UgCS Photogrammetry Technique for UAV Land Surveying Missions
UgCS is easy-to-use software to plan and fly drone survey missions, it supports almost any UAV platform, providing convenient tools for areal and linear surveys and enabling direct drone control. UgCS enables professional land survey mission planning using photogrammetry technique.
How to plan photogrammetry mission with UgCS
Standard land surveying photogrammetry mission planning with UgCS can be divided in following steps :
Step one: Obtain data input
To reach the desired result, first - input settings have to be defined:
GSD and area boundaries usually are defined by the customer's requirements for output material parameters (for example by scale and resolution of digital map). Overlap should be chosen according to specific conditions of surveying area and requirements of data processing software.
Each data processing software (e.g., Pix4D, Agisoft Photoscan, Dronedeploy, Acute 3d) has specific requirements for side and forward overlaps for different surface. To choose correct values, please refer to documentation of chosen software. In general 75% forward and 60% side overlap will be a good choice. Overlapping should be increased for areas with small amount of visual cues - for example for deserts or forests.
Often aerial photogrammetry beginners are excited about the option to produce digital map with extremely high resolution (1-2cm/pixel) and use very small GSD for mission planning. This is a very bad practice, as small GSD will result in longer flight time, hundreds of photos for each acre, tens of hours of processing and heavy output files. GSD should be set according to output requirements of the digital map.
Other limitations can occur. For example, GSD of 10cm/pixel is required, but it is planned to use Sony A6000 camera. Based on mentioned GSD and camera's parameters the flight altitude would be set to 510 meters. In most countries maximum allowed altitude of UAV's (without special permission) is limited to 120m/400ft AGL (above ground). Taking in account maximum allowed altitude, the maximum possible GSD in this case could be no more than 2.3cm.
Step two: Plan your mission
Mission planning consists of two stages:
First step is to set surveying area using UgCS Photogrammetry tool. Area can be set using visual cues on underlying map or using exact coordinates of edges. The result - survey area is marked with yellow boundaries (see Figure 1).
Figure 1: Setting survey area using UgCS Photogrammetry tool
Next step is to set GSD and overlapping for camera in Photogrammetry tool's setting window (Figure 2).
Figure 2: Setting camera's Ground Sampling Distance and overlapping
To take photos, in Photogrammetry tool's setting window define the control action of camera. In example (Figure 3) Set camera by distance triggering action with default values is used.
Figure 3: Setting camera's control action
At this point, initial route planning is completed. UgCS will automatically calculate photogrammetry route (see Figure 4).
Figure 4: Calculated photogrammetry survey route before optimization
But in most cases the automatically calculated photogrammetry route will not be optimal and in some cases even dangerous to fly (for drone or surrounding). Therefore proceed to optimization stage.
To optimise the route, it's calculated parameters should be known: altitude, estimated flight time, number of shots, etc.
Part of route's calculated information can be found in Elevation profile window. To access Elevation profile window (if it is not visible on screen) click Parameters icon on the Route card (lower-right corner, see Figure 5) and from the drop-down menu select Show elevation:
Figure 5: Accessing elevation window from Route cards Parameters settings
Elevation profile window will present estimated route length, duration, waypoint count and min/max altitude data:
Figure 6: Route values in elevation profile window
To get other calculated values, open Route log by clicking on Route status indicator - the green check-mark (upper-right corner, see Figure 7) of the Route card:
Using route parameters, it can be optimized to be more efficient and safe.
Figure 7: Route card and status indicator, Route log
Survey line direction
By default UgCS will trace survey lines from south to north, but in most cases more optimal will be to fly parallel to the longest boundary line of survey area. To change survey line direction edit Direction angle field in Photogrammetry tool. In the example, by changing angle to 135 degrees - number of passes is reduced from five (Figure 4) to four (Figure 8) and route length is 1km instead of 1.3km.
Figure 8: Changed survey line angle to be parallel to longest boundary
UgCS Photogrammetry tool has the option to define how to trace route according to altitude - with constant altitude above ground (AGL) or above mean sea level (AMSL).
Please refer to your data processing software requirements which altitude tracking method it recommend.
UgCS Team's experience is that the choice of altitude type depends on desired result - for orthophotomap (standard aerial land survey output format) it is better to choose AGL to ensure constant GSD for entire map. If the aim is to produce DEM or 3D reconstruction, use AMSL so the data processing software has more data to correctly determine ground elevation by photos to provide more qualitative output.
