Guide Mission Planning for Aerial Imaging/Mapping
In this section: Overlap requirements Camera orientation considerations Camera cycle time considerations Recommended GeoSnap settings Mission Planner example DJI Ground Station example
Overlap requirements Overlap between images is a very important consideration in mission planning. Overlap can be split into two types: overlap (along-flight-line overlap) and sidelap (between flight-line overlap), and is typically measured in percent of an image. Sidelap Overlap When it comes to stitching the imagery collected in an aerial imaging mission, most photogrammetry software has fairly high requirements for percent overlap and percent sidelap of the images. For aerial mapping missions, Agisoft Photoscan Pro recommends 80% overlap and 60% sidelap. Camera orientation considerations It is recommended to orient the camera in the aircraft so that the top or bottom of the camera is pointed in the direction of flight (landscape orientation). This allows you to fly less flight lines to cover an area since the smaller dimension of the field of view is in the flight line. Camera cycle time considerations When using the GeoSnap to trigger your camera at a distance interval, the maximum ground speed that should be allowed during your flight is a direct function of your desired trigger distance interval and the cycle time of your camera. Every camera has a limit on how fast it can take pictures (ranging from sub-second to over 4 seconds, depending on the camera), so, if you are trying to take images at consistent distance intervals, you have to fly at a speed at which the camera can keep up. The equation to use is: Maximum ground speed [m/s] = Trigger distance interval [m] Camera cycle time [s] The cycle time of a camera is dependent on its buffer size, processor write speed, image size, SD card write speed, etc. To figure out the cycle time of your camera, set the GeoSnap to time triggering and set the time interval to a reasonable guess for the camera s cycle time (for most DSLR-style cameras, a good starting point is 0.8 sec). Point the camera at a complex scene (outside the window works well) as this can affect the file size of the jpegs, and thusly the cycle 2
time of the camera. Start time triggering with the GeoSnap and let the camera trigger for several minutes until you are confident that it can keep up with the commanded cycle time. If the camera starts lagging or missing images, try increasing the time interval. If the camera is keeping up, try reducing the time interval. Continue this testing until you are fairly confident you have found the camera s fastest cycle time at which it can sustain triggering indefinitely. We then recommend doing a long test (at least the length of your mission) to make sure that the camera keeps up over the mission length. Recommended GeoSnap settings For the most efficient mapping mission, you want the images taken at evenly spaced intervals so you get consistent overlap. To accomplish this with the GeoSnap, it is recommended that the distance triggering option be used. With a distance-based trigger interval set, the GeoSnap will command the camera to take an image every time the aircraft travels the specified number of meters. To set this up, in the CONFIG file, under the Triggering Options section, set the Trigger Mode to distance (option 2) and set the Trigger Distance (m) to your desired trigger distance (you will get this from the flight planning stage it will be covered in the next sections). 3
Mission Planner example If you are using a Pixhawk/APM autopilot, or don t have a mission planning tool for your autopilot, a good option is to use Mission Planner for your mapping mission flight planning (it is a free download). Within Mission Planner there is a camera grid flight plan tool (found in the Flight Plan tab by creating a polygon then right-click > Auto WP > Survey (Grid)). Under the Simple tab, you can set your desired AGL altitude for the mission (you also need to make sure the Advanced Options box is checked to access the Grid Options and Camera Config tabs). Under the Grid Options tab, you can set your desired overlap and sidelap. Under the Camera Config tab, you can set the focal length, image dimensions, and sensor dimensions of your camera (these parameters can be found by looking up the camera s specifications online). You can also load in a Sample Photo to auto populate several of the fields. From this information, Mission Planner will automatically create the flight plan with the correct flight line spacing to get your specified sidelap. (NOTE: If your camera is set up in a portrait orientation, you will have to uncheck the box on the Simple tab that says Camera top facing forward.) Mission Planner also calculates the trigger distance interval necessary to obtain your specified overlap and this can be found under the map at Distance between images. In this example, it is 14m. So, in the case shown in the screenshot, you would input 14 into the Trigger_dist_m field in the GeoSnap CONFIG file. Before generating a flight plan, you also need to set the Flying speed (m/s) on the Simple tab. For example, with a camera cycle time of 0.8sec, and a trigger distance of 14m, the max speed allowed should be (14/0.8)=17.5m/s. So, we would input 17 into the Flying speed field here. (CAUTION: This value may be different for a fixed wing setup fixed wing aircraft introduce other considerations such as stall speed and the effect of wind on ground speed). 4
DJI Ground Station example If you are using a DJI aircraft/autopilot, it is recommended that you get their DJI Ground Station software for a computer. Within the software there is a Photogrammetry Tool (under Toolbox > Photogrammetry Tool) for planning mapping missions. Within this tool, you can input the focal length and sensor height and width of your camera [Focal Length (mm), Sensor H(mm), and Sensor W(mm)], the AGL altitude at which you want to fly [Fly Height(m)], and your desired overlap and sidelap [H Overlap(%) and W Overlap(%)]. The ground station will then automatically create the flight plan for you with the correct flight line spacing, and calculate your trigger distance to input into the GeoSnap CONFIG file. (NOTE: this is assuming a landscape orientation of the camera with respect to the direction of flight. If your camera is set up in a portrait orientation, you will have to switch your H and W values for the sensor.) This trigger distance value can be found as the Shooting Distance(m). So, in the case shown in the screenshot, you would input 14 into the Trigger_dist_m field in the GeoSnap CONFIG file. Before generating the flight plan, you also need to set the flight speed [H Speed(m/s)]. For example, with a camera cycle time of 0.8sec, and a trigger distance of 14m, the max speed allowed should be (14/0.8)=17.5m/s. So, we would input 17 into the H Speed field here. It is also recommended to set the Turn Mode to Adaptive_Bank_Turn. 5