The Litto3D Project. L. Louvart (Author) 13, rue du Chatellier BP Brest cedex

Transcription

1 The Litto3D Project L. Louvart (Author) 13, rue du Chatellier BP Brest cedex C. Grateau (Co-Author) 2, avenue Pasteur Saint-Mandé Abstract In 2003 the French Institut Géographique National (IGN) and Service Hydrographique et Océanographique de la Marine (SHOM) have been tasked by the Prime Minister to join efforts to produce together a seamless, modern, precise topographic and bathymetric model, including possibly the tides, of the entire French coasts. The area envisaged should extend from the 10 metres contour line inland to the distance of 10 kilometres seaward, or 6 nautical miles from the coastal baselines. This project was created to meet hundred or more requirements expressed by coastal managers concerned by the protection and exploitation of the littoral and by users of geo-referenced data; it should become the core of all future integrated coastal management projects. The preliminary study conducted in the Golfe du Morbihan, Southern Brittany, has already proven that France should be spared the strenuous geodetic problems met elsewhere. Thanks to the Napoleonic tradition of keeping common geodetic references inland and at sea, the historical database Histolitt" could be assembled fairly quickly, leaving in the poorly surveyed areas gaps that could be filled efficiently by modern technologies (laser bathymetry and topography, MBES, RTK, aerial orthophotos, permanent digital tide gauges), allowing metric accuracy on the plane and decimetric precision of heights and depths (fig. 1). A first lidar survey has already been planned in the summer of 2005 in the Golfe du Morbihan to conduct further tests and generate a precise Digital Terrain Model (DTM) at different resolutions (2x2 m and 4x4 m). The survey strategy comprises three different approaches, depending on the bathymetric interest, the near shore seabed complexity and the Rapid Environment Assessment military program requirements. 1 MANY NEEDS ON LITTORAL Coastal management is an essential objective to satisfy many needs : public maritime delimitation, littoral protection (change of coastline due to erosion, fauna and flora protection ), risks prevention (floods, pollution, safety at sea, natural disasters ), regional development (ports, tourism and industry), mineral and living resources concern, research and scientific studies, military needs (inshore patrol, search and rescue, amphibious operation, mine warfare ). More than a hundred of applications have been registered. Larger concentration of people and coastal housing justify a specific cartography of this area. Density of population on the seaside is twice and a half bigger than it is inland. Littoral actors need a very good description of the coast : dense everywhere, precise, at low cost, rapidly, open to everybody and fully shared. Apart from that the areas of interest don t matched, there is no difference between civil and military needs. 2 PRESENT CARTOGRAPHIC DEFICIENCIES Sea charts and nautical documents are not fully suited to theses needs as they are mainly dedicated to seamen. The information is condensed on the navigation roads and ports. Moreover, for legibility reason, information is less dense

2 than it is in bathymetric surveys (fig. 1). Access to these plotting sheets is not easy and many people have no idea that hydrographic services get much more information than printed on charts, in a digital format. As a consequence, some people lose time and money digitizing charts and they get very poor (and even false) results if they use only charted information to describe and to model coastal phenomena. Figure 1: printed sea charts depths (black) with bathymetric survey sheets (red and yellow) On land, height information usually comes from digitized charts contours (1: at most) and photogrammetric interpretation. There is a good density but accuracy is not sufficient to describe precisely the coast and to match with depths (fig. 2). Figure 2: lidar altimetry model (cell size = 2m, óz : 15cm ) and IGN BDAlti ( cell size = 50m, óz = 2m)

3 At the end, the intertidal area is not very well described as it is very difficult to make surveys in very shallow waters (not enough water, too many rocks and breakers) and to get good photogrammetric restitution (wet and flat areas especially). That's the reason why, in many places, there is no data at all and no continuity between the sea and the land (fig. 3). Figure 3: Fusion of lidar heights (green), multibeam survey (blue) and old vertical sound depths (yellow). The cross section makes appear a large gap between data (black hole ~50m large). 3 EUROPEAN RECOMMANDATION AND FRENCH RESPONSE Erika (December 1999) and Prestige (November 2002) disasters have pointed out the lack of coastal information : for instance it was quite impossible to estimate environmental damages due to pollution and to take the best decisions (which beach to protect or to evacuate, which port of refuge to be chosen ) as we don't know exactly the coast relief and the currents nearby. On May 30 th 2002, the European Parliament made a recommendation aimed to European members to start the inventory of the littoral and to carry out an integrated coastal management. On April 29th 2003, CNIG (Conseil National d Information Géographique) and CIMER (Comité Interministeriel de la MER) took it in turns and recommended that SHOM and IGN prepare this inventory, achieving together an altimetric continuous model for metropolitan coast and over seas French subdivisions. On this model, all the different thematic layers will be based on and will constitute what is called RGL (Littoral Geographic Reference). 4 PRELIMINARIES During a preliminary study (Marsh 2003), SHOM and IGN have evaluated the different issues to solve before getting a seamless database with continuous relief between water and land. In addition to the previous observations ( 2), it appears too that there is no difficulty to merge information as SHOM and IGN data can in most cases share the same geodetic reference level : there are known correspondences between sea chart datum (LAT) and land datum. These first results have been reported to CIMER who encouraged SHOM and IGN to keep on (September 14 th 2004). By the way, the enlargement of SHOM and IGN' original attributions have been justified. Within their present responsibilities, SHOM and IGN have all the necessary skills to contribute to this project: bathymetric, geodetic and cartographic expertise. Notably, a French tide model has been implemented with a precision compatible with Litto3D requirements. The very first an easy stage is to promote and to provide existing digital information through a new product called Histolitt (2006). Thus, a part of littoral requirements will be rapidly satisfied.

