3DroneMapping completes PPK trials

3DroneMapping has been testing a PPK system for the past year. The point of PPK (or Post Processing Kinematic) is not just to improve internal photogrammetric model orientations, but also is general recognised as a way of undertaking full-scale surveys at the same accuracy but without having to place time-consuming ground control points. 3DroneMapping setup a calibration site to test the claims made by manufacturers and ultimately for our own quality assurance that what we publish is indeed right the first and only time.

Ground control points traditionally have become a sore topic with RPAS aerial surveyors as they take time to construct, survey and verify. These points are needed in a much greater density than tradition aerial survey methods, due to the non metric cameras often used in RPAS. Locations where the terrain changes rapidly, homogenous surfaces, and densely vegetated areas all require a higher density of control points to assist the photogrammetric software to locate accurate ties points and stop the model from deviating. Undertaking a photogrammetric survey without control, can lead to disastrous results, as outlined in a previous article, “GROUND CONTROL VS NO CONTROL

There are a number of issues with placing and surveying control points. The obvious one being that another piece of survey equipment is needed to accurately record the positions. This comes at a great cost as survey grade GPS systems can run into tens of thousands of dollars. Then the areas for the control need to be identified. These areas are often located at the fringes of the site to be surveyed as well as in the center. This means that the site needs to be traversed (often on foot) to place marks and survey them. In some cases it can take a few days to complete the marking for fairly small sites due to the terrain, vegetation. Control points are also not permanent. They often go missing due to weather, theft for vandalism and cause problem for the surveyor back in the office when the surveyed points are not visible on the imagery.

Another problem with using control only for surveys using low-level photogrammetry is that the consumer grade cameras used are not metric and are prone to distortion. While control can eliminate this to an extent, it is the areas between the control points that often are forced to fit alignment. This can lead to errors especially regarding heights.

PPK is setup to have an accurate onboard GPS system that records a high frequency measurement tracklog of an RPAS flight. The system notes every time the camera takes an image and logs it. Another GPS is setup on the ground to record satellite information during the flight. Using some Post Processing software, the relative satellite positions between the devices can be compared and reduced. The results are very accurate coordinates for each image captured.

 

The photogrammetric model generated from using PPK enhanced images gives a much more consistent and accurate rendering of the terrain. Tie points have a much greater certainty of intersection, speeding up the matching processes. This reduction in uncertainty means fewer resources are required, leaving an opportunity to increase the density of other processes like point cloud generation, geometrically verified matching, etc.

Trials for PPK testing began 3 weeks ago with a test calibration site. Over 200 tradition ground control points were surveyed over a 780ha site, some of which were placed in very challenging positions that like close to tall structures, under power lines, close to dense vegetation and visibly homogeneous surfaces. The site was flown over a 3 week period with a number of flights to determine the optimum settings and processing requirements. At the end, the most consistent and repeatable results were chosen for the optimised settings for our company and RPAS setup.

Control points were digitised from resulting point clouds and orthophotos and compared to the previously surveyed. These positions showed almost no difference to the control as published. Areas that have previously been a problem for photogrammetry showed up even and consistent between surveys when drawing profiles or cross sections. The 200 control points varied only a few centimeters over the entire site giving a very accurate and tight model.

What this all translates to is that surveys can now be undertaken with even less time spent in the field and can result in even more accurate and consistent data.

 

 

Drone mapping a private tropical island

3DroneMapping had the privilege of mapping a tiny private island in the Indian ocean off the Zanzibar coast in the Indian Ocean. This tiny (15ha) island is approximately 5NM from the mainland is is completely isolated from the rest of the world. Surrounded by live coral reefs and sand bars, the island is home to an exclusive, high-end resort. The resort prides itself in being completely self-reliant and offers guests individual service that would rival some of the best hotels. The island limits it number of guests to 20 to maintain the feeling of being remote and disconnected from the world.

Guests have private “bandas” or thatched bungalows that allow the gentle wind through the casuarina trees, while waves lap at the shore. Each meal is 3 courses, each planned carefully to suit guests needs and local availability. There is a butler per banda that does everything for guests from serving meals to putting out slippers . This is luxury and attention to detail that goes above and beyond any other resort.

The island resort prides itself in offering its guests privacy and high quality service. To maintain this feeling, the accommodation is spread out over the island in such a way so as to not intrude on adjoining structures or guests. Behind the scenes, and army of 65 staff on average also reside on the island. Besides accommodation, kitchens, storehouses, generators, saltwater desalination plants, boat sheds, and various other structures need to be hidden from guests.

The main problem of shielding guests from the day to day operations is space. The island has limited space as it stands and is now in threat of it shrinking further. As currents have changed, part of the island is eroding at a very fast pace, So much so that the developers are concerned that some of the newly proposed structures will be washed out to sea in a few years. Since no plans or maps of the island have ever been drawn or surveyed, they felt it was important to give some scale and dimensions to the architects for the master plan.

The plan of the island was to include all existing structures, pathways, major trees, visible services, high tide marks, levels and contours. This needed to be done in a very tight timespan as the island closed for renovations  in 3 months. Another major concern was that the survey could not disturb any guests while being carried out.

Using a multirotor drone with high resolution camera allowed us to operate high enough to still obtain images with good detail but far away enough from guests to not bother them. Total time for the flights was 30min, also reducing the time spent flying over paying residents. Control points were placed over the island at strategic places to not be visible to the public but in a fashion that met basic survey principles. The GPS used for the control points was the tiny L1 Reach made by Emlid. These cost effective units have proved themselves over and over to be some of the cheapest and accurate single frequency receivers about and are perfectly suited to this type of survey.

