UgCS Photogrammetry tool for UAV Land Survey Missions
- UgCS Photogrammetry tool for UAV Land surveying missions
- Main UAV advantages compared to traditional survey solutions
- Flight planning workflow in UgCS
- Step one: Specify the desired result
- Step two: import accurate map and elevation data to UgCS (optional)
- Step three: Plan your mission
- Step four: deploy ground control points (optional)
- Step five: fly your mission
- Step six: PPK processing and image geotagging (optional)
- Step seven: data processing
Unmanned Aerial Vehicle (UAV) technology revolutionizes mapping services and helps generate accurate orthomosaics, point clouds, and elevation models faster. Drones and cameras evolve amazingly quickly, however, the accuracy and precision of resulting products strongly depend on the flight planning software.
- Faster data acquisition. With real-time kinematic (RTK) and post-processed kinematic (PPK) technologies, there is no need to place ground control points (GCP) for many applications
- UAV equipment is cheaper than traditional land survey equipment
- Drones can approach areas unreachable by foot or by car
UgCS by SPH Engineering is a globally known UAV mission planning and flight control software solution. UgCS supports almost any UAV platform, provides convenient tools for areal and linear surveys, and enables direct drone control. UgCS ensures professional land survey mission planning using the photogrammetry technique.
- Specify the desired result
- Import accurate map and elevation to UgCS (optional)
- Plan your mission
- Deploy ground control points (optional)
- Fly your mission
- Create image geotags (optional)
- Process data
Define input settings first:
- Required GSD (ground sampling distance)
- Survey area boundaries
- Required forward and side overlap
GSD is the spatial resolution of a sensor, it is the distance measured in centimeter units between the centers of two neighboring pixels in the image on the ground surface. GSD and area boundaries usually are defined by customer requirements for output material parameters (for example, digital map scale and resolution).
GSD affects the level of detail on acquired data. Below are examples of how a car may look like in pictures with different GSDs
The pilot can either draw survey boundaries manually in UgCS or import from a KML, which will be explained later.
The overlap should be chosen according to specific conditions of the survey area and requirements of drone data processing software.
By default, 60% forward and 30% side overlap are the minimum recommended settings. Overlap can be increased for areas with vertical surfaces or high uniformity of visual surfaces (desert, snow).
Very often aerial photogrammetry newbies are excited about the option to produce a digital map with extremely high resolution (1-2 cm/pixel) and use very small GSD for mission planning. This is very bad practice, as small GSD means longer flight time, hundreds of photos for each acre, tens of hours of processing, and heavy output files. GSD should be set according to the output requirements of the digital map.
Other limitations may also apply. For example, a GSD of 10 cm/pixel is required, but it is planned to use a Sony A7 III camera with a 24 MP sensor. To meet GSD requirements, the drone has to fly at 510 meters above the ground. The problem is that in most countries maximum allowed UAV altitude (without special permission) is limited to 120 m/400 ft AGL (above ground level). Taking into account maximum allowed altitude, the maximum possible GSD, in this case, could be no more than 2.3 cm.
By default, UgCS provides access to a variety of online tiles services and SRTM elevation data. The pilot may also have orthomosaics and/or elevation data from surveys or satellite data providers. If that’s the case pilot can upload this data into UgCS and make more accurate flight planning.
Open Map layers to upload a map or elevation data (Figure 1).
Figure 1 Open Map layers to upload a map or elevation data into UgCS
Open Map and Elevation tabs to Add new sources and Upload orthomosaics (GeoTiff) or elevation data (Figure 2).
Figure 2 Adding new sources and uploading orthomosaics (GeoTiff) or elevation data to UgCS
Video 1 Import of accurate map and elevation data into UgCS
Mission planning consists of two stages:
- Route validation and tuning
Video 2 Basics of the photogrammetry mission planning with UgCS
The first step is to specify your drone survey area using the Photogrammetry tool: put visual cues on the underlying map or input exact coordinates of edges. The resulting survey area is marked with yellow boundaries (Figure 3).
Figure 3 Setting survey area using UgCS Photogrammetry tool
Another option is to import area boundaries from KML: add a new route, select “Import from file”, and then select using Photogrammetry for all LineRing objects in the KML file (Figure 4).
