Drone mapping and surveying is the acquisition of hundreds or thousands of images from UAVs flown along a specific path to create a large areal map. Drone mapping offers huge benefits compared to the methods that were previously used for aerial surveys acquired by satellite or airplane. It’s cheaper, not limited by cloud cover and delivers more frequent, up-to-date data.
In this article, I’ll go through what the drone mapping process involves, handy tips, terms you need to know, equipment and, in the next article, UAVs commonly used for drone surveying.
By the end of this article, you will know whether you want to attempt drone mapping or outsource it to a drone surveying company.
What is drone mapping with remote sensing?
Drone mapping with remote sensing is the use of UAVs to photograph the terrain. Remote sensing can include many types of data collection such as multispectral information for heat stress of plants, LIDAR, temperature data and radiation.
For more on the different types of remote sensing such as NIR, multispectral and thermal take a look at this post on Drone Imaging.
Drone Mapping v Drone Surveying
The difference between drone mapping and drone surveying is accuracy. Drone surveying uses ground control points (GCPs) and checkpoints. These are surveying tools that mark a coordinate on the ground which is identified in drone photos to correct the location. Using GPS alone isn’t accurate enough for drone surveying.
Survey drone maps are more accurate, they’re designed to be used for taking measurements and calculate areas for measure stockpiles etc. Drone surveying uses RTK (real time kinematic) or PPK (post processing kinematic) which are GPS correction technologies that can be used without ground control points.
Drone mapping is a more general term meaning the use of drone photos that have coordinate metadata from the location they were captured. These photos are then stitched together with drone mapping software to create one big map.
RTK vs PPK : Which is better?
RTK corrects the location in-flight, as the ‘real time’ part of the name suggests. PPK corrects the data after it is all collected and uploaded for processing. While the time saving benefit of RTK may seem like the better option, it depends on the location that the data is being acquired.
This is because RTK requires a constant connection, if the location that the drone is being flown has a lot of interference (buildings, cell towers etc) then PPK is a better option.
What is drone mapping software?
Drone mapping software is used to turn drone photos into a variety of 2D maps and 3D models. Drone mapping software is made for different purposes so most basic software won’t process all data types.
Some common drone mapping software programs are:
- Drone Deploy
- DJI Terra
- Pix4D
Pix4D is a good example of fit-for-purpose software. Pix4Dreact and Pix4Dmapper are two packages aimed at different purposes. Pix4Dreact is designed for first responders that use drones for search and rescue. This software produces super quick orthomosaic maps from drone photos used in emergency response situations like post-earthquake rescue missions.
Pix4Dmapper is more advanced. It produces 3D models and 2D surveys with higher accuracy. Pix4Dmapper drone software is also compatible with more data types such as thermal and multi-spectral.
What is the drone mapping process?
The drone mapping process involves 3 main steps; planning the flight path, setting up and surveying the area, and post-flight processing.
The flying component of drone mapping is generally automated using drone mapping software.
These are the common steps in the drone mapping process:
1. Select the mapping software
Some good examples are Drone deploy, DJI Terra and Pix4D. The software needs to be compatible with the data type you are collecting.
2. Design the flight path
Drone surveys will need a few additional measures like ground control points and RTK base stations.
Setting up the flight plan for surveying includes:
- Set the Altitude: Drone mapping is generally done at a fixed height above ground level. The altitude is important because it affects the resolution of the photos.
- If using Drone Deploy the resolution is selected in inches per pixel, this level of detail is set by a sliding bar that interactively gives the altitude required. The closer to the ground the drone flies the higher the resolution (greater detail).
- However, there are some issues with setting the altitude too low such as obstacles in the flight path, increasing flight time and affecting accuracy.
- Set waypoints: these are the GPS coordinates along the flight path that tell the drone where to fly and the order.
- Program the overlap: overlap is the amount that the drone photos overlap when stitched together. This tells the drone how frequently to take photos. The recommended minimum is 70% for front overlap and 60% for side overlap.
- Camera tilt: For orthomosaic photos, the camera is programmed to tilt 90 degrees (directly under the drone) once it has reached the required altitude.
4. Pre-Flight Checks
- Check the battery is charged and extra batteries are packed.
- Make sure the SD card has enough storage for the mission (usually around 100GB of available space is enough but this depends on lots of factors).
- Test and set the focus and camera settings manually if needed to get better quality photos before take-off.
5. Deploy the drone keeping visual line of sight
Generally this is the easy step if all of the set up has been well planned.
