Category: Mapping

01 Jun 2020
Aerial Map For Topographic Survey

Topographic Survey Using Drone Photogrammetry a Case Study

We were invited by a local council to demonstrate our aerial topographic survey capabilities for a small culvert project. This was challenging as the route passed through particularly dense tree coverage. Typically multi return LiDAR would offer better penetration to identify true surface through vegetation and is typically chosen in this situation (See our article comparing Photogrammetry and LiDAR). In this instance we wanted to show photogrammetry as a valid option during winter months which would have a much smaller cost to the client over LiDAR data collection (typically costing thousands a day). The survey manager was specifically interested in a topographic drone survey to see how this compared with existing measurements of the site. The culvert ran next to an existing tarmac walkway under some small tree’s densely packed together.

A processed orthomosaic of the 400m long site taken in February 2020.

The Survey

In February we returned to site to conduct the survey. This was to be a geo-referenced survey and we chose to use ground control points and manually collect their locations using a survey pole and base station for later post-processing.

Firstly, the base station was set up to collect satellite data for 2 hours. In this case we used the Emlid Reach RS GNSS that we use for our own RTK drone setup. However, typically we use which ever survey equipment clients prefer such as those from Leica and Trimble. In our test we knew the accuracy of the Emlid would still be several magnitudes better than the accuracy of positioning information available from the drone. We laid out 12 ground control points along the length of the route with square/diamond patterns indicating the center point. Tthe position was then taken from this point with the rover pole and later tagged in the drone imagery.

GCP’s along the route to georeference the point cloud with some used as validation points.

In this case since the mission was a demonstration and close to residential property so we chose to use the DJI Mavic 2 Pro. Typically we would use a drone platform with higher image quality such as the DJI Matrice or heavy lift drone with a full frame camera. For mission planning we chose Drone Deploy with a one pass Nadir grid with standard settings at a low altitude of 20m. The mission was flow collecting 199 images of 5472×3648 resolution. Some blurring was observed due to poor winter light. After the mission was flow we measured the position of the GCPs before clearing up.

Drone Survey Data Processing

Later the survey data was post processed using RTKLib and local RiNEX data to provide accurate positions of each GCPs. This provided an accuracy beter than 2cm X,Y and under 4cm in height. The images and GCP locations were input into Pix4D and the mission processed to obtain a classified point cloud, DEM, DTM, mesh model and contour map as show below. This had an ground sampling accuracy of 0.81cm/px over the area of 2.47ha. The absolute camera position uncertainties X,Y,Z(sigma) were 0.112(0.021), 0.317(0.028) ,0.71(0.051)m. The error of a couple of the GCP measurements was high as the rover had poor line of sight to the base station on the hill through vegetation.

Rendered DSM with contour map overlay derived from Pix4D.

From the results we observed holes in the point cloud due to the dense wood branches over the pathway. This led to a poor contour model as seen below. This was disappointing but highlighted the importance of planning in accordance with the task which Drone Deploy alone didn’t consider.

Improved 3D Survey Model

We returned and manually flew and acquired photos around the problematic area this time taking additional oblique images perpendicular to the walkway. Processing these images provided an improved point cloud with the walk way visible through the trees to measure the topography along the route.

Classified point cloud derived from oblique images with vegetation.

Extra processing was done to classify the point cloud this time so that high vegetation could be classified and removed from the point cloud to provide a more accurate DSM for measurement and topographical contour creation.

Classified point cloud derived from oblique images with high vegetation removed.

This example from the earlier batch of processing without point cloud classification and high vegetation removal shows peaks along the DSM profile. The profile was much more accurate after point cloud classification.

Profile derived from the DSM along the proposed culvert route used to determine if the run off is sufficient.

Our conclusion is that during winter months photogrammetry can provide actionable topographic survey data over woodlands, an area where LiDAR is typically required. With anticipated or non essential delivery schedules this can provide huge financial savings. Sky Tech are happy to work with local surveyors on projects such as this where drones can provide more rapid data collection over convential site walks. Please get in touch if you would like to work with us:

Quick video overview of the outputs from the topographical survey.
15 Feb 2020
Llaca Glacier

Mapping Glaciers From Edinburgh To Peru

Sky Tech was approached by Ph.D student Rosie Bisset from Edinburgh University School of Geosciences last year to assist with a custom build drone. After a year of bad press for drones and the climate emergency frequently in the news we were motivated to help with a drone mapping project.

