How to measure distances using drones.
Using drones for aerial surveys is big business. The use of a drone as opposed to the traditional 'boots on the ground' method has meant that a survey that could have taken days the old way, now takes a matter of hours and even minutes in some cases! We often get asked how can a drone determine a distance from just a camera? It's a great question that is often harder to explain on the spot (often to someone who has never picked up a drone in their life), so below we have outlined the basic workflow behind the technology.
Setting out GCPs: Ground control points (GCP's) are intentional markers placed on the surface of the earth by the operator to where a known distance, or accurate GPS coordinate is known. These are often marked with a cross that will be clearly visible from the captured data once viewed in the processing stages.
Mapping data: All mapping projects start out with an autonomous flight that captures multiple amount of photographs along its course. These can range from as little as 20 images to thousands of images, depending on the area that is to be mapped. Images are taken at regular intervals with the drone travelling at a constant and steady speed. All variables like flight altitude, front and side overlaps and camera angles are set in the flight plan before launching the aircraft. For 2D Othomosaic maps, a side to side grid mission can be flown to gain enough data. For 3D modelling a 3D cross hatch style grid mission is required due to the added information needed from different angles to model the 3rd dimension.
Processing: Processing has many stages depending on the kind of output that is needed for the end user. All projects start with image alignment where the software will look for points in the images called 'tie points'. These are distinctive features within the images that the allows the software to recognise and 'tie' together. The images metadata is also used to determine the drones position and location in space at the time the image was captured. From here several steps are taken to create what is called a point cloud (pixels in space) and also to create the triangulation process which generates meshes and textures. These meshes and textures are what brings the project to life to add realistic features to the project.
Geo-referencing: This process is key to producing accurate and measurable data. The GPS metadata rooted in the image files is only as accurate as where the drone thought it was at the time of the shot. Most drones use GPS units that are generally accurate to within 5m. When we need to measure the supplied data, this tolerance is far from what we would deem adequate. This is why we Geo-reference our mapping data. This can be done in 2 ways:
1st - Using software with the ability to overlay a Google satellite image is our most common technique. We pick out key points on our 2D orthomosaic, and pick out the very same points on the overlaid Google map, at anywhere between 10-30 points depending on the project size. Our software will then accurately scale and adjust our map until is is in perfect sync with Google satellite imagery.
2nd - Although the second choice comes at an added fee, the data is a lot more accurate and can be classed as 'survey grade'. Going back to the use of GCP's: We would set out various GCP's in the field and have these locations recorded and logged by a chartered land surveyor. We would then input the recorded data into our software that would use the GCP's position on the map, and precisely correct this to the GPS data recorded by the land surveyor.
Delivery - Once processing and Geo-referencing are complete, we can now supply the data to our client. We can overlay measurements and save these onto the Orthophoto to clearly show various measurements. Alternatively we can supply the data in various CAD software readable formats for the end user to load and measure at their own will. 3D models can be uploaded and viewed from a web-based location allowing 3D viewing and measuring possible. 3D models also hold the capabilities for volumetric calculations to be taken. All Airmap Limited's processed data can be stored on a secure platform at an agreed annual fee. Airmap Limited only use desktop processing software for added client data security.
Feel free to contact the team to find out more...
Airmap Limited CEO