Part 1 – Project Setup
Chapter 1.3 – Ground Control Points
Overview
In this chapter, the goal is to add ground control points (GCPs) and use them to refine the relative and absolute orientations of the photos with reference to a real-world coordinate system.
Importing GCPs
MetaShape will accept GCP files in a range of different formats and file types. For this exercise, the GCPs are in a comma-delimited format (CSV) which can be imported directly by MetaShape. In the Reference tab, select Import Reference, navigate to where your GCP file is stored (i.e. Boundary.CSV) and click Open. Adjust the Coordinate System, Delimiter and Columns as per Fig. 8, paying close attention to the Start import at row value (2 in this case) and click OK.
Click Yes to All when asked whether to “Create new marker?”. You should now see a view similar to Fig. 9 showing the initial locations of nine control points. If the markers flags are not visible in your image, click Show Markers in the main toolbar.
Placing GCP markers
In the Reference pane, notice the list of nine markers with their corresponding Easting, Northing and Altitude. Right-click on a Marker (e.g. 1) and select Filter photos by markers. Now, in the Photos pane along the bottom of the main display window, all the photos that GCP 1 appears in are shown.
Double-click the first photo and zoom in on the flag for GCP 1. It may be that the actual GCP is not visible in that photo! The flag can initially be several meters from the target due to the absolute positional error of the RPAS GPS. Select the next photo if that’s the case.
The goal is to left-click and drag the marker flag to the centre of the checkered GCP as per Fig. 10 below.
Notice now that the columns Error (m), Projections and Error (pix) now have values in them indicating the relative (pix) and absolute (m) error associated with placing that marker. In order to get a true sense of these errors, at least three GCPs need to be placed in a minimum of three photos (projections) followed by a least-squares adjustment. Double-click on the next photo and repeat the placement of the flag in the center of the target. Do this for at least six photos so as to build some redundancy. You may also find that the ground control targets are not easily visible in some photos.
Once you’ve placed GCP 1 into six photos, note the Error (m) and Error (pix) values. For example 0.897622m and 123.862 pixels, respectively. This means that even though we placed marker 1 in the center of the same target in all six photos, it is displaced by just over 123 pixels over the six photos. In an absolute sense, the coordinate value at the center of the ground control target, has an error of just less than 90cm over the six photos. These may seem like high error values, but recall that the last step is perform a least squares adjustment in order to distribute and minimize these errors. To do this, click the Update Transform icon. You should see a significant reduction in the errors now