with two flights had more variance in the Y direction than the data with a single flight.  That is why it is lower than the first data set that was just one flight.

Conclusions:
In order to get highly accurate positional data from UAS data, GCP's must be incorporated especially if a local datum is not available.  If taking flights in a remote country or location GCP's may be the only option to tying the images to the earth.  The use of GCP's create more accurate data that is highly beneficial to the user in the long run.