When collecting GCPs observe the following requirements:
- As much as possible use should be made of suitable existing features on the ground. This will ensure that target maintenance is not required and allows targets to be re-surveyed if necessary. All artificial targets should be of durable nature so that they are still well visible from the air by the time the flights are taking place e.g. manhole covers, pavements, etc.
- When choosing a specific part of a feature make sure it is well defined. To remove any ambiguities as to which feature exactly was surveyed, take three (3) illustrative photos of the target at the time that it is occupied by the survey rod – i.e. showing the survey rod on the target. To avoid confusion it is very good practice to allow the cellphone to geotag the photos. That way photos and corresponding targets can be unambiguously linked. For example the north- western corner of a painted stop bar at a road intersection is a well-defined point; the center of a large manhole however is not because it relies on guess work by the surveyor as well as the person who is going to observe the target in the imagery. Similarly, the outer apex of a V shaped target is better defined than the intersection of two lines forming a cross. Well composed cellphone pictures give a much better definition of the exact location of the target than a verbal description can do. However, where necessary verbal descriptions should be used to augment the documentation.
- Targets should preferably be on flat ground. The three-dimensional definition of a target is important. So if a target is suitable for horizontal purposes make sure that there is no ambiguity as to exactly where the height of the target was referred to. For example, if the corner of an elevated concrete slab was chosen make sure that there is a record of whether the height refers to the top of the concrete surface or to ground level at the corner.
- Targets should be visible from as low a horizon as possible. Targets near trees or buildings will be obscured from several aerial camera positions. Targets must appear on as many of the aerial images as possible.
- When installing targets in parking lots bear in mind that parked cars may obstruct a target during the course of the image acquisition flights.
- The best contrast is provided by black and white. Green and white – such as a white cross on a green lawn is also very good. Orange and red are bad colors because they tend to “run” in the images. So don’t use orange when spraying a cross on asphalt. Rather use white.
- Edges of lines or figures used in the definition of a target should be sharp. Spray cans without the use of templates produce fuzzy edges.
- The resolution of PLACE Aerial images can be 3cm or better at low altitudes so targets should be detectable at that resolution. For example, if painting a cross or a V on asphalt, make sure that the arms of the cross or V are 6cm wide. Circular or square targets should have a diameter/side of about 15cm.
- Target coordinates should be as accurate as practically possible. Processing is sensitive to target errors of less than 3cm. GNSS RTK or PPK usually suffices but it is important to survey each point at least twice and to accept only fixed solutions.
- Unless 100% sure that the rod bubble is in perfect adjustment, take 2 measurements per occupation. To eliminate the bubble error provide two measurements with the rod turned 180° between the two measurements. The mean of the two measurements will be free of bubble misalignment. Note that a pair of measurements of this kind are considered as a single measurement of the point.
- All the points MUST be on the same coordinate system. Unless the chosen coordinate system is well defined in relation to WGS 84 or any of the ITRF epochs, a separate survey will be needed to establish transformation parameters between the chosen system and WGS84/ITRF.