Ground Control points are a method of achieving survey grade relative and absolute accuracy in drone scans. They are physical markers on the field that have had their location measured using specialised survey equipment to know their exact geospatial location.
Using this workflow while process the data, Sitemark can use this information to achieve very high levels of both relative and absolute accuracy in the data output.
How many GCPs do you need?
A question that we get asked quite often is how many GCPs we need for an X-hectare site or how many GCPs we need per hectare.
Unfortunately, it is not that simple to have a set number of GCPs per hectare. It is important to know that for GCPs to work on a site, there needs to be a minimum of 4 GCPs; any less than that, and the dataset will not georeference.
This being the case we would always advise to use a minimum of 5 on a site, this provides a redundancy should there be an issue with the coordinate of one of the GCPs.
Below we’ve provided some schematics which help to explain setting out of GCPs on a site.
If you have a relatively uniform shape to your site, such as a rectangle as shown above, then for an area of between 0ha and 15ha then 5 GCPs should suffice. When the site starts becoming larger, then you would want to distribution more GCPs across the area. What you are looking for with GCPs is to have a regular and even distribution across the site. You will see that the GCPs are placed along the edge of the site, with a couple in the middle.
In the second image below we’ve illustrated a site with an irregular boundary. However, the principles of GCP placement remains the same. Placing of them along the edge of the site and some also within the centre.
When picking locations to place your ground control, always pay attention to what the chosen area would look like from your drone. A GCP is useless if it is obscured by trees, buildings or fences in the final imagery.
GCPs should always be measured at ground level to minimise the risk of elevation error in the terrain of the final 3D model.
The GCPs need to be surveyed using survey grade equipment that can obtain an Absolute Accuracy of a couple of centimetres (eg Emlid Reach RS2+) by using either a base station setup on a known point or an RTK/PPK solution.
GCPs in the Sitemark platform
You can tag GCP's yourself, right after the upload of your photos. For more information on how to tag GCP's yourself, please refer to the following article Tagging Ground Control Points (GCPs)
You can assess the accuracy of the GCPs in the platform in the GCP tab of the operation. You can see the error is displayed in meters or feet depending on the CRS used.
Control points can also be marked as "checkpoints" when they are being tagged in the platform. These control points are not used to build the accuracy of the photogrammetry, but are only used to verify accuracy when the photogrammetry is finished.
As the GCP chosen to be a checkpoint will not be taken into account while processing it's important that you have sufficient GCPs and ideally when on-site survey create a GCP with the purpose of being a checkpoint.