Figure 4 Importing KML to use the boundaries to set drone survey area
The next step is to set GSD and overlap for the camera in the Photogrammetry tool's setting window (Figure 5).
Figure 5 Setting camera's Ground Sampling Distance and overlapping
To take photos, set the camera control action in the Photogrammetry tool's setting window. In the example (Figure 5), Set camera by distance triggering action with default values is used.
At this point, initial route planning is completed. UgCS will automatically calculate the photogrammetry route.
But in most cases, the automatically calculated photogrammetry route will not be optimal and in some cases even dangerous for drones or surrounding. Therefore, proceed to the next steps.
UgCS generates a trajectory that will be uploaded to the drone. The pilot can see it as a green 3D polyline and check other parameters of the calculated flight plan.
The first part of this information can be found in the Elevation profile window. To open the Elevation window (if it is invisible on the screen), click Parameters icon on the Route card (lower-right corner, see Figure 6) and select Show elevation from the drop-down menu:
Figure 6 Accessing Elevation window from Route cards Parameters settings
The elevation profile window will present estimated route length, duration, waypoint count, and min/max altitude data (Figure 7).
Figure 7 Route values in the elevation profile window
To display other calculated values (Figure 8), open Route log by clicking on Route status indicator - the green check-mark (upper-right corner, see Figure 6) of the Route card.
Figure 8 Route card and status indicator, Route log
To optimize the performance and improve the efficiency and safety of the drone, use parameters of the route.
UgCS Photogrammetry tool has the option to define route tracing method according to altitude - with constant altitude above ground (AGL) or above mean sea level (AMSL).
Please refer to your UAV data processing software requirements which altitude tracking method it recommends.
UgCS Team's experience shows that the choice of altitude type depends on the desired result - for the orthophoto map (standard aerial land survey output format) it is better to choose AGL to ensure constant GSD for the entire map. If you want to produce DEM or 3D reconstruction, use AMSL to provide the data processing software with more data to correctly determine ground elevation by photos and obtain a higher quality output.
Figure 9 Elevation profile with constant altitude above mean sea level (AMSL)
In this case, UgCS will calculate flight altitude based on the lowest point of the survey area (Figure 9).
If AGL is selected in Photogrammetry tool settings, UgCS will calculate altitude for each waypoint but in this case, terrain-following will be rough if no “Additional waypoints” are added (Figure 10).
Figure 10 Elevation profile with AGL without additional waypoints
Therefore, if AGL is used, add some “Additional waypoints” flags, and UgCS will calculate the flight plan with elevation profile accordingly (Figure 11).
Figure 11 Elevation profile (AGL) with additional waypoints
By default, UgCS will generate forwarded passes oriented along the longest side of the area. To change this default setting, a pilot may change the Direction angle field of the Photogrammetry tool and use a rotating arrow on the polygon itself to adjust the same parameter (Figure 12, 13).
Figure 12 Direction angle = 246 degrees from North
Figure 13 Changed survey line angle to be parallel to the longest boundary.
New direction angle = 328 degrees from North
Most autopilots or multirotor drones support different turn types in waypoints. DJI drones, which are one of the most popular, have two turn types (Figure 14):
- Stop and Turn: drone flies to the fixed point accurately, stays at that fixed point, and then flies to the next fixed point.
- Adaptive Bank Turn: the performance is similar to Bank Turn mode (Figure 14) but the real flight routine will be more accurate.
Adaptive Bank Turn should be used with caution because drones can miss waypoints and no camera triggering will be initiated.
Figure 14 Typical DJI drone trajectories for Bank Turn and Adaptive Bank Turn types
The Adaptive Bank Turn type may be used to shorten the flight time compared to the Stop and Turn. Combining the Adaptive Bank Turns with Overshot (see the Overshot section) is recommended for the photogrammetry area.
Initially overshot was implemented for fixed-wing (airplane) drones to have enough space for maneuvering a U-turn.
Overshot can be set in the Photogrammetry tool to add extra segments to both ends of each survey line.
Figure 15 Adding 40 m overshot to both ends of each survey line
Figure 15 shows that UgCS added 40 m segments to both ends of each survey line (comparing to Figure 13).