After fieldwork, the photogrammetric process was a fairly simple affair with 600 images collected and control added to the model. A high over and sidelap was required to obtain ground strikes between the vegetation. The ground strikes where then extracted from the dense pointcloud using specialised 3d pointcloud editing and classification software. These were fairly easy to identify given the flat nature of the terrain on the island. Within a 3d viewer and GIS environment, the other required features were also extracted and exported directly to a CAD.

All files were handed to the client via an online GIS platform as well as AutoCAD files for the master planners to work with. Such a survey could not have been possible without drones. Using this technology kept the pricing low for the customer, allowed us to obtain additional byproducts for the delivery such as orthomsoiacs and pointclouds for rendering, kept our time on the ground and disturbance to guests to a minimum and provided a very quick turnabout time.

Drone Mapping in Gorongosa National Park

Gorongosa National Park is a 4000 square kilometer nature reserve in central Mozambique. In the 1950s it boasted some of the greatest animal densities in Africa, with over 500 bird species, abundant carnivore populations, as well as reptiles. The park originally began as a hunting reserve in the 1920’s and became a popular destination very quickly. A camp was built in 1940 for administration and tourists but had to be abandoned in 1942 due to flooding. This remaining structure from that era has now become a famous  landmark in the park, known as “Lion House”, due to a pride that has taken up residence among the ruins.

During the late 1960s, the camp now relocated to slightly higher ground at Chitengo had grown so big that it served over 400 meals a day to tourists that visited it. the camp had 2 swimming pools, a bar / nightclub, petrol station, post office and beds for over 100 guests.

Disaster struck the park between 1981 and 1994 with the famous civil war in Mozambique. In December 1981 the Mozambican National Resistance (MNR, or RENAMO) fighters attacked the Chitengo campsite and kidnapped several staff, including two foreign scientists. The violence increased in and around Gorongosa. In 1983 the park was shut down and abandoned. For the next nine years Gorongosa was the scene of frequent battles between opposing forces. Fierce hand-to-hand fighting and aerial bombing destroyed buildings and roads. The park’s large mammals suffered huge losses. Both sides in the conflict slaughtered hundreds of elephants for their ivory, selling it to buy arms and supplies. Hungry soldiers shot many more thousands of zebras, wildebeest, buffalos, and other ungulates. Lions and other large predators were gunned down for sport or died of starvation when their prey disappeared.

A cease-fire agreement ended the civil war in 1992 but widespread hunting in the park continued for at least two more years. By that time many large mammal populations—including elephants, hippos, buffalos, zebras, and lions had been reduced by 95 percent. Surveys counted just 15 buffalo, 5 zebra, 6 lions, 100 hippos, 300 elephants and just a handful of wildebeest.

In 2000, Mozambique experienced the worst floods in 50 years. over 700 people were reported killed and thousands of square kilometers of land was affected. The flooding resulted from 5 weeks of intense rainfall, causing rivers to burst their banks and overflow.

Despite all the challenges the park has endured, there is now a renewed push to restore the park to its former glory. With funding from foreign investors as well as the Mozambican government, the Gorongosa is quickly becoming a popular tourist destination again. So much so that the old camp of Chitengo has outgrown itself, and new luxury, remote camps are now in demand. Many companies are eager to be a part of this big business but are wary of the environmental risks previously experienced.

One of those intrepid business is The Gorongosa Collection, which aims to establish luxury tented camps in strategic points within the park. However, the developers insisted that the area be higher than the flood levels recorded in the 2000 floods. Since the ruins of “Lion House” still exist, the high water mark is still very much visible on the structure walls on this very flat plain. This mark to be transferred to the proposed new camp site, 3.5km away to ensure that the structures remain high and dry.

3DroneMapping was called in to map those proposed areas and compare the levels from the floods in 2000 with the planned new locations. Since this section of Gorongosa is so flat, it is next to impossible to accurately measure levels without the aid of instruments. Using post processed GPS data with accurate satellite ephemeris data, the high water mark was determined and then compared to various other sites in the vicinity as well as being used for accurate control points for photogrammetry.

Our inhouse fixed wing mapping RPAS was deployed at suitable elevations to mapped the various areas after control points where placed and measured with the accurate GPS system. From the images, orthophotos and point clouds where produced. The points clouds contain nearly 1 billion points of the terrain surface that contain colour, horizontal and vertical measurements. These points where then both manually and automatically classified into various levels of vegetation, structures and bare ground. From the bare ground points, an accurate 3d model of the terrain can be made from which contours linking areas of equal elevation can be produced. Without classification of pointcloud data, the terrain data would be incorrectly include the vegetation, giving false representations of the topography.

Chitengo airstrip was also surveyed by 3DroneMapping. This was done to ensure that the airstrip is indeed above the high water as well as a overview of the slope of the strip as it undergoes a surface upgrade. From our 0.25m vertical intervals, engineers could ascertain the slope and where the cut and fill is required to ensure good water drainage as well as an even camber.

Using RPAS for this project not only ensured that the client receives the data in the most cost effective fashion, but also allowed for flexibility in the field to allow for last minute changes to flight paths to accommodate the scope requirements. Other deliverables such as 3d renders for visualizations and marketing purposes were also generated.