4 Last, the commercial policy is not yet defined as these data belong to SHOM, IGN and others organisms. Many agreements must be found before to make these data available to everybody. 5 DEMONSTRATOR IN THE GOLFE DU MORBIHAN In the same time, SHOM and IGN must show that it's possible to achieve together a continuous altimetric model in a small area, combining different and new acquisition means. Thus, a laser airborne system provides an accurate, rapid, safe and cost effective method of surveying coastal areas. This system has been used by governments and commercial organisations over the last decade to conduct surveys for nautical charting, coastal zone management It seems particularly suited to fill the gap between former sea charts and Litto3D (fig. 3 cross-section). The objectives of the laser demonstrator are : - To know the performances and the limits of airborne laser survey - To compare and to merge MBES (multibean echo sounder) and laser data - To be up to the highest hydrographic and cartographic standards - To lay down standards to Litto3D partners : data acquisition methods and qualification rules The Golfe du Morbihan offers a huge variety of relief and thematic. It's a good spot to achieve a demonstrator as it concentrates all difficulties : 0-50m depths, turbidity, currents, wide inter-tidal area, flat sandy beaches and rocky coastlines are most representatives of the French littoral. This is also a highly frequented boating spot in summer and SHOM ought to refresh nautical documents there. Another point is the demonstrator for the navy. SHOM try to get on a well known landing area, a very precise description of the seabed and to elaborate some very specific products, dedicated to amphibious operation. If the demonstrator is convincing, a navy military concept will be set up and could be replicated to military areas of real interest. This survey takes place in summer Survey objectives There are two different areas to be surveyed : a global survey of the whole gulf and a landing spot. Small scale survey of the whole gulf : each depth will individually fulfilled the S44 order 1 requirements (absolute accuracy: vertical = 50cm and horizontal = 5m) and the area will be incompletely surveyed, as shallow waters will be not exhaustively detected. Spatial resolution will be 1 measure every 4x4m laser spot spacing and there will be 20% of overlapping between tracks. Data processing will be done in batch mode. Large scale survey of a landing beach area (fig. 4) in respect to S44 order 1 (absolute accuracy: vertical = 25cm and horizontal = 2m) with a full exploration of the seabed (all cubic obstructions detected). This survey will be split in two separate surveys; A first flight, lengthwise to the beach, will be done in order to determine the best landing way (1 measure per 4x4m cell size). Data processing will be achieved in quite real time (some hours). A second flight, perpendicular to the beach and in accordance with the best landing way determined just before, will be done with a spatial resolution of 1 measure per 2x2m laser spot spacing with a 60% overlapping tracks. Data processing will be done in very short time (few hours). In the landing area, 2 cubic targets will be dumped at 5 and 10 meters deep, and must be detected.

5 Figure 4 Flight plan on the landing beach The industrial team in charge is committed to survey the whole coverage but not to get the bottom everywhere, as it is too dependent on turbidity and hazardous meteorological conditions. 5.2 Survey equipment There will be a laser bathymeter (SHOALS 1000-T) boarded on a twin-engine aircraft (CESNA 404) (fig. 5). This laser works with a near infrared (1064nm) and a green (532nm) light. There are two distinct working modes: - Hydrographic and topographic mode at 1kHz frequency - Topographic mode at 10kHz frequency Figure 5 Aircraft and inside racks The aircraft will be located through at least 3 different ways: - Real time DGPS: For navigation and data pre-processing, OMNISTAR differential corrections will be used; in case of lose of signal, inertial central data will be used too. - Batch stage KGPS: For data post-processing, both raw GPS data and inertial central data will be computed in L1/L2 mode to get a Kinematic GPS location.