Adding the overshot is useful for copter UAVs in two situations:
- When Adaptive Bank Turns are used (or a similar method for non-DJI drones), adding the overshot will increase the chance that the drone will precisely enter the survey line and camera control action will be triggered. UgCS Team recommends setting the overshot to be approximately equal to the distance between parallel survey lines.
- When Stop and Turn type is in use in combination with an action to trigger the camera in waypoints, there is a possibility that before making the shot, the drone will start rotation to the next waypoint, thus resulting in wrongly oriented or blurred photos (Figure 16). To avoid that, set shorter overshot, for example, 5 m. Don’t set too short a value (< 3 m) because some drones could ignore waypoints that are too close.
Figure 16 Example of a blurred image taken by a drone in a rotation to the next waypoint
UgCS supports 3 camera control actions:
- “Set camera mode” to make the shot exactly in a specified point
- “Set camera by the time” to make shots every N seconds
- “Set camera by distance” to make shots every N meters
Here are some benefits and drawbacks of all three methods:
Doesn't require a lot of additional waypoints and has quite good precision.
Precision depends on the selected turn type (see Turn type preference below) and distance calculation algorithm of certain autopilot
The only method that takes shots in the planned locations.
Requires a lot of additional waypoints. Many DJI drones have a limit of 99 waypoints. Keep this in mind
Doesn't require a lot of additional waypoints
The precision of this method is hard to predict because it depends on UAV’s actual speed, which depends on the wind, temperature, payload weight, acceleration/deceleration, etc.
In most cases for a photogrammetry survey camera shall look straight down. Please make sure that the “set camera” attitude action with a 90-degree tilt angle is selected for the photogrammetry segment (Figure 17).
Figure 17 Setting the camera's attitude and position with a 90-degree tilt angle
For urban areas with vertical surfaces, we recommend making another pass over the areas with the camera tilted at 45 degrees (Figure 18). That will enable to capture of the walls and increase the quality of the point cloud.
Figure 18 Setting the camera's attitude and position with a 45-degree tilt angle
As described above, this waypoint should be set as Stop&Turn type, otherwise, the drone may skip this action. To set the camera to a horizontal position, select the last survey route waypoint, click Set camera attitude/zoom, and enter "0" in the "Tilt" field.
The photogrammetry tool has a magic Action Execution parameter with three possible values:
- Every point
- At start
- Forward passes
This parameter defines how and where camera actions specified for the Photogrammetry tool will be executed.
The most useful option for photogrammetry/survey missions is to set Forward passes: drones will only take photos on survey lines and will not take excess photos on perpendicular lines.
Complex survey areas
There may be a situation when the photogrammetry/survey mission has to be planned for irregular areas. For example, if two fields connected in a “T” shape are marked as one Photogrammetry area, the route will not be optimal regardless of any survey line direction (Figure 19).
Figure 19 Complex survey area before optimization
UgCS functionality enables you to combine any number of photogrammetry areas in one route, avoiding splitting the area into separate routes. Survey lines for each area can be optimized individually (Figure 20).
Figure 20 Optimized survey flight passes for each part of a complex photogrammetry area
Double grid is a very efficient way of increasing the point cloud quality in urban areas and areas with complex elevation or high vegetation. Use Double grid option in photogrammetry settings (Figure 21).
Figure 21 Double grid option enabled in photogrammetry settings
Flying at a constant altitude above ground level will lead to excessive generation of waypoints. The waypoint capacity of the autopilot is a limiting factor so the pilot may want to reduce this number. Reducing the trajectory level of details will also result in smoother drone movements (compare Figure 22 and Figure 23). AGL tolerance parameter of the photogrammetry survey can be used for AGL altitude mode. AGL tolerance specifies vertical corridor. As long as the trajectory is within the upper and lower limits of this corridor, no additional points are generated by UgCS.
Figure 22 AGL Tolerance = 2 m
Figure 23 AGL Tolerance = 10 m
Generally, the higher the flight speed, the less is the flight time. But high speed in combination with large camera exposure can result in blurred images. In most cases, 10 m/s is the best choice.
