What are Photogrammetric Support Points and what is their relationship with RTK/PPK Drones?
The PAF: Photogrammetric Support Points, or also known as GCP (Ground Control Points in English) are those points that are used to adjust the photogrammetric block. On this occasion, we will study its relationship with RTK/PPK Drones. The PAF coordinates will allow calibrating the scale of the stereoscopic model and leveling it.
In Geometry it is shown that three points are enough to determine a plane, however, to transfer the square figure of an aerial photo to its corresponding plane projection, the number of points used is at least four, which should preferably be located near the corners.
However, in conventional photogrammetry, the minimum number of points to carry out this operation is four: three points on the X, Y, Z axes (Planimetric-Altimetric) to be able to adjust the scale of the model and a fourth in the Z axis (Altimetric) so that they make a total of three points of known coordinates.
In Photogrammetry with RTK/PPK Drones this number of points is considerably reduced, you can even do without them and use them as quality control. Even if support points were used with PPK Drones, the tests indicate that adjusting the block with a single point is sufficient, since the model is highly accurate and in the worst case scenario would only need a 2D or 3D translation.
The position of these points is determined by GNSS observations or with Total Stations. These points can be linked to an official network, for example, a geographic or military institute, therefore, the photogrammetric block would also be.
The support points can be observed before the flight, in order to indicate their positions so that they appear marked in the photos, but it is more frequent to select them and observe them after the flight.
It is essential to choose, as such reference and control points, some that are clearly identifiable in the photograph. At the very moment of making the observation, each point is marked on the photo with a small mark, indicating its surroundings with a figure, for example, a circle, square, triangle, among others, in addition to a serial number; and you can also make a reference sketch.
The PAF must have the following characteristics:
- Sufficient size and proper shape: For a flight at an altitude of 80 meters with a 14 megapixel camera and an 18mm focal lens, a square control point half a meter on each side will occupy an area of 16 x 16 pixels.
- Have a clearly defined visual center with sufficient contrast: The only thing that matters about a control point is the position of its center. An X made with two lines, a square divided into four quadrants, a letter T, a target, among others, are equally valid.
Recommendations when designing the PAF:
- In general, avoid spots with a uniform color or excessively irregular shapes.
- Do not use colors that blend in with the environment. A white control point within an area of light calcareous rock will likely cause environmental elements to be identified as control points.
- Use materials that are visible in different lighting conditions. And we mean visible from the point of view of the drone, not at ground level. What is absolutely visible at ground level will not necessarily be visible from the sky.
- Place them on flat surfaces free of obstacles. This will improve the probability that the checkpoint will appear complete and with sufficient size. Avoid trees, vegetation, buildings, etc., as the results after capture are often that the control point is visible in far fewer images.
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It is important to consider the impact that RTK/PPK Drones cause in reducing the number of Photogrammetric Support Points – PAF that we will need in our field projects, and with it, the benefit in saving time and resources.
It is possible that you have a drone that does not have an integrated RTK/PPK system, and you consider that investing in new equipment with this technology is a considerable investment or in any case, perhaps you have a Phantom 4 Pro that you bought some time ago, and you feel that it is no longer as useful as before because you do not have this system, for both cases we have two solutions for you: our GNSS X5 PPK Module for DJI Phantom and the PPK KIT for use with the GNSS Emlid Reach M2, non-intrusive, which you can easily install on your DJI Phantom 4 Pro and turn it into a GNSS PPK equipment, at a price very comfortable.