To obtain the highest amount of global accuracy in your drone mapping project, the most common and effective
method is to use Ground Control Points.
Let�s first explore what Ground Control Points are.
GCPs are marked points on the ground that have a known geographic location.
This means that each point will have 2D or 3D coordinates assigned to it depending on how you capture the point
The accuracy of the captured points can be as good as in the range of millimeters to as way off as a few meters
again depending on what instrument you use to capture the point.
In your drone mapping experience, you will come across projects that require global accuracy along with local accuracy.
I�ve explained the difference between global and local accuracy in part 1 of this course series.
Let me go over it again:
There are two types of survey accuracies - relative accuracy and absolute accuracy:
Relative (or local) Accuracy: Relative accuracy is the extent to which a given point on a map
ss accurate relative to other points within the same map..Whereas absolute or global accuracy is the extent to which a point on a map corresponds
to a fixed coordinate system in the real world.
Examples of relative accuracy.
if the distance between two points measures 2 meters in the real world,
it should also measure 2 meters on your map.
On the other hand, If a map has global accuracy, the latitude and longitude of a point on that map should
correspond accurately with actual GPS coordinates.
So how relatively accurate is your drone survey and what does it depend on?
One of the main factors that determine the accuracy is Ground Sampling Distance (GSD).
GSD is the distance between two consecutive pixel centers measured on the ground.
The bigger the value of the image GSD, the lower will be the resolution of the image and the less visible are the details
This will also reduce the relative accuracy of measurements.
Also remember that, the GSD is related to the flight height: the higher the altitude of the flight, the bigger the GSD value.
Here's a simple formula-
The horizontal relative accuracy is 2 times the GSD and the vertical relative accuracy is 3 times the GSD.
For example, if GSD is 3 cm per pixel, horizontal relative accuracy is 6 cm and vertical relative accuracy is 9 cm.
The project's target relative accuracy needs
be chosen depending on the project accuracy requirements.
So, when explaining to your customer about accuracy, try to first assess what type of accuracy they need
and to what extent. ..This will help you determine how to go about planning the flight and processing the images.
Now, moving on to global accuracy. If you haven�t come across a customer requirement that needs global accuracy,
global accuracy, and are wondering when such scenarios arise, here are some examples :
Property Boundary Surveys - When you have to map a property boundary with the real world,
you�ve got to be extremely precise with the coordinates of the boundary lines and points.
GCPs are most definitely needed for such kind of surveys.
Subdivision Surveys - When properties need to be subdivided accurately as per real world conditions.
Georeferenced Overlays - If the customer needs your outputs to be overlaid with other globally accurate spatial layers.
Also, if the survey data needs to be legally binding or signed off by a certified surveyor, it�s best to
use GCPs, even if global accuracy is not needed.
This is because, there is a certain confidence that people have in data that is captured with GCPs as against without them.
You can even demonstrate the accuracy of your results by measuring distance between any
two GCPs or between a GCP and any other check point on your outputs vs the real world measurement at site.
Do make sure you understand the project requirement well enough to take a call as to whether you need GCPs or not.
Using GCPs will increase time spent on the site and during processing.
It will also increase your cost as you may need to hire or use specialized equipment for this.
This will increase the cost you quote to your customer.