Mission Planning Guide

Guide
Mission Planning for Aerial Imaging/Mapping
Mission Planning
In this section:
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Overlap requirements
Camera orientation considerations
Camera cycle time considerations
Recommended GeoSnap settings
Mission Planner example
DJI Ground Station example
Mission Planning
Overlap requirements
Overlap between images is a very important consideration in mission planning. Overlap can be
split into two types: overlap (vertical overlap) and sidelap (horizontal overlap), and is typically
measured in percent of an image.
Overlap
Sidelap
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. Agisoft Photoscan Pro recommends 80% overlap and 60% sidelap.
Camera orientation considerations
It is recommended, if at all possible, 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
within 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 intervals, you have to fly at a speed at which the camera
can keep up. The equation to use is:
𝑀𝑀𝑀𝑀𝑀𝑀𝑀 𝑔𝑟𝑟𝑟𝑟𝑟 𝑠𝑝𝑝𝑝𝑝 [𝑚/𝑠] =
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𝑇𝑇𝑇𝑇𝑇𝑇𝑇 𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑 𝑖𝑛𝑛𝑛𝑛𝑛𝑛𝑛 [𝑚]
𝐶𝑎𝑚𝑚𝑚𝑚 𝑐𝑦𝑦𝑦𝑦 𝑡𝑖𝑖𝑖 [𝑠]
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Mission Planning
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).
It is also desirable to have as few excess images as possible (excess images = more time to
transfer/sort/process). To help with this, you can limit when the GeoSnap triggers the camera
based on attitude (found in the Trigger Limiters section of the CONFIG file). For instance, if you
set the Roll Limit to 30 degrees, the GeoSnap won’t trigger the camera when the camera is at a
roll angle of 30 degrees or greater. This keeps the system from taking images during turns, etc. It
is recommended to set the pitch and roll limits to values that make sense for your setup (you
will refine this as you perform more missions).
You can also limit triggering based on altitude so that the GeoSnap won’t trigger the camera
below the specified altitude. While this does help eliminate some excess images, it is
recommended that you do not use the altitude limiting feature, at least initially. This is due to
the fact that, with the altitude limiting enabled, there is no way to check before taking off that
the GeoSnap is triggering as you think it should (sometimes the wrong setting gets selected in
the CONFIG file, or something similar). And, as it is better to have a few extra images than not
get any images at all, it is best, especially while learning the system, to not use the altitude
limiting feature.
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Mission Planning
Mission Planner example
If you are using an Ardupilot-based system, or don’t have a mission planning tool for your
autopilot, we would recommend using 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
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Mission Planning
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 nformation,
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 the flight plan, you also need to set the Flying speed (m/s) on the Simple tab,
which will be a function of the trigger distance and the camera’s cycle time as described in the
Camera cycle time considerations section. So, 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).
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.
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Mission Planning
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 speed [H Speed(m/s)], which will be a
function of the trigger distance and the camera’s cycle time as described in the Camera cycle
time considerations section. So, 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.
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