Drone technology is the new tool on the block when it comes to aerial survey and has been delivering impressive results. Accuracy is of upmost importance to any survey, without accuracy the information collected by your aerial survey drone is of no value. Any reasonable drone can be turned into an aerial survey drone with the right software. But is the data accurate?
A request has come through to obtain the volume of a stockpile for a public company. Shiny new aerial survey drone in hand, the data is collected and the information handed over. The information is used as part of valuing the company. The information is found to be incorrect and the companies value has been affected. A letter arrives from the company’s solicitor.
Bad process goes in, bad results come out.
The position of any one point collected by an aerial survey drone is not necessarily as accurate as other survey means. The way we improve the accuracy over the entire survey is by the sheer number of points collected.
Setting a high overlap allows unique points to be collected in multiple images, allowing them to be matched in post-processing software to triangulate a position. The more points in multiple images, the more accurate the data becomes. Additionally, without a million-dollar sensor on board there are camera issues to contend with.
If the quality of our data depends on good overlap, the image position is also very important. To gain the correct overlap the images must be captured where the aerial survey drone’s software has anticipated. As a pilot, this means planning the flight to fly crosswind, ensuring the ground speed of the aircraft is the same along the survey flight lines, resulting in equidistant images. This is essential for aerial survey drones taking images by time instead of distance.
Ground sample distance
The closer the aerial survey drone is flown to the ground the better the spatial resolution or more pixels that will appear in the image. In some aerial survey drones the ground sample distance (size of one pixel) is set by the height flown, in others the height is dictated by the ground sample distance. Depending on the aerial survey drone being flown there may be a minimum height that can be flown for a given speed and camera setting before the images become blurred.
If the terrain being captured by the aerial survey drone is undulating, the ground sample distance and the overlap continue to change. As the aerial survey drone gets closer to the ground the spatial resolution gets better but the overlap gets worse. As overlap is important to the quality of the data it is important to allow for the terrain. Some aerial survey drones will allow the aircraft to follow the height of the terrain by following the elevation data contained in the satellite image or a custom map imported by the user.
The type of shutter on board the aerial survey drone will change the way it captures the image. Rolling shutter is when only a line of the image is collected at a time, to combine to create the entire image. When the aerial survey drone is moving the camera position for the next line has moved.
Global shutter is when the entire image is collected at once. The shutter type will change the way the images need to be processed to ensure accurate results.
Image position errors
After programming the aerial survey drone with the correct overlap, image position, ground sample distance and allowing for terrain the image position is still not correct. This is because there is a large error in the position information acquired by the Global Navigation Satellite System (GNSS). To correct for GNSS error Ground Control Points (GCP), Real time Kinetics (RTK) or Post Processing Kinetics (PPK) needs to be used.
These methods create a “known” point, one we can compare to our image positions and correct the GNSS error.