6 - Real time GPS-RTK: ground stations will provide differential corrections through GSM antenna. This third way is too dangerous for real time navigation as there are some risks (data communications losses) but it will be used for post-processing and quality insurance. 5.3 Survey strategy Many strategies are possible, only few are optimal. In water it depends of tide and turbidity for laser water penetration. On land it depends of tide. In all cases it depends of weather conditions. High-pressure condition, no rain, light wind (<15 knots): the best weather period seems to be summer. July and August must be avoided, as there are too many navigators in the gulf. The progress survey state of the gulf is typical of a multi-captors Litto3D survey. There is already a land laser survey of the gulf so the strategy is to make sure that there are smallest but exiting overlapping between this land survey and the new one, to be able to do seamless products: high water rather than low water. There are already MBES surveys in navigation channels, the deepest waters of the gulf. So the strategy is to cover the unsurveyed areas rather than to penetrate deep water again: topographic laser mode in low water and bathymetric laser mode in high water at springs. Turbidity is the conjunction of 3 factors : chlorophyll content, organic materials content and suspended sediments in water. The best period for chlorophyll and organic materials are summer and winter. The optimal period for sediments is the less turbulent current periods : slacks at neaps. Costs must be taken into account too. Strategy must minimize operational risks and acquisition duration : 2 weeks. The general strategy will be the following one : - Bathymetric surveys will stand during slacks of high water at springs: turbidity is not optimal but rather good at slacks, deep depths can t be reached but there are some MBES surveys to fill in the gap. - Topographic surveys will stand during slacks of low water. - Flights will take place nights and days in order to get best laser measure (no sun interferences) and not to waste slacks. 5.4 Flight tactic Flight tracks are oriented perpendicular to tide propagation into the gulf and will follow the slacks. There is a tidal model of the gulf with 143 tide harmonics every 200m. It will be used to get predicted tide information all along tracks and to follow the tide with the aircraft in real time (fig. 6). Figure 6 Evolution of tide height along flight tracks (heights in meters)

7 As a hydrographical rule, 10% tracks will be perpendicular to regular survey tracks to permit error detection and S44 qualification. 5.5 Quality insurance Laser calibration (static and on the fly), GPS (Ground Control Point and levelling) and tide monitoring (SHOM) are carried out all along the survey. Everyday, environmental information, geometric calibration measures and laser data are downloaded to a ground control system (GCS). GCS involves a signal processor to discriminate sea or land surface and sea bottom from the different signals. It contains automatical algorithms that calculate the exact location of laser signal from GPS and inertial central attitude data: X, Y, Z and a confidence interval is given for each measure. Doubtful signals are underlined and submitted to human control (fig. 7). Some video camera information will be helpful to eliminate some artefact in signals: birds, boats, shore breaking At the end of the process, data will be provided into XYZ format and related to RGF93 datum. The same tide model used before for flight plan, is reused to reduce laser data into depths. In a first stage depths will be relative to lowest astronomical tides (LAT), then to chart datum (CD) and at the end to land survey datum (IGN69). Tide model makes this possible as it contains differences models between all these vertical references (fig. 7). H BV (X,Y,t) Sounding Area Reference Port H GPS (X,Y,t) H PIR (X,Y,t) äh S Corrected level Modeled level äh R Mesured level Modeled level H S (X,Y,t) H R (t) H(X,Y,t) H AS (X,Y,t) H M (t) N S (X,Y) N R H BV -H PIR äz S LAT CD äz R LAT CD Z /CD = H (H BV -H PIR ) Ellipsoïd Ellipsoïd H GPS Z /WGS84 = H GPS -H BV Figure 7 Propagation of tide observations at depth sounding area throughout model application: z S X, Y N S X, Y and hs h z N R R R Laser data will be compared to land surveys (laser and GPS levelling) and MBES surveys. Data will be merged in accordance with the different accuracy and uncertainty of these different surveys. Many methods could be applied: GIS methods or geo-statistical criteria (CUBE algorithms for instance). 5.6 Products First of all, if some dangers are found out during the laser survey, they will be automatically reported to SHOM and noticed to mariners. The results acquired within this demonstrator allow SHOM and IGN to have a good idea of the feasibility, the costs and the real interest of laser. Results should be extrapolated quite easily to other metropolitan parts, within Litto3D project. Basic products are commonly the following ones:

8 - Raw laser data - GPS and inertial central data - Calibration data - Seamless numerical models (2x2m) with uncertainty information on nodes - Mosaic of digital images (day surveys only) - Charts - Final report included procedures and operations description Validated data will be stored in bathymetric SHOM and IGN databases for general public use. The first customer of these data will be surely all the different littoral agents of the Golfe du Morbihan and SHOM cartographers. As early as these basic products will be produced, many modern and accurate by-products could be done (fig. 9). Figure 9 Sea flood scenario 6 CONCEPTION ISSUES AND PRODUCTION PLAN The replication of this demonstrator all along the metropolitan coast and French subdivisions request new know-how: in laser survey domain mainly. SHOM and IGN are not the only organisms concerned by the project Litto3D. This project has also a big impact on French industry and on research field. Subcontracting and cooperation are essential to constitute Litto3D products and services. In addition to public investment, one must go through European funds possibilities as this initiative is spreading in Europe. At the moment, there are still many open issues SHOM and IGN should resolve during the conception phase: - Data acquisition: Do we purchase a national and independent laser captor or do we rent it? Do we acquire the complete mastery or do we remain project manager? - What are the geographic priorities? - The demonstrator Golfe du Morbihan costs 500 k. A first estimation of the whole project costs gives 25 millions euros for 10 years. Who is ready to fund a part of this project? State-government, Region, Inter-Region? - What does the commercial policy look like? Public data must be reused by all according to the European law (November 17 th 2003) and the French ordnance in progress. - The update of the product? It is quite hard to evaluate as it depends both on environmental considerations (such as variability and anthropogenic features) and technological possibility.