Relative accuracy, Absolute accuracy & Ground Sampling Distance (GSD).  A simplified explainer.

Written by David Holmes

When you engage with a drone company that offers “survey” grade work you need to be mindful of some of the terminology and really what you can expect.

When it comes to drone mapping, there are two ways to think about accuracy: relative accuracy and absolute accuracy.

Relative (or local) accuracy: Relative accuracy is the degree to which a given point on a map is accurate relative to other points within that same map. In other words, if a distance between two points measures 10.00m in the real world, it also measures 10.00m on the provided output.

Absolute (or global) accuracy: Absolute accuracy, on the other hand, is the degree to which a point on a map corresponds to a fixed coordinate system in the real world. If a map has a high level of global accuracy, the latitude and longitude of a point on that map will correspond accurately with actual GPS coordinates.

Let look at these two ideas in more detail.  We need to also be aware of the idea of GSD or Ground sampling Distance as this sets our relative accuracy.

Ground Sampling Distance (GSD)

GSD is the distance each pixel is equivalent to over the ground.   We commonly refer to GSD as resolution. The smaller the resolution, the more detail you will see in your outputs or digital model.  In drone mapping, GSD is controlled by the drone camera type as well as the flying height.  Propeller Aero have a useful GSD calculator here:  https://www.propelleraero.com/gsd-calculator/

Flying lower over the asset produces more detail and better resolution (we can achieve a GSD of 0.15 cm/px @12m which is really the best resolution currently available by enterprise drones); however there is a cost in terms of capture time, amount of data collected and processing speeds.

Relative Accuracy

Also known as internal accuracy, relative accuracy is the accuracy between two points within the digital outputs or model.

Generally speaking, the relative accuracy of the best quality outputs is 3 times the GSD. For example, if the GSD of the model is 2 mm (or 0.20cm/px) per pixel, the relative accuracy should be within 6 mm. That is across all coordinates – X,Y and Z.

Several additional factors determine relative accuracy, including how the data the captured data.  Good data (i.e. high quality images with correct exposure, no motion blur etc) will ensure a better reconstruction, offering better accuracies.

Absolute Accuracy

Also known as global accuracy, this refers to how close the measured position on an output is to the absolute real world position.  In practice this is much harder to achieve with drone outputs even when capture conditions are ideal.

Many variables need to be to considered in order to determine absolute accuracy. Most important is the quality of drones GPS system to achieve global positioning.  You can increase accuracies by using enhanced positional information from RTK or PPK systems (onboard the drone) as well as surveyed ground control points (GCP).  For any project when absolute accuracy is going to be important then always speak to your drone team to ensure that they are using the correct number of GCPs (for a given land / building area and shape) and that they understand the positioning requirements.

Quoting absolute accuracy should always be considered against the backdrop of the GSD.  Absolute accuracy can never exceed the relative accuracy and is usually quoted from a quality report from the photogrammetry software.  Again, think of this as a statistically estimated level of absolute accuracy rather than the “correct” value.  As a client you should ensure that you see this report and also ask for “check-point” data as well.  This will allow you to see differences from known points on the ground to calculated theoretical positions – adding to your evidence portfolio around positioning accuracy.

Be cautious

Drones have tremendous ability to disrupt traditional workflows and speed up data capture as well as make it much safer.  In many applications the data from drones should be used to augment rather than replace the other methods of surveying.

As a client you should always ask questions about relative and absolute accuracy and in particular the expected GSD.  At InView Assets we never claim to be surveyors, instead we are technologists who capture data with a known resolution.   That’s worth remembering.

Email: info@inviewassets for a more detailed discussion of these concepts and our technical document.