3DroneMapping was approached to assist in a topographic and bathymetric survey of 2000ha in Northern Zanzibar . This semi-autonomous island lies off the coast of Tanzania in East Africa.
The land has been provisioned for development of various resorts, marina, airfield and golf course and is the biggest Zanzibar has seen with many investors and shareholders throughout the world.
Site identification and Control
The cadastral extents of the site had previously been surveyed by government operators and markers were placed on the ground. This made the process of adopting a survey coordinate system easier as the markers needed only be confirmed. However, little had been surveyed in terms of a datum. Given the low lying nature of the island, it was imperative that formal geodetic measurements be taken. A Leica L1/L2 GPS recorded raw data for 10 hours and was processed against Continuous Operating Reference stations in Ethiopia and South Africa. An excellent result was achieved of 15mm in standard deviation for our ellipsoidal levels. The modern gravimetric geoid model of Tanzania by J.G. Olliver (2007 was then applied to obtain accurate orthometric levels. Various control points designed for photogrammetric uses where then placed over the site using the Leica GPS in RTK mode, all under 10mm in standard deviation.
Drone Mapping
Once the control was placed, our fixed wing mapping drone, “Inyoni” was deployed on a prescribed flight path to obtain vertical images of the site. The flight planning required 75% sidelap and overlap at an elevation of 600ft. Despite the terrain only varying by 15m in height, a high degree of overlap was necessary to achieve ground points between the closely spaced vegetation. Conditions on the day chosen for flying were not favourable with winds in excess of 5m/s, low cloud cover and intermittent rain. As the day progressed, the weather conditions improved, allowing for sections to be re-flown with better results. A total of 4 flights were done in 4.1 hours flying, travelling 210km. A total of 1500 images were collected, each 28mp.
Bathymetric Survey
The developers ask for 900ha of marine area to be mapped for a proposed marina and exclusive lodging area. To achieve this, a standard logging sonar receiver was mounted to a tradition wooden dhow boat. The tracklog of the vessel was recorded using a L1 only GPS unit powered by and Edison computer. This tiny GPS device was coordinated on land to the same survey system that was used for the photo control, ensuring continuity between the terrestrial and marine surveys. The roving unit was mounted to the dhow above the sonar receiver and the offsets recorded. For robustness of the final tracklog accuracy, it was decided to post process the L1 data from the receiver and base and not use RTK methods. Data was collected for 9 hours at 10hz.
Processing of data
The images were run though Pix4d, photogrammetric software to obtain an orthomosaic and pointcloud. Our in house workstation processed the data in 32 hours at maximum data resolution, delivering 50mm orthomosaics and dense point clouds. The point clouds were classified in GlobalMapper and LASTools and further cleaned manually in ArcGIS to achieve bare earth measurements. All features were then captured in ArcGIS and drafted to an AutoCAD drawing for the developer and engineers to use.
The L1 data from the GPS was post processed to derive and accurate tracklog. However, at some point in the survey, the antenna had slipped, shielding some of the raw measurements. The results still processed at 200mm confidence level, being sufficient for the purposes of this survey. The tidal range at this particular part of the coast varied approximately 2.8m, making an accurate tracklog essential for the long duration of the survey. The rough seas encountered past the reef also meant that the sonar device varied in heights. This was taken into consideration when mating the timestamps from the sonar data to the GPS post processed tracklog.
Results and comments
The efficiency of the combination of fixed wing drone and ground control is unrivalled for the accuracy and level of detail obtained. The total time spent on the project by a single person, from control placement to delivery for 2000ha, was just 7 days. Having the correct tools and knowledge makes this type of project very economical for the developer as well as lucrative for the surveyor.