At Inflights, we request our pilots to do a position check on a known benchmark before starting to install the Ground Control Points. In this article you can find some more information about how you can do that.
It is good practice to locate the closest benchmark to the working area, and measure it before and after the survey.
When using Ground Control Points in a drone survey, the whole accuracy and precision of the survey will depend on the accuracy and precision of the installed GCP’s. If the GCP’s are measured incorrectly, the whole project will be incorrectly georeferenced. Therefore it is of the utmost importance to make sure that the GNSS receiver used to measure the GCP’s is performing well and setup correctly.
Therefore we request our pilots to use their GNSS receiver to get the position and the elevation of a known benchmark and compare that position with the official values for the benchmark.
Furthermore, it gives an independent proof that the ground control points that are measured are correct, and it will give the pilot invaluable information if something is actually wrong, not to mention the gain of time by catching on early if something is wrong.
When using a local base station, measuring on a benchmark is indispensable to ensure the base station is correctly configured, a small mistake is easily made.
To make sure your settings inside the rover are correct
Geodetic transformations applied correctly
Correct coordinate system
Control the correct functioning of the RTK correction
No unknown errors
Of course once you turn off your receiver there is never a 100% guarantee that everything will be correct again when it restarts, but 99.9% will be good enough.
Locate a known point that is accessible, close to the site, and can be easily measured. Obtain the official coordinates for that point. The best points will be the ones from an official source.
Set the antenna over the mark as accurately as possible in order to reduce centering errors, average the measuring over at least a minute.
After measuring, compare the measured values with the existing values. Investigate in case there are errors. A lot depends on the quality of the benchmark as well.
Possible errors: if you have more than 25cm of difference, chances are quite substantial that something is not correct in your setup. If it is the altitude: is your antenna height correctly entered? For horizontal errors, double check your transformation parameters.
Smaller errors can be due to a change in the position of the official benchmark. Some of these are quite old and can have shifted horizontally or settled vertically since they were last measured. The state of the benchmark itself will give you already some indication to the probability of that. Always take a picture of the benchmark you are measuring. It’s always a nice addition to a survey report, and it can clear up any questions regarding slight errors in the position.
Where to find official benchmarks?
This depends on the country you are in. Contact a local surveyor, or check with the national geodetic service, they should be able to help you further. In some countries, like Italy, you will have to pay to obtain the benchmark sheets, but in most countries these are freely available.
We listed some official sources for Belgium/the Netherlands/France/UK/USA to help you find official benchmarks there.
Navigate to the application of the national geographic institute: here.
Zoom in onto your area of interest, click on a planimetric point, an altimetric point or a 3D point (they can be visualised in the layer settings on the left). Now you can show the datasheet of the point close to your worksite.
The datasheet will give you coordinates and elevations that you can compare with your own measurements. (planimetric points are only valid in x,y/altimetric points are only valid in z).
Geodetic points (mostly applicable for altitude only) can be found on: https://www.pdok.nl/viewer/. Add the datasets for NAPinfo and RDinfo to visualise the points in your area of interest.
Now you can click on a point and in the bottom right of the screen on Object Informatie you can visualise the values for the point.
Another interesting link to locate points is here.
Navigate to the bottom of the page, zoom in on your area of interest and click on the points. The Red ones (Stations) have the most information available.
Go to the application of the national geodetic institute: here.
Zoom in on your area of interest, make sure at the left side of the screen the necessary points are visualised, as below. Then click the rectangle on the top of the screen to select the points in an area.
Now in the bottom left the points will appear in your panier. You can click the PDF to download the datasheets.
Beware that some points are only valid for altitude, while others are only valid in the horizontal system. The precision is also usually listed, so don't expect 1cm accuracy on your measurement if the precision of the point is 50cm. The sheet also shows you when the point was installed. The longer ago it was, the less trustworthy it will be.
Go to the application of the Ordnance survey. You will need to have or make a (paid) account. The benchmarks in here can only be used for height purposes.
Go to places on the top menu, activate benchmarks and trigpoints.
Now you can zoom in on your area of interest and click appropriate points. The information of the marks will be shown in the left panel when clicking the more info button.
The application of the government is very easy and well explained when you open it first. Navigate to your area of interest, click find marks on the left, click the mark you want on the map, and you can download the datasheet.