Always keep visual line of site and be prepared to take manual control in case of technical fault or emergency. There are 3 options if the drone mapping flight needs to be interrupted:
- Stop the drone in place and hover.
- Return home; This sets an automatic flight path for the drone to send it straight back to the start location.
- Activate manual control. Abandon the automated flight path and steer the drone out of harm’s way.
6. Download the photos to a tablet or computer
The photos can usually be stored offline if you’re in a remote area and later uploaded to cloud storage.
What is the output for drone mapping?
The raw output after a drone mapping survey is a folder of photos with coordinates in their metadata. These are imported to a mapping software that lays them out in 2D with a point at the centre of each image showing where the drone was when each photo was taken.
You’ll also need to import the ground control point data coordinates which can just be a simple text file, this also contains your control points so these need to be specified as the correct type when importing.
Then select the centre of each control point in the photos to correct their location. Each ground control point will be in multiple photos but you don’t need to correct every single one, about 10 will do.
Drone mapping tips
- Don’t rely on obstacle avoidance:
- Keep the drone within visual line of sight; The camera will be set to point at the ground so the drone pilot won’t be able to see unexpected obstacles from FPV like birds.
- Minimum Obstacle Clearance Avoidance (MOCA); When setting the altitude for the flight path consider trees, power lines, buildings and any other objects. Even if the flight path is programmed to avoid these, in an emergency or malfunction the ‘Return Home’ feature can be activated meaning the planned route is abandoned and the drone flies back to the deployed location regardless of obstacles in the way.
- Weather: If you have some flexibility on when the drone survey is acquired, choose a day when the weather is cloudy. This minimizes sharp shadows and gives an even white balance, the clouds diffuse the light.
- Setting up: Find an open space with no obstacles when choosing a start point or ‘return home’ location. This is important for getting the best possible signal transmission as big buildings can cause interference. Also keep in mind that if the auto-return home feature is activated, the drone could collide with trees or buildings in the flight path near the home location.
What are ground control points (GCPs) for drone mapping?
Drone mapping control points are stations on the ground that have known coordinates. Usually, these are vertical poles inserted into a base mat or target. Drone mapping ground control points are a surveying method used for additional accuracy, this process is known as photogrammetry.
A minimum of three GCPs should be set up inside the perimeter of the area being mapped, not along the boundary.
Often checkpoints are also set up. These are specific points where the coordinates are known but unlike GCPs, checkpoints are not used to correct the orthomosaic.
Checkpoints are used as a tool to QC your final map. They are a way of comparing the location of the checkpoint in the survey to the real location so we know the amount of error and the accuracy of the map.
What are drone maps used for?
- Progress reports for mining, building or construction sites. This can be useful for stakeholders that are interstate or overseas
- Photo-based evidence of conditions at specific intervals. This can be useful in construction where planned timelines are often delayed due to weather. Regular drone surveys show the conditions at specific timed intervals such as flooded roads that have been inaccessible due to factors out of the construction company’s control. This photographic evidence comes in handy for mediation where timelines affecting budget are questioned.
- Create a map that can be overlaid with infrastructure plans or diagrams to check the position of site work as the area is being developed
- Check crop and plant health in agriculture. Drones are often used in agriculture to identify issues with irrigation and areas where weeds or insects need to be treated.
What is drone mapping orthomosaic?
A drone map orthomosaic is hundreds or thousands of drone images stitched together to create a map also known as orthophotos.
Orthophotos are different to regular photos, they are geometrically corrected for lens distortion and other effects to produce an accurate representation of the Earth’s surface.
Similar terms that you may hear are orthorectify & georectify. Orthorectification is a more accurate image correction than georectification but both are basically a way of applying the spatial location to an image.
Orthorectified images correct for terrain and perspective so that they show objects in their true location. Orthorectification is required for drone maps and surveys that are used for measuring distances and calculating areas.
What is drone mapping overlap?
Drone mapping overlap is the amount that a flight plan is programmed for orthomosaic photos to overlap each other. This is important for accuracy when the images are stitched together to create a large map.
Wrap up on Drone Mapping & Surveying
Drone mapping and surveying are becoming important tools across many industries but before you invest in a drone for mapping consider accuracy, the time it takes to learn this new skill and the cost involved in getting started. It might make more sense to hire a drone mapping company.
If you have the time and interest to have a go at drone mapping yourself, this article on what drones are used for mapping helps break down the features to look for.
