Processing Pix4D Imagery with GCPs

Introduction:

The purpose of this activity was to compare the accuracy between processed UAS data imagery from Pix4D that utilized Ground Control Points (GCPs) contrasted to the imagery that didn't from the week previous.  A GCP is, according to Pix4D, "a characteristic point whose coordinates are known.  GCPs are used to georeference a project and reduce the noise." Pix4D recommends users to incorporate GCPs, as the angle differences in a given image set which have GCPs enable the images to adjust properly to project the data accurately in a 3D model.   

Methods: 

To start out, the data imagery gathered from the DJI Phantom 4 drone were uploaded to Pix4D.  When brought in, the shutter method defined for imagery capture was designated to "rolling shutter".  After the images were all imported, users were then asked to navigate to "GCP MTP Manager" import 16 individual GCPs which were collected at the Litchfield mine.  Once these were brought in, they were displayed as blue crosses atop the red circles which represent image locations displayed below in figure 1.

Figure 1

After the GCPs were imported, the Basic GCP/MTP Editor was used to mark the exact center of each GCP location in order to ensure the highest degree of location accuracy and representation.  Figure 2 below shows an example of a GCP being previewed for marking. 

Figure 2

This process was repeated for the next 15 GCPs.  Following that, point cloud, mesh, index, DSM and Orthomosaic are all ran.  Upon process completion, a quality report is generated, shown in figure 3 below.

Figure 3

After the processing was completed, the pinpoint accuracy of GCP marking can be shown in figure 4 below through the point cloud imagery.

Figure 4

Results

Once processed, the geotiff file created in Pix4D can be brought into ArcMap.  Figure 5 below shows a hillshade model of the GCP UAS Imagery.

Figure 5

It was then compared to the previous imagery that didn't utilize GCPs to assess overall accuracy (shown in figure 6 below).  The biggest distinguishing factor between the two seemed to be with the difference of elevation.  Notice how the max value reads 247 meters for the GCP map, and the non-GCP reads 108 meters.