It is important to check the take-off area at the site before flying any mission! To better explain the best practice of setting Take-off point, let's first review an example of how it should not be done. The take-off point in the example mission (Figure 24) is marked with an airplane icon, and the drone pilot will upload the route on the ground by setting an Automatic mission for automatic take-off.
Figure 24 Take-off point example
Most drones in automatic take-off mode would climb to a low altitude of about 3-10 meters and then fly straight towards the first waypoint. Other drones would fly towards the first waypoint straight from the ground. Looking closely at the example map (Figure 24), some trees between the 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. Drone manufacturers may change elevation behavior in drone firmware, therefore, it is recommended to check the drone's automatic take-off mode after firmware updates.
It is also very important to note that most small UAVs use relative altitude for mission planning. Altitude counted relatively according to the first waypoint is the second reason why the actual take-off point should be near the first waypoint and on the same terrain level.
UgCS Team recommends placing the first waypoint as close as possible to the actual take-off point and specify safe take-off altitude (~30 m will be above any trees in most situations, see Figure 25). This is the only method that guarantees safe take-off for any mission. It also protects against any weird drone behavior, unpredictable firmware updates, etc.
Figure 25 Route with safe take-off
In the previous example (Figure 25) you can notice that after adding the take-off point, the route survey grid entry point was changed because, if an additional waypoint is added next to the Photogrammetry area, UgCS will plan to fly along the survey grid starting from the nearest corner to the previous waypoint.
To change the survey grid entry point, set an additional waypoint close to the desired starting corner (Figure 26).
Figure 26 Changing survey grid entry point by adding a waypoint
If no landing point is added outside the photogrammetry area after the survey mission, the drone will fly and hover in the last waypoint. There are two landing options:
- Take manual control over the drone and fly to the landing point manually
- Activate the Return Home command in UgCS or from Remote Controller (RC)
If a radio link with a drone is lost, for example, if the drone survey area is large or there are problems with Remote Controller, then one of the following can occur depending on the drone and its settings:
- The drone will return to the home location automatically if the radio link with the ground station is lost
- The drone will fly to the last survey area waypoint and hover. If the battery charge level is sufficient then:
- the drone will perform emergency landing or
- the drone will try to fly to the home location
We recommend adding an explicit landing point to the route to avoid relying on unpredictable drone behavior or settings.
If a drone doesn’t support automatic landing or the pilot prefers manual landing then place the last route waypoint over the planned landing point with an altitude that ensures comfortable manual drone descending and landing and is above obstacles in the surrounding area. In general, 30 m is the best choice.
Previously this step was mandatory if the survey output map should be precisely aligned to coordinates on Earth. Now it’s up to the pilot and the customer whether to deploy ground control points or use just PPK. However, GCP is still the only way to ensure accuracy.
Data processing software like Agisoft Metashape, Pix4d, etc. will produce very accurate maps using geotagged images but you will never know the real precision of your map without ground control points.
Ground control points should be used to create Survey-Grade results. If you need a map with “good enough” precision, you may rely just on RTK GPS and data processing software capabilities.
This is the most straightforward step for a carefully planned mission. Mission execution will not be described here, as it may vary depending on the type of UAV and equipment in use (please refer to equipment and UgCS Manuals). Important notes before flying:
- In most countries, there are more or less strict regulations for UAV usage. Always comply with the regulations! Usually, these rules can be found on the local aviation authority website.
- In some countries, special permission for any kind of aerial photo/video shooting is required. Please check local regulations (fines and other consequences can be costly).
- Even if you have created an accurate plan in your office, always validate it in real conditions
Image geotagging is optional for modern drones, as many professional drones with an integrated camera can geotag images automatically during the flight. In other cases -after landing, the images can be geotagged in UgCS.
Also keep in mind PPK processing, which is sufficient in most cases. Use your favorite tool, for example like this.
However, only ground control points ensure the desired accuracy.
For data processing use third-party software or services available on the market. For fast processing in the field, it is recommended to use UgCS Mapper, as it works without an internet connection. For flight fidelity processing Pix4D Mapper, Agisoft Metashape, or other professional tools should be used, which support dense point cloud generations and GCPs.
The article is written in collaboration with Filippo Fiaschi ILERON, sharing professional experience on using UAVs for land surveying and photogrammetry technique.