From fields to forests
Aerial photography has long been used for land surveying and analysis. With the increased availability of unmanned aerial vehicles (UAVs or drones) the cost is lower and the accuracy is higher than ever.
This article covers the benefits of large area drone surveys, the limits and benefits over manned aerial flights and some of the technical aspects of what it takes to perform a survey.
Manned vs Unmanned Flight
Manned: Usually manned flight cannot fly below 500ft so will need a large, long lens camera to pick-up details but it can record more in each frame.
Unmanned: UAVs can’t normally fly above 400ft without permission from the Civil Aviation Authority. The UAV cannot carry a very big lens but data can be captured very frequently and frames stitched together to make a very high definition large image.
Manned: Aircraft with specialist cameras, attached to gimbals and high precision GPS receivers are expensive.
Unmanned: Professional UAVs have a high quality gimbal which can point straight down and embed GPS coordinates into image metadata as standard. Survey cameras (NDVI) are relatively easy to buy and fit to the gimbal.
Manned: Aircraft can cover large areas in a small amount of time.
Unmanned: Without permission from the CAA, UAVs can only cover a maximum of 500m in any direction from take-off but can be carried to the next area in a rucksack.
Fixed Wing vs Multirotor
UAVs come in many different forms but in general there and fixed wing and multirotor survey aircraft. Either type can fly an automated flight path and capture images on a gimbaled camera (steady-cam). Multirotor UAVs have the advantage of vertical take off and landing, whereas fixed wing usually require a take-off and/or a landing strip. Fixed wing aircraft are much more energy efficient and therefore can cover more land area and take more photos than multirotor aircraft but they have a large turning circle and have to travel at speed to stay in the air. Mutlirotor UAVs can stop and hover to take higher quality photos but will only fly for 15 minutes or so, which is usually enough to record a 1km diameter grid section.
UAVs in the UK and most other countries, have to stay in visual line of site (VLOS) and not fly further than 500m away from the pilot. Larger areas are surveyed in a grid of 500m (or so) blocks. The images and data can be automatically stitched together to make a continuous survey over a very large area.
Here in the UK, beyond visual line of site (BVLOS) operations are possible with permission from the CAA but it’s usually easier to work in the 500m limit and grid survey that to apply for the BVLOS exemption.
Using a NDVI camera, it is easily possible to fly over fields and forests and survey the health of the vegetation.
Skip over the next bit if you are not feeling geeky….
Vegetation reflects different frequencies of light depending on the health of the plant. For healthy plants, more near infra-red (NIR) light is reflected. From the amount of NIR light that is reflected across a field of a single crop, “heat” maps can be created showing the differences in relative plant health.
Unmodified standard cameras are set-up to capture images best for the human eye. The sensors are capable of capturing the NIR light but the camera contains a filter over the sensor because the NIR light can spoil the visible light and saturate the photograph.
Most cameras can be modified to remove the filter and allow NIR light to be recorded. Here at Crystalmark, we use DJI Inspire 1s with modified X3 cameras. These cameras are very light weight and capture data of excellent quality.
Another type of survey we are asked to perform is to find and report on fallen trees in large forests. When tree farming, fallen trees can harm other trees around them so quick forest management is required to minimise damage.
Fast response on-site surveys can be used to find fallen trees and send forestry management crews to the exact area without having to wait for an external survey. The UAV and operator can go out on the day with the forestry management crew. We at Crystalmark are on call to perform such surveys, please email email@example.com for advice and cost estimates.
Larger surveys can be performed in a grid and the report viewed online within a couple of days. The report includes an interactive map with GPS coordinates of highlighted areas of interest.
Flights are automated to ensure full coverage of the survey area. Automation also allows repeat surveys with results that can be compared to previous reports. Our online report maps can be layered with a series of reports, making changes over time easy to see.
The pilot will stand at the centre of a 1km wide circle and fly in a pattern which covers as much area as possible. Most surveys can be automated, especially those on flat ground, but others that have varying ground height or are near structures like wind turbines or power lines must be flown manually.
Every survey must be rigorously planned, not just for safety reasons but to ensure efficient and full coverage of the surveyed area.
Ideally the camera should be a constant height above the vegetation, in practice this is difficult to achieve since we only really know the height above the take-off point. For a grid survey we calculate the optimum height to allow good clear images to be captured while minimising the overlap of images which can lead to a distorted survey. At the centre of each image the camera is looking straight down while at the edges, the camera is at an angle to the vegetation so when two images are merged, the vegetation does not look even. Depending on the type of survey and any time constraints, the UAV can fly higher, producing lower quality of image, or take more photos with lots of overlap.
There’s a whole science to do with stitching images together. We can provide a range of solutions to match the client’s budget:
- simply handing over the raw data
- interactive Google Maps PhotoOverlays
- full orthomosaic reports with highlighted areas of concern and condition change over time
- 3D models of landscapes, buildings or structures (we can even 3D print some of the models, which is quite good fun!)
In every operation, safety of public, staff and equipment is priority. We are happy to trek trough the darkest forests, splash through the deepest mud and climb the highest Munro but UAVs can be dangerous so we will complete a risk assessment and ensure that no-one will be hurt.
Sorry but only a qualified pilot can fly the drones, so, no you can’t have a go!
Not everywhere is perfectly flat and we have found that capturing images at an exact hight is more of an art than science, especially when the trees and other vegetation is not of an even height. Surveys can either be at the height of the highest object in the survey area or a fixed height above the ground, which depends on the client requirements. If the survey is not to your satisfaction, we are happy to repeat the flight, maybe even at a discount – customer satisfaction is very important to us.
The UAVs are pretty hardy, they can fly in 25mph winds and hot/cold temperatures but they don’t like precipitation.
We bring our own tea but accept offerings of biscuits.
There’s usually two Crystalmark staff around for a survey but this can vary depending on conditions and the operational area. Survey costs usually reflect number of staff required.
Flying over fields and forests is usually hassle free but sometimes we have to apply for permission to fly, especially if there is a town or nearby an airport.
Email us for a chat, we are very friendly and like talking to people. If you just wanted to see if we can help, please get in touch. firstname.lastname@example.org