Figure 9: Elevation profile with constant altitude above mean sea level (AMSL)
In this case UgCS will calculate flight altitude based on lowest point of survey area.
If AGL is selected in Photogrammetry tool's settings, UgCS will calculate altitude for each waypoint. But in this case terrain following will be rough if no “Additional waypoints” are added (see Figure 10).
Figure 10: Elevation profile with AGL without additional waypoints
Figure 11: Elevation profile with AGL with additional waypoints
Therefore, if AGL is used, add some “Additional waypoints” flags and UgCS will calculate flight plan with elevation profile accordingly (see Figure 11).
In general - if flight speed is increased it will minimize flight time. But high speed in combination with large camera exposure can result in blurred images. In most cases 10m/s is the best choice.
Camera control method
UgCS supports 3 camera control methods (actions):
Not all autopilots support all 3 camera control options. For example (quite old) DJI A2 does support all three options, but newer - starting from Phantom 3 and up to M600, support only triggering in waypoints and by time. DJI promised to implement triggering by distance, but it’s not available yet.
Here are some benefits and drawbacks for all three methods:
Table 1: A Benefits and Drawback for camera triggering methods
To select triggering method in UgCS Photogrammetry tool accordingly, use one of three available icons:
Most autopilots or multirotor drones support different turn types in waypoints. Most popular DJI drones have three turn-types:
It is advisable not to use Bank Turn for photogrammetry missions. Drone interprets Bank Turns as “recommendation destination waypoint” - the drone will fly towards this direction but will almost never pass through the waypoint. Because drone will not pass the waypoint - no action will be executed, meaning - camera will not be triggered, etc.
Adaptive Bank Turn should be used with caution because drone can miss waypoints - and again - no camera triggering will be initiated.
Figure 12: Illustration of typical DJI drone trajectories for Bank Turn and Adaptive Bank Turn types
Sometimes Adaptive Bank Turn type has to be used to have more short flight time comparing to Stop and Turn. When using Adaptive Bank Turns it is recommends to use Overshot (see next chapter) for the photogrammetry area.
Drones, e.g., DJI Phantom 3, Phantom 4, Inspire, M100 or M600, with integrated gimbal have the option to control camera position as part of automatic route plan.
It is advisable to set camera to nadir position in first waypoint and in horizontal position before landing to prevent lenses from potential damage.
To set camera position, select waypoint preceding the Photogrammetry area and click Set camera attitude/zoom (Figure 13) and enter "90" in the "Tilt" field (Figure 14).
Figure 13: Setting camera attitude
Figure 14: Setting camera position
As described previously, this waypoint should be a Stop&Turn type, otherwise the drone could skip this action.
To set camera to horizontal position - select last waypoint of survey route and click Set camera attitude/zoom and enter "0" in the "Tilt" field.
Initially overshot was implemented for fixed-wing (airplane) drones to have enough space maneuvering a U-turn.
Overshot can be set in photogrammetry tool to add extra segment to both ends of each survey line.
Figure 15: Adding 40m overshot to both ends of each survey line
In the example (Figure 15) can be seen that UgCS added 40m additional segments to both ends of each survey line (comparing to Figure 8).
Adding overshot is useful for copter-UAVs in two situations:
Figure 16: Example of blurred image taken by drone in rotation to next waypoint
It is important to check take-off area at site before flying any mission! To better explain best practice how to set Take-off point - first discuss an example how it should not be done. Supposing that the take-off point in example mission (Figure 17) would be from the point marked with airplane-icon and drone pilot would upload route on the ground with set Automatic mission for automatic take-off.
Figure 17: Take-off point example
Most drones in automatic take-off mode would climb to low altitude about 3-10meters and then fly straight towards first waypoint. Other drones would fly towards first waypoint straight from ground. Looking closely at the example map (Figure 17), some trees between take-off point and the first waypoint can be noticed. In this example, the drone more likely will not reach safe altitude and will hit the trees.
Not only surrounding can affect Take-off planning. Also the fact, that drone manufacturers can change drones elevation behavior in drone firmware, therefore after firmware updates it is recommended to check drones automatic take-off mode.
Also a very important consideration - most small UAVs use relative altitude for mission planing. Altitude counted relatively according to first waypoint is a second reason why actual take-off point should be near the first waypoint and on the same terrain level.
