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Spatiotemporal physical modeling of tropical islands within the Digital Ecosystem Avatar (IDEA) Project

Armin GRUEN, Matthias TROYER, Tao GUO

ETH Zürich, Institute für Theoretische Physik · agruen@geod.baug.ethz.ch

Extended Abstract

The Moorea Island Digital Ecosystem Avatar (IDEA) project has been initiated in 2013 by a group of international researchers to build a virtual representation of Moorea and Tetiaroa Islands, French Polynesia, South Pacific. The main aim of the project is to model an entire ecosystem, observe the changes through it and be able to predict future changes reliably. The Moorea IDEA project incorporates observations, experiments, data, and theory across a coupled 3-D marine-terrestrial landscape to model where physical, chemical, biological, and social processes interact to shape the island’s phenotype.

Both islands are of volcanic origin. Moorea (132 sqkm) has an extremely rugged and com-plex topography with mountains ranging up to 1200 m (see Figure1), while Tetiaroa (6.5 sqkm), as a pure coral island, is completely flat.

Figure 1: Moorea island. Cook´s Bay

In order to generate the 3D physical model of the Islands, multi-sensor data with varying accuracies, timestamps and spatial and spectral resolutions need to be fused. For this pur-pose we are using the following sensors and raw data:

- High resolution optical satellite images (Pleiàdes and WorldView) for terrain modeling and landuse/landcover classification

- LIDAR data over land and water

- Aerial film photography extracted and scanned from archives for change analysis over land

Hanke, K. & Weinold, Th. (Hrsg.) (2017): 19. Internationale Geodätische Woche Obergurgl 2017. © Herbert Wichmann Verlag, VDE VERLAG GMBH, Berlin/Offenbach. ISBN 978-3-87907-xxx-x.

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- Underwater sonar measurements for modelling the bathymetry

- Underwater photogrammetry for monitoring the coral growth in 3D

- UAV flights recording of archaeological sites, for mapping the lagoon habitat and 3D building modeling

This paper describes the project and addresses the processing methods and the problems en-countered during the processing of multi-sensor and multi-resolution data. The resolution of the image data spans from about 1 m GSD to a few millimeters. The resolution of point clouds from one point per 25m (wide range sonar) to a millimeter spacing (3D coral models).

High resolution DSMs and orthoimages have already been generated using Pleiàdes imag-es with 70 cm pan resolution acquired over Moorea and Tetiaroa in summer 2014. The images have been acquired in triplet mode.

Figure 2: Pleiades multispectral imagery acquired over Moorea (left) and Tetiaroa (right) Islands in June 2014 with a GSD of 70 cm.

For the georefencing Rational Polynomial Coefficients (RPCs) provided by the satellite operator are used and their accuracy is improved using ground control points. Both in Moorea and Tetiaroa georeferencing could be done with subpixel accuracy.

DSMs of both islands have been generated automatically using eATE under LPS and edited manually using “interactive terrain editing” under LPS. Further manual editing has been done in stereo. With Terrain Prep Tool under LPS, a grid DSM has been generated with 60 cm (later also 70 cm) resolution.

Ortho images have been produced in LPS Ortho rectification. The ortho images have been used for texture mapping (ArcGlobe, PCI, ERDAS) and the textured 3D model (Figure 3) is available for analysis, visualization, simulations, etc.

Bathymetry data has been acquired by two sonar systems (one producing a 20mx20m grid, with depths ranging from to 4074 m, the other resulting in a 2mx2m grid, with depths from 10 m to 200 m) and by one aerial green laser scanner, providing 4 points per sqm (Figures 4 and 5).

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Figure 3: Textured 3D model of Moorea

High resolution bathymetry data is used for different purposes, e.g. for flow modeling in the lagoon and along the reef. It has also been integrated into the generated land DSM.

The final physical 3D model, amended by landuse data and other semantic information will provide a presentation and geospatial analysis platform to the project participants from many other disciplines.

Figure 4: Moorea bathymetric map acquired by aerial laser scanning (RIEGL VQ-820-G green laser)

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Figure 5: Point cloud of Moorea Gump Station, acquired by aerial laser scanner

UAVs are used extensively for four different purposes. Firstly, they are employed to meas-ure and model archaeological sites, so-called marae. These are ancient temples of the Poly-nesian culture (see Figure 6).

Figure 6: UAV image of Marae Nuu Roa

Secondly, the whole north-coast of Moorea has been imaged by a UAV-based HR video sequence. This is a 35 minutes video, covering 24 km of coastline.

Thirdly, UAVs are used for habitat mapping in the lagoon. They are launched from a boat, flying at a height of 10m above the water level and image the bottom of the lagoon. Subse-quently classification algorithms are applied for mapping.

Finally they will be used for 3D building modeling.

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An important application is the measuring of the growth rate of corals. Especially in times of El Nino and the widespread destruction of corals it is important to understand the recov-ery mechanism. We produce 3D coral models over a number of test areas by using under-water photogrammetry. Single cameras, a 2-camera stereo arrangement and a 5-camera head system are used by scuba divers to produce images of the corals, which then are turned into 3D surface models largely automatically (Figure 7). This is done several times at different time instances which allows us to compute the yearly coral growth rate.

Figure 7: 3D model of part of a coral testfield, produced by underwater photogrammetry

This project, in its physical modeling part, combines a number of photogrammetric and remote sensing sensors and techniques to generate the base data of a tropical island and its surroundings for a variety of different disciplines. It is a continuous effort and may serve as a model approach for other areas of similar character worldwide.

The project also shows how photogrammetry and remote sensing can contribute to the solution of some pressing problems of our planet´s land and ocean.

References

Cressey, D. 2015. “Tropical paradise inspires virtual ecology lab.” Nature 517: 255–256. doi:10.1038/517255a

Davies, N., D. Field, D. Gavaghan, S.J. Holbrook, S. Planes, and M. Troyer. 2016. “Simu-lating social-ecological systems: the Island Digital Ecosystem Avatar (IDEA) consortium.” GigaScience 5:14. doi 10.1186/s13742-016-0118-5

Guo, T., A. Capra, M. Troyer, A. Gruen, A. J. Brooks, J. L. Hench, R. J. Schmitt, S. J. Holbrook, and M. Dubbini. 2016. “Accuracy Assessment of Underwater Photogrammetric Three Dimensional Modelling For Coral Reefs.” International Archives of the Photogram-metry, Remote Sensing and Spatial Information Sciences XLI-B5: 821-828. doi:10.5194/isprsarchives-XLI-B5-821-2016

Moorea IDEA. 2016. “Moorea Island Digital Ecosystem Avatar.” Accessed 11 December 2016. www.mooreaidea.org.