Ground control points (GCPs) can enhance your drone mapping accuracy

Wanting to make decisions using your drone mapping data? If so, understanding your location precision and accuracy is paramount, and ground control points – GCPs – might be your answer.

As aerial surveys become increasingly integral to various industries, ensuring accuracy in data processing and location information is essential. Let’s delve into the significance of GCPs and their role in enhancing accuracy. We’ll look at their placement strategies, required equipment, and how to integrate them into your data processing workflow.


What are GCPs?

Ground control points are points or features on the surface of the earth (or even underwater) with a known location. They can be physical or painted markers that you deploy temporarily for your survey duration, or distinctive infrastructure or natural features in the environment. Critically, they must be visible from the imagery you are capturing, be that drone, traditional aerial survey, or satellite. 

Once in place, we record the accurate and precise location (e.g. latitude, longitude, elevation) of each GCP using a GNSS device (AKA GPS). We can then use these reference points and their location to accurately georeference remotely sensed data. This location information provides crucial ground validation data, aligning and calibrating imagery with real-world coordinates.

Why are they important?

Not all remotely sensed data platforms capture locational information to high levels of accuracy. The co-ordinates provided in many consumer/prosumer drones, for example may be several meters away from their true location. Depending on the nature of your work, this could be a very big deal!

Using ground validation data and the known location of reference points allows you to correctly georeference your mapping data, and ensure that it aligns well with other spatial data layers.

Calibrating your data with reference points can significantly reduce distortions, errors, and inaccuracies inherent in drone imagery, resulting in highly accurate spatial data for analysis, modeling, and decision-making.

How to deploy GCPs

Effective placement of GCPs is critical for achieving optimal mapping accuracy. When planning a survey, consider distributing a minimum of five GCPs evenly across the entire area of interest, ensuring adequate coverage and representation.  For example, place one in each corner of the survey and one in the middle.

If you are likely to return to the same site for a repeat data capture, consider using stable, non changing features in the environment in your mix of reference points.

Don’t forget where you placed them, and remember to retrieve them at the end of your mission.

Using Properller Aeropoints as GCPs
Using Properller Aeropoints as GCPs

What equipment do I need?

To implement GCPs effectively, you’ll need a few bits of kit:

  • High-accuracy (preferably) GNSS receiver: Ensures accurate GCP location recordings.
  • Surveying tools (e.g., total station, RTK GNSS): Determine accurate positioning of GCPs on the ground.
  • Durable ground markers: Clearly identifiable objects used as GCPs, such as surveying targets or painted markers. Alternatively, if there are features in the environment that are easily distinguishable, you can use those. These are called ‘natural GCPs’. Make sure you take note of which particular feature you are using though!
Dr Javier Leon, University of the Sunshine Coast surveying in a natural GCP
Researchers from James Cook University set up a base station on Taylor Cay, Great Barrier Reef

How to integrate GCPs into a drone mapping workflow

The workflow is simple, but does require a number of steps:

  • Pre-survey planning: Determine GCP locations based on survey objectives and terrain characteristics.
  • Deploy GCPs: Position using surveying equipment where possible, ensuring accurate placement.
  • Data capture: Conduct drone surveys (or other airborne or satellite mission), capturing imagery of the survey area.
  • Post-processing: Use GeoNadir or other photogrammetry software to process and georeference aerial imagery, incorporating GCP coordinates for precise alignment.
  • Quality assessment: Validate mapping accuracy by comparing GCP-derived coordinates with surveyed ground validation data.

Processing drone mapping data with GCPs

There are a variety of different options for photogrammetry software to process you drone mapping data with GCPs. Each package will have a slightly different workflow. Here’s how we do it in GeoNadir:

  • Obtain your GCP data, which is a csv file containing the name of each point and it’s corrected x,y, and z information.
  • Match each GCP with its location in at least three individual photos captured on the mission. 
  • Upload the matched GCPs with your drone mapping data to GeoNadir.
  • Sit back while we take care of the processing

Watch the video below for a full demonstration of this workflow.

Incorporating GCPs in your workflow may seem daunting, but it can be achieved in a few easy steps. And it really does make a difference to the accuracy of your final product.

To learn more about getting the most accurate location information possible, check out these other articles:

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Topographic map with dots displaying data points of environmental drone mapping data. Three drone mapping overlays in different environments

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