UgCS Team recommends to place first waypoint as close as possible to actual take-off point and specify safe take-off altitude (≈30m in most situations will be above any trees, see Figure 18). This is the only method that warrants safe take-off for any mission. It also protects from any weird drone behavior, unpredictable firmware updates, etc.
Figure 18: Route with safe take-off
Entry point to the survey grid
In previous example (see Figure 18) can be noticed, that after adding the take-off point, route's survey grid entry point was changed - because, if additional waypoint is added next to the Photogrammetry area, UgCS will plan to fly the survey grid starting from nearest corner to previous waypoint.
To change the entry point to survey grid, set additional waypoint close to the desired starting corner (see Figure 19).
Figure 19: Changing survey grid entry point by adding additional waypoint
If no landing point will be added outside photogrammetry area after survey mission, the drone will fly and hover in the last waypoint. There are two options for landing:
In situations when radio link with drone is lost, for example if the survey area is large or there are problems with Remote Controller. Depending on the drone and it's settings one of these actions can occur:
Recommendation is to add explicit landing point to route, to avoid relying on unpredictable drone behavior or settings.
If drone doesn't support automatic landing or pilot prefers to land manually, place route's last waypoint over planned landing point with altitude not only for comfortable manual drone descending and landing, but also above obstacles in surrounding area. In general 30m is best choice.
Photogrammetry tool has a magic parameter “Action Execution” with three possible values:
This parameter defines how and where camera actions specified for Photogrammetry tool will be executed.
Most useful option for photogrammetry/survey missions is to set Forward passes - drone will make photos only on survey lines, but will not make excess photos on perpendicular lines.
Complex survey areas
UgCS enables photogrammetry/survey mission planning also for irregular areas, having functionality to combine any number of photogrammetry area in one route, avoiding splitting the area in separate routes.
For example, if mission has to be planned for two fields connected in a shape of “T” and if these two fields are marked as one Photogrammetry area - the whole route will not be optimal regardless any direction of survey lines.
Figure 20: Complex survey area before optimization
If the survey area is marked as two photogrammetry areas within one route - survey lines for each area can be optimized individually (see Figure 21).
Figure 21: Optimized survey flight passes for each part of a complex photogrammetry area
Step three: deploy ground control points
Ground control points are mandatory if survey output map has to be precisely aligned to coordinates on Earth.
There are lot of discussions about necessity of ground control points in cases, when drone is equipped with Real Time Kinematics (RTK) GPS receiver with centimeter-level accuracy.
It is useful that RTK GPS enables to define coordinates of drone location with centimeter-level accuracy. But the drone coordinates are not enough, as for precise map aligning image center coordinates are necessary to be provided.
Data processing softwares like Agisoft Photoscan, Dronedeplay, Pix4d, Icarus OneButton and others will produce very accurate maps using geotagged images, but real precision of the map will not be known without ground control points.
Conclusion: ground control points have to be used to create Survey-Grade result. For a map with approximate precision it is sufficient to rely just on RTK GPS and capabilities of data processing software.
Step four: fly your mission
For carefully planned mission, flying it is the most straightforward step. Mission execution differs according to the type of UAV and equipment used, therefore it will not be described in detail in this topic (please refer to equipment's and UgCS documentation).
Important issues before flying:
Step five: image geotagging
Image geotagging is optional if ground control points were used, but almost any data processing software will require less time to process geotagged images.
Some latest and professional drones with integrated camera can geotag images automatically during flight, in other cases images can be geotagged in UgCS after flight.
Very important: UgCS uses telemetry log from drone, that is received via radio channel, to extract drone's attitude for certain moment (when pictures were taken). To geotag pictures using UgCS assure robust telemetry reception during flight.
Step six: data processing
For data processing use third party software or services available on the market.
From UgCS Team experience most powerful and flexible software is Agisoft Photoscan (http://www.agisoft.com/), but sometimes too much user input is required to get necessary result. Most uncomplicated solution for users is online service Dronedeploy (https://www.dronedeploy.com/) that can produce required results in most cases. All other software packages and services will fit somewhere between these two in terms of complexity and power.
Step seven (optional): import created map to UgCS
Should the need arise for the mission to be repeated in the future, UgCS enables to import GeoTiff file as map layer and use it for mission planning. More detailed instruction can be found in UgCS User Manual. See the result of imported map created using UgCS photogrammetry tool imported as GeoTiff file (Figure 22).
Figure 22: Imported GeoTiff map as layer. The map is output of a Photogrammetry survey mission panned with UgCS