One of the key points in drone mapping is accuracy. Having an accurate map is crucial for further analysis and for projection. Keep in mind that drone technology, like other emerging technologies is not a complete solution for the land surveyor. There are times when drone surveying can eliminate repetitive and time-consuming data gathering processes, while on the other hand the drone is not a good choice for certain applications. Several factors such as wind, rain, temperature, vegetation and even shadows can make the drone a poor choice. However, before we move on to solving the accuracy question, let’s take a brief look over the working mechanism of drone mapping.
A rough sketch of the workflow looks like this. Drones have a camera which can be pointed directly at the ground surface. You plan the flight beforehand, using special software so that the drone covers the whole working area. The drone’s onboard camera takes pictures during the mission. Multiple photos of each feature are taken, as frequently as desired. The goal is to produce an orthomosaic map afterward. An orthomosaic map is sort of a puzzle, made from high resolution photos of the area stitched together and geometrically corrected. The rest lies in post-processing.
The only question is, how to stitch the photos together accurately relative to the Earth’s surface and to the objects in the photos? This is where ground control points help.
A ground control point, or GCP, is to help you align the aerial map and position it accurately relative to the state plane coordinate system. To create an accurate orthomosaic map, you will generally need from four to ten GCP’s, depending on the landscape and size of the site. The final map’s accuracy is, among other things, dependent upon accurately surveyed GCP’s. The GCP’s are surveyed and marked before the flight
Some recommendations for creating good ground control points:
1) Ideally, they should be shot with a robot and levelled. GCP’s shot with RTK should be set with 3-to-5-minute observations and then levelled.
2) Use a bipod to ensure the rod remains still while recording the point.
3) If painting the GCP, make sure all the marks are the same size and shape with a clearly identifiable center. Commercial targets will be provided to aid in this manner.
4) GCPs should be spread out evenly, usually in a 100-150 ft. grid, with a minimum of 5, in each corner of the site and in the center area. GCP density depends on the surface relief. If there’s a big change in elevations within the site, set GCPs more often to make sure the elevation changes are considered. If it is safe to access, place GCPs on the lowest and highest points of the surveyed site.
There are three software applications used to get from start to finish for a given drone mapping project.
The first is mission planning software. “Dronelink” is one of many examples of flight planning software available for drone mapping. For the desktop application, Dronelink allows the user to create a project from which a flight mission can be created. This work is best done from the desktop application and will automatically program the drone to make the necessary flight pattern around a given site, set the altitude, camera settings, launch site location and drone speed. It will also be useful to determine if there are any tall objects or obstructions in the flight path.
The second software requirement is the Dronelink phone app. Once the mission completed and saved with the desktop app, any user with an Android cell phone or iPhone can use the Dronelink phone app in the field and load the previously created mission. Next, the drone gets placed at the launch site, turned on and it will automatically fly its preplanned mission with the press of a button. Very limited user skill is required to fly a pre-planned drone mission. All the image data is stored on an on-board SD card which can be downloaded later for processing.
The last phase of the project involves post processing. Again, there are many software applications which offer photogrammetry post processing. For our purpose, we already have a highly rated application from within our Civil3d subscription package called Autodesk ReCap Photo. The process involves uploading all the imagery, defining the state plane values of all the control points, and digitizing the control points which appear in each image so the software can align, pull, and stretch the images to result in a fully rectified orthomosaic image on the state plan coordinate system. The resulting image yields amazing detail and accuracy. Additionally, the software post processing will also create a 3d model of the site which can be used to create contours.
Well, first and foremost one needs a drone. Although there are many commercial selections, one quadcopter has dominated the mapping market. The DJI Phantom 4 is a commercial aircraft that provides great camera imagery, and great flight stability given its larger size and weight. Average flight time is under 30 minutes on one battery, so multiple batteries and flights will be required on many missions. As with any piece of survey equipment, there are many accessories that are required before the first flight. Battery management is also crucial to successful drone mapping.
Pilot Requirements
Since drone mapping requires a larger size commercial drone, the commercial drone pilot is required by the FAA to possess a Remote Pilot Certification, commonly known as “Part 107 certification”. To receive the Part 107 certificate, the drone operator is required to pass a test by the FAA. This is a multiple-choice test with approximately 60 questions. Questions cover pilot safety procedures, operation over human beings, weather, aviation map reading. Don’t be fooled by the multiple-choice format as the FAA makes the choices very tricky! Studying for the Part 107 test is required. To start the process, one needs to create an account with the “Integrated Airman Certification and Rating Application” (IACRA) The website is here. Once you have an account established, you can study and then schedule a test at one of the several FAA testing locations.