Hydrographic Use of Satellite Imagery in South Pacific01/01/1970 |
| The French Hydrographic Office (SHOM) uses remotely sensed visible imagery for the cartography of atolls and reefs. It is considered as an efficient and relatively inexpensive method to collect information where data is sparse. |
| Michel Le Gouic, Michel Even and Jean-Paul Tournay, SHOM, France |
Remotely sensed visible imagery is used in Pacific French Overseas territories for the cartography of atolls, reefs et cetera. All shallow-water areas may be involved, in particular when the cost of usual hydrography would be out of proportion with maritime traffic. Remotely sensed imagery is mixed with all field information available (bathymetry, buoys,
This spatiochart also provides assistance in the search of waterways before an in situ survey, for example in lagoons scattered with coral structures. Images The remote-sensing images used by SHOM (French Hydrographic Office) are mainly SPOT imagery provided by SPOT IMAGE (www.spotimage.fr). If there is no image of the relevant area in the catalogue or if the quality is not adequate, for example when there are clouds, a programme planning is ordered. SPOT imagery uses the visible spectrum and the colour of water gives information about the depth. The horizontal resolution before SPOT 5 was 10m x 10 m or 20m x 20 m. The image can be purchased with differing levels of treatment: from 1A (homogenisation of the Charge-Coupled-Device1 measurements sticks) to 2B (with position rectification). SHOM chooses 1A to apply its own precise positioning and bathymetric models. Field Measurements The precise positioning of the image is obtained by site observations. Using zooms of the image, a team is in charge of determining the real position of pixels. If a pixel is identifiable without doubt (crossroads for example) a differential GPS position is sufficient. Otherwise, the operator follows a remarkable feature, for example, coastline, limit of vegetation, to continuously record his DGPS position: the comparison of the recorded path with the zoom at the same scale allows fixing this. The geometric rectification carried out by SHOM software makes possible generation, with a single base point, of a corrected image with a precision of the size of the pixel. However, usually SHOM uses the position of 5 pixels for an image in order to have redundancy and accuracy control. The bathymetry model is derived from the colour of the water. To initialise the model a minimal survey must be conducted: at least one line from the shoreline toward the sea. If the conditions, nature of the sea-bottom, turbidity and so on, are not homogeneous over the entire image complementary surveys must be carried out. Processing The software used by SHOM comprise packages dedicated, respectively, to geometric and radiometric processing. The steps involved in this processing are:
De-striping Due to the sensor (CCD sticks), there are stripes on some images, a difficulty in water areas due to the low radiometric dynamics. This noise must be removed before calculating the bathymetric model. Geometric Correction While being taken by the satellite the image is distorted as a result of certain factors such as yaw, pitch, roll, altitude and orbit parameters. The purpose of georeferencing is to transform the image coordinate system into a specific map projection. Orbit parameters provided by Spot Image and the geodetic field measurements are used to model and rectify distortions. The precision of positioning is about 10 metres. Pre-processing To check the quality of the image and characteristics of detected objects the following processes are run:
Building Masks Extracting Land-area At a scale of 1/50,000, nautical charts do not need details over land area. Hence, a uniform mask is built from the image. Depending on image quality and atoll geomorphology, SHOM uses either the NIR band of the NDVI band or a band resulting from PCA analyses for extraction of a land-mask. This mask is then compared with the Hue band. Extracting clouds and their shadows Due to the spectral bands of SPOT (in the visible spectrum) clouds may mask useful information. The shadows of such clouds minimise the true radiometric value of the pixels, thus affecting the accuracy of the bathymetric model. At SHOM, clouds and their shadows are merged as a unique cloudy mask. Calculating the Model of Bathymetry The signal received by the satellite is a composite of reflection from the water surface, water volume and the sea floor, leaving aside atmospheric interference. To get a reading on depth, at least some sunlight must be reaching the bottom and then being reflected back through the water towards the satellite. Water absorbs light, such absorption increasing with the wavelength until the light is almost totally absorbed in the NIR. So to extract depth contours several wavelengths are useful, with visible Blue going deepest, visible Green less deep, visible Red less deep and NIR not deep at all. SHOM uses the visible Green and Red bands of SPOT. Bathymetry is calculated in shallow waters, over the atoll's lagoon and off the cloudy mask. The model used at SHOM is classically based on the Lyzenga model: Z = A * LN (R1 - R1inf) + B * LN (R2 - R2inf) + C where Z The calculated depth of the pixel R1 The radiometry of the pixel in the visible Green band R2 The radiometry of the pixel in the visible Red band R1inf The radiometry, in the visible Green band, over a deep-water zone (where the radiometry is free from bottom effects) R2inf The radiometry, in the visible Red band, over a deep-water zone A, B, C The modelŐs coefficients. Thirty or more sounding measurements allow accurate calculation of these coefficients and thus calibration of the model The quality of the model depends on several factors:
Over a homogeneous atoll, such as Ouvea in New Caledonia, the relative error is lower than 10% for depth between [5-20] metres but increases for depth between [0-5] metres. This increase is due to the slope, to correlation of the visible Green band with the Red one (after 5m visible Red light is totally absorbed) and to a more heterogeneous feature bottom. La Spatiocarte Marine The land-mask, inter-tidal-mask, cloud mask and bathymetric model are merged as a thematic map. Bathymetry is mapped as coloured depth ranges of 5m, between (0-20) m. The scale of the spatiochart is about 1/50,000. Three negative films are generated by a laser-plotter:
These films and the thematic map are provided to the cartographic department for production of the navigational chart. Conclusion The processing of satellite images in areas of clear water provides an efficient and relatively inexpensive method of checking on an isolated item of information, optimising the work of survey missions, completing a partial survey or updating an old document. It is the method currently utilised by SHOM. When included in a nautical chart, information derived from satellite imagery is clearly presented in a different way than is ‘S44 hydrographic data’ due to differences in accuracy and reliability and in order not to confuse the mariner. But it is helpful information in huge areas where the data is scarce and expensive to collect: in Rangiroa atoll a pass to a tourist island was detected on a spatiochart and then checked by the SHOM survey team, allowing access to large cruise liner. This is just one example among numerous daily others. Notes
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| Biography of the author Ingénieur en général Michel Le Gouic joined the French HO in 1978 and surveyed at sea for seven years. He occupied various posts at the Main Establishment of SHOM in Brest engaging in R&D, including remote sensing and database management as deputy director. He is presently head of the Bureau for general affairs (external relationships and strategy) at SHOM Direction in Paris. Ingénieur chef Michel Even joined the French HO in 1991. He carried out surveys at sea for six years. At the Main Establishment of SHOM (EPSHOM) in Brest, he worked on remote sensing, database management, and various applications of GPS for positioning and tide measurement. He is now at SHOM Direction in Paris, where he heads the bureau that organises and controls the activities of various SHOM elements. Jean-Paul Tournay became a hydrographer in 1990 and contributed as a programmer to the development of multi-beam echosounder processing lines. In 1998 he was posted to SHOM in Brest, where he is involved in imagery processing, especially for spatiocharts, and in many studies on remote sensing. |
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| Archive |
| Archive > June 2004, Volume 8, Number 5 > Hydrographic Use of Satellite Imagery in South Pacific |
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