What is the best accuracy that can be achieved?

Accuracy depends on many factors among which, the flight height, type of camera used, the accuracy of the ground control points…Accuracy can be as good as 5 mm. Although, this accuracy is obtained thanks to the latest (and expensive) technology. It also requires much more time for the setup, the flight and the process. The absolute accuracy can only be as good as the accuracy with which the ground control points are collected. It thus depends on the tool that is used to measure the gcp’s: GNSS receiver or total station.

You should count on an average accuracy of 2 cm horizontal and 3 cm vertical.

Read Pix4D’s -leading photogrammetric software provider- white paper about accuracy.

The relative accuracy however can be as low as a mm. Indeed, ground control points are used to calibrate the model but not to position it in a reference frame. Aetos drones achieved an a survey of the cooling tower of Tihange Power Station.

Is a survey always possible?

A survey is not always possible. Indeed, certain countries don’t allow the usage of drones. A non objection request can often by-pass this.

In other countries, strict regulation doesn’t allow the use of drones in areas near airports, prisons, nuclear plants, … however, it is possible to ask an exemption to the law.


Aside the law, the weather is also a limiting factor. It is impossible to survey during rainy days. Some drones can handle rain but no software can build a 3D model based on pictures containing water droplets.

Also, high winds are to be avoided (higher than 10 m/s).

Finally, it is impossible to map water surfaces. Indeed, as the software that builds the 3D model looks for characteristic points, the water surface always in movement or reflecting sun rays does not contain such points.

How much time should I allow for the whole process?

Depending on whether the project is in a controlled area or not, availability of pilots and the area to cover, it can take two days to two weeks. On earthwork projects, a survey will take 30 min / 10 hectares. The post-processing takes about 8 hours / 10 hectares.

The analysis and volume computation depends on the complexity of the task and whether the model needs to be cleaned up from cars, trees, containers… Count 2 days on average.

What price can I expect?

This depends on the area to cover, the accuracy required, the location of the project and the requested analysis/delivrables.

In Belgium for instance, a 270€ exemption is to be requested when inside a controlled area. (blue zones)

Small interventions that can be completely done in a day will be around 500€ as for larger projects it will depend on the final product that has to be delivered.

Why are the Z value of my DJI pictures lower than the actual flight altitude.

This is a firmware error. To rectify this error, there is a fast and easy way. 

Download the following exe file.

Rename it exiftool(-GPSAltitude=xx).exe where xx is your absolute altitude you’ve flown at. (not the altitude above ground level).

Example: I’m flying at a location 68 meters above sea level and I’m flying at 40 meters altitude. I’ll rename the tool exiftool(-GPSAltitude=108).exe

Make a copy of all the pictures you want to rectify. Drag and drop all those copies onto the exe icon.

Verify in the properties of the picture that the altitude has changed correctly.