Modern commercial drones have made mapping any area or an object a matter of few minutes or hours, instead of the typical days or weeks it used to take before.  A small commercial drone can now let a qualified operator achieve survey-grade accuracy in a photogrammetric map or 3D model.

But just how accurate can these surveys be? How is accuracy defined and measured in these drone based aerial surveys?

SURVEY ACCURACY VS. PIXEL SIZE

One of the most important parameters of a survey is the spatial resolution. Spatial resolution is described as GSD (Ground Sampling Distance). In literature, it is defined as the distance between two consecutive pixel centers measured on the ground. In practice, it is simply the size of the pixel in the field.

The GSD depends on the parameters of the camera being used (mainly camera resolution and focal length) and the flight altitude. For example, to achieve 1 cm (3 inches) pixel size using a smaller professional drone like the DJI Phantom 3 Professional or DJI Mavic Professional, you will need to fly at an altitude of 25 m. Using a larger professional device like PrecisionHawk’s Lancaster 5 will let you achieve 1 cm GSD at 60m.

RELATIVE ACCURACY VS. ABSOLUTE ACCURACY

Relative accuracy is the measurement of how objects are positioned relative to each other in a reconstructed model (Orthophotomap, Digital Surface Model or 3D point cloud). Absolute accuracy refers to the difference between the location of the objects on the reconstructed model and their true position on the Earth (or a geodetic coordinate system). For most small areas and simple use cases, the relative model works just fine. It accurately measures distances, volumes and height differences and allows for operations like applying vegetation indexes to your data.

However, if professional photogrammetric survey documentation is needed, or if the data is to be combined with other data layers (e.g. GIS vector data) or do any operations with geographic coordinates in a geodetic reference system, the absolute orientation of the reconstructed model will need to be determined by using Ground Control Points or GCPs. They help to increase the accuracy of your drone survey and allow you to fit the model in a geodetic coordinate system. These points must be measured by a land surveyor using professional equipment, such as the RTK GPS or a Total Station.

WHAT RELATIVE ACCURACY CAN BE ACHIEVED IN DRONE PHOTOGRAMMETRY?

When it comes to relative accuracy (Surveys without GPCs), the literature says you can expect an error of 1-3 times the pixel size for a correctly reconstructed model, both horizontally and vertically. This means that for a 2 cm GSD, one should be able to achieve the accuracy in the range of 2-6 cm.

CONCLUSION

The accuracy requirements of any survey depend on its final application and the use case. In many cases, 30 cm (1 foot) will be more than enough, while on other projects 2 cm is the maximum acceptable error. Using a small professional drone without GPCs placed by land surveyors can still provide a highly accurate product for many standard use cases.