Creation and Placement of Ground Control Points

Introduction:

When Pix4D performs bundle-block adjustment on aerial images, the horizontal and vertical accuracy are dependent on the on-board GPS. Dependence on the on-board GPS can cause distortion of the resulting products. By recording the positions of recognizable features within the study area, it is possible to rectify the bundle-block adjustment to the known points and thus reduce distortion. For these points to be effective, the image analyst needs to be able to identify the exact location of the control point.

Methods:

The Litchfield mine site has been used for several of the previous labs, so it seemed prudent to place permanent GCP markers around the location, to ensure the accuracy of future flights. The markers were constructed of black plastic, commonly used to make the 'boards' surrounding hockey rinks. The plastic is flexible and weatherproof, so they should survive the elements, as well as any vehicles running them over. The GCP markers were spray painted with bright yellow spray paint, so a large 'X' is visible in the center of the marker. This 'X' will allow the analyst to precisely identify the point later when performing bundle block adjustment. After marking the 'X' on all of the markers, they were each labeled with individual letters of the alphabet, so they may be more easily identified in the field during later data collection.

On April 11, 2016, we travelled to the Litchfield site, and placed the GCP markers around the site. Before placing the markers, we removed sticks and flattened the soil/rock-material to ensure they aren't moved by wind. All of the selected locations had good aerial visibility, and were outside of well-travelled areas. The markers were evenly placed throughout the mine area, with some around the edges and in the center.

Discussion:

The control points also need to be recorded at a higher level of accuracy than the GPS on the imaging platform, otherwise they may actually decrease the accuracy of the resulting imagery. The markers were placed outside of well-travelled areas so they wouldn't interfere with mine operations, and to reduce the chance they will be buried, destroyed, or moved by mine equipment or material. It was important that all of the marker locations had high aerial visibility, because markers with poor aerial visibility would only be accurately identifiable in some of the images, reducing the effectiveness of the bundle-block adjustment. The spacing of the GCPs was extremely important, as they increase the accuracy of imagery between them, and can cause distortion if the density of them differs within the study area.

Conclusion:

GCP marking has been one of the most discussed topics within the UAS community as of late, and this exercise provided good insight into GCP installation and creation. The placing of GCP markers is the most time-consuming portion of the data collection process, so by installing permanent markers, it will allow future flights to be performed more quickly without having to sacrifice positional accuracy.