GCP Distribution and Count
Establishing how many Ground Control Points you need to use and exactly where to place these points across your area of interest can make or break your survey accuracy values. Here are some general guidlines to consider in planning your GCP data.
How many do I need?
Having to few Ground Control Points across your survey area can lead to inaccuracies in select regions of your survey. It is important to consider the full size of your area of interest in the planning of these GCP's. Often times we use tools like Google Earth to create a full plan for our Ground Controls before ever going out to the field.
- Typically you will need atleast 4 ground control points in your survey to get good accuracy for any sized survey. But for larger areas more Ground Controls should be considered.
- For larger areas no more than 10 Ground Control Points are needed.
It is difficult to build a template for number of ground control points needed in relation to the size of the survey. Much of this decision will depend on the shape of your survey. A long rectangular survey will need more GCP within the survey area than a square survey of 50 acres.
The distribution of your GCP's over your area of interest is even more important than using the right number of Ground Controls. Even with 10 ground control points in a small survey, if you decided to place all of them in one corner of the area, your accuracy will be limited outside of that particular corner.
Imagine that you are draping a tarp over a large pile of dirt. It is very windy out and you must secure the tarp with some weights in order to keep it from blowing away. You should have a similar way of thinking when you are placing your GCP locations across your area of interest. Here are a few guidelines to consider in this placement decision-
Evenly distribute GCP across the area- Do not bunch Ground Controls into one particular area
Do not place GCP in the immediate edge of the survey- Keep the GCP atleast 20 meters from the edge of your survey area. These areas on the immediate edge are known to have lower overlap in the source imagery which will cause problems in the GCP tagging process.
Place the GCP target in the open- These must be visible in your source imagery so make sure there is no overhead obstructions that would prevent this visibility.
If you are using 3 Dimensional Ground Control Points- Consider the elevation changes throughout your area of interest. Just like the horizontal distribution of GCP you must distribute the GCP vertically for accuracy in elevation values.
Example of Good GCP Distribution
Example of Bad GCP Distribution
Your ground control targets must be apparently visible in the source imagery in order to tag them in the upload process. If you decide to use a target that is not distinguishable in the source imagery the GCP could be wasted.
There are an endless amount of objects that you could use as a Ground Control Point in this process. However to ensure that this GCP tagging process is done accurately and efficiently see our recommendations below for the ideal GCP target.
There should be a large X of some kind on the GCP target. Instead of tagging just anywhere on the target you want to use the center of that X for maximum precision.
Target should be 3-4 feet in width for good visibility in the source imagery
The target should have high contrast against the environment of the survey area
Number your GCP target and correspond this labeling to the GCP file containing the GPS of each ground control. Without a unique identifier between GCPs you could easily confuse two targets with one another during the tagging process if they are close enough together.
GCP should be placed at ground level on a relatively flat surface.
It is not recommended to use natural features as GCP targets
Make sure the GCP target is weighted or tied down, any movement will defeat the purpose of the target
Mapping your GCP's
Once you have laid your GCP targets with good distribution you must gather accurate GPS data for every GCP target that was laid out. This GPS data should be gathered with a high accuracy RTK or PPK GPS system for good results. This GPS data must be recorded in a .csv file for upload with the source data in the PrecisionViewer program. For full instructions on this upload process see our Processing in PrecisionMapper with Ground Control Points article.
The Phantom 4 Pro has a GPS accuracy of +/- 1-5 meters. This does not mean that every processed survey from the drone is within +/- 1-5 meters of accuracy, it means that each individual capture tags a GPS location of this accuracy level. There is some inaccuracy that can be introduced in the stitching process to raise this accuracy value. Without the use of Ground Control Points you could be anywhere from 1-5 meters in accuracy on the final Orthomosaic results.
Choosing a high accuracy GPS system for the collection of GCP's is very important. The Phantom 4 Pro has a GPS accuracy of +/- 1-5 meters. GPS accuracy from your average smartphone is only around 5-8 meters depending on your device. With this said if you are not using a survey grade GPS system like RTK or PPK GPS you are really wasting your time with the collection of Ground Control Points. You could potentially make your accuracy worse by using a smartphone or low accuracy GPS for the collection of these points.
When collecting data with the GPS System be sure to measure the GPS location of the very center of the X on the target. Most GPS systems will have the ability to collect the GPS data in many different Geographic Projection Systems.
For compatibility with PrecisionMapper the data must be recorded in WGS84 Geographic Projection and NAD83 Vertical Datum (For GCP with elevation data). If your GPS system does not collect in these projections there are ways to convert these values after collection.
To upload with PrecisionMapper the GCP data must be saved as a csv file with the exact columns and headers seem below. Attached is the sample gcp.csv file you can use to to assist in this formatting.
Type- Should be "gcp_2d" or "gcp_3d"
- gcp_2d- Latitude and Longitude only
- gcp_3d- Latitude, Longitude,and Elevation
- Latitude and Longitude- GPS coordinates in WGS 84 geographic coordinate system
- Elevation- In meters above mean sea level
- Tolerance- The accuracy of the GPS unit in meters