No matter the scale, flight planning has always been key to any successful aerial photogrammetry/remote sensing project. The rules for UAS are no different. Additionally, a successful flight plan is characterized by knowing the specifications of the camera/sensor and having a UAS that is capable of flying with that sensor mounted.
Part of the planning process involves defining a number of specifications for the desired end-product before going into the field. Some basic considerations include:
- Image resolution: How much detail is desired? A smaller ground sample distance (GSD or pixel resolution) will have a higher resolution compared to a larger one. The flying height and focal length of a cameras sensor determine the pixel resolution.
- Image overlap and sidelap: Image overlap and sidelap are essential for the establishment of tie points that orient the images and create the 3D model. Traditional standards call for 60% overlap and 30% sidelap, however, UAS acquisition usually requires higher variations of both.
- Sun angle and weather: Reducing the amount of shadows is very important when capturing aerial imagery. Having the sun at it’s peak height in the sky minimizes shadows and allows the sensor to receive as much light as possible. Wind also plays an important factor. How powerful is your UAS? How powerful of wind gusts can in handle and still maintain a level position?
So you are ready to make a successful flight and have all of the flight planning figured out. The next question, however, is how geographically accurate should it be? If one is only interested in a product that is marginally accurate, say within 5-10 meters, than the GPS from the flight log file or camera would be sufficient.
For survey grade accuracy however, having a set of survey control points measured from a professional land surveyor is necessary. Control points are the most accurate way to ground truth any georeferenced product. Land surveying is expensive, however, so a product with an accuracy within 5-10 meters may be enough for some.
For flight planning, I use Mission Planner, sponsored by 3D Robotics. It is free to download and very easy to use. I have an ideal ground station by combining this software and the supplied telemetry antennas that came with the 3DR IRIS. With a few simple clicks, Mission Planner is able to display flight data in real-time through a series of interchangeable gauges.
The Flight Plan tool allows the user to plan varies autonomous missions and tasks by establishing GPS waypoints. For my project, I used the survey (grid) tool. Within minutes I had my area of interest (AOI) polygon, entered my camera specifications (the iPhone 5s’ focal length is 4.12 mm) and set my overlap and sidelap (70 and 60 percent).
On top of flight planning, Mission Planner also acts as the control station for the IRIS. By using the supplied telemetry kit, my laptop was able to send and receive data in real-time to the copter. A log file was also recorded storing all of the flight parameters.
The desired pixel resolution for this project was 1 cm. Considering the highest resolution from traditional manned aircraft missions averages between
1-3 inches, resolution to this degree from a consumer grade product like an iPhone is quite remarkable.
During the flight, the camera shutter was activated using an intervalometer feature from an app called CameraSharp. The app allows you to set the seconds interval at which you wish to snap photos. For this project, I went with one photo every second. I have yet to determine the exact amount of overlap I averaged, but considering the flying speed of the IRIS was well below the necessary limit for 70%, I’m guessing I achieved an overlap somewhere in the 75-80% range. I’ll post those results in a few months when my research is complete.
The flying height was set at 28 m to obtain a pixel resolution slightly better than 1 cm at the sites lowest point. The figure to the right illustrates the iPhone 5s and flight specifications.
Having a professional land surveyor measure ground control points (GCPs) is the best way to accurately ground truth any geospatial data. Thankfully, I have some friends who had experience and equipment to measure GCPs for me at the Lime Ridge site. The control was measured using a Trimble real time kinematic (RTK) device. The RTK provides real-time corrections of GPS information by way of a network base station enabling the measurements to be accurate with 1 cm.
Aerial targets were fabricated using some poster board that were cut 5 cm wide at the edges. Nine targets were placed around the site providing a sufficient ground truth throughout. The targets were measured during image processing.