Peru glacier retreat - Two study sites marked shown.
Peru glacier retreat – Two study sites marked shown (Rosie Bisset).

Rosie’s glaciology project was to map an area of the Llaca and Shallop glaciers in Peru. Previous studies had shown large reduction in glacier area in the Cordillera Blanca from 1987 to 2010. These glaciers are an important source of water for the area and research had already shown a reduction of fresh water during the dry season.

One part of the study was to examine surface debris depth and how it affected melt rate. A camera drone needed to be flown in a grid pattern to build a 3D model of the surface. A FLIR thermal camera on the drone would also be used to measure surface temperature differences. In particular to draw links between the thickness of the surface debris and elevation changes, surface movement and water pooling.

Custom Build Drones

This was a challenging custom drone project as the equipment needed to be ready and tested in a few weeks. Sky Tech also needed to provide sufficient training for Rosie and her assistant to fly the drone on the glacier. It had to be a simple to use and quick to deploy. There was some hiking to do to the glacier so the drone needed easy to transport in a backpack. The operating conditions for the drone at high altitude (4600m) and in subzero temperatures were also challenging. Finally a sound methodology to operate the drone and collect thermal data to create a 3D model also had to be addressed.

Losing weight by decasing the geotagger and trimming wiring.

The students already had access to a DJI Phantom 4 drone so we chose to modify that given there was insufficient time to build a custom drone on this occasion. We had our concerns it would be able to fly at an altitude of 4600m as it was close to the manufacturers recommended limit. Effectively propellers would need to spin faster in thinner air to create lift, depleting the battery faster. To our advantage we had the fact the air was cold making it denser and easier to create lift. A second thermal camera also had to be attached at additional weight. And finally a requirement to fly for enough time to cover a substantial sized grid that was marked out for study below.

Drone Mapping Training

We started off providing some training on different mapping packages such as Drone Deploy and Pix4D. In the meantime we got started designing a payload package for the FLIR VUE 640 Pro R thermal camera that would be light. We had to speculate if the drone could handle the thin air at higher altitudes (not easy to test in UK airspace) and have a backup plan. We worked on a set procedure for operation in the expected environment, up most was always personal safety on the glacier.

Sky Techs monster Frank.
The creation of Frankenstien (Frank) made from parts of three Phantom 4 drones.

Disaster Strikes

A few days before the trip there was a training mishap where one of the test drones fell out of the sky and crashed. We suspect this was from a manufacturers fault allowing the battery to fall out. The drone was destroyed. In some respects we were lucky as the much more expensive FLIR thermal camera wasn’t attached at the time. (At time of writing DJI have not yet offered a replacement to the students).

Not all was lost, calling around local suppliers we found local store Kooltoyz had a spare frame that had been returned and was missing a camera. This gave an opportunity to make the drone lighter without the stock video camera always being attached. Given all the modifications the new drone was ready – nick named Frank (for Frankenstein).

Local Trials

Longniddry Drone Testing - Thermal gradient as tide receeds
Longniddry Drone Testing – Thermal gradient as tide receeds shown right.

At Longniddry beach we performed some trials at low tide as the surface texture here was a mixture of sand and rocks similar to the surface of a glacier. We also provided advice on use and transit of the equipment and batteries on airlines – not an easy trip and kit has been know to be confiscated.

Frank Mapping Glaciers
Frank mapping glaciers in Peru and Iceland (Rosie Bisset).

Rosie and her assistant left for their month in Peru and we got news back the trip was a success. So much so Frank earned a second trip to Iceland. The required data had been collected. Frank had made it back from his trip half way across the globe. This was fantastic news that the study was a success. Secondly for Sky Tech that our customisations and training worked perfectly for the students to complete their task. Analysis of the data continues at Edinburgh University Geospatial Department. What a fantastic opportunity for us to work with others on a custom drone project. It was rewarding to see others gain confidence with this drone technology.