Proceedings of the Symposium on Global and Environmental Monitoring: Proceedings of the Symposium on Global and Environmental Monitoring

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Multivolume work

Persistent identifier:: 856665355

Title:: Proceedings of the Symposium on Global and Environmental Monitoring

Sub title:: techniques and impacts ; September 17 - 21, 1990, Victoria Conference Centre, Victoria, British Columbia, Canada

Year of publication:: 1990

Place of publication:: Victoria, BC

Publisher of the original:: [Verlag nicht ermittelbar]

Identifier (digital):: 856665355

Language:: English

Document type:: Multivolume work

Volume

Persistent identifier:: 856669164

Title:: Proceedings of the Symposium on Global and Environmental Monitoring

Sub title:: techniques and impacts; September 17 - 21, 1990, Victoria Conference Centre, Victoria, British Columbia, Canada

Scope:: XIV, 912 Seiten

Year of publication:: 1990

Place of publication:: Victoria, BC

Publisher of the original:: [Verlag nicht ermittelbar]

Identifier (digital):: 856669164

Illustration:: Illustrationen, Diagramme, Karten

Signature of the source:: ZS 312(28,7,1)

Language:: English

Usage licence:: Attribution 4.0 International (CC BY 4.0)

Editor:: International Society for Photogrammetry and Remote Sensing, Commission of Photographic and Remote Sensing Data

Publisher of the digital copy:: Technische Informationsbibliothek Hannover

Place of publication of the digital copy:: Hannover

Year of publication of the original:: 2016

Document type:: Volume

Collection:: Earth sciences

Chapter

Title:: [TP-4 SOILS]

Document type:: Multivolume work

Structure type:: Chapter

Chapter

Title:: EROSION RISK MAPPING IN THE TROPICS. J. D. Flach, S. M.White, W. G.Collins, T. R. E. Chidley

Document type:: Multivolume work

Structure type:: Chapter

3. PRODUCTION OF THE MODEL INPUT DATABASE The production of data for use as input to the erosion risk model can be separated into three main sources of data : thematic maps, topographic maps and remote sensing. All these data were converted into a common form using the PC based ICONOCLAST Image Processing and Spatial Information System, developed by Image Technology Systems Ltd. Prior to the digitisation process the effective scale of the required data was chosen. As the input to the model and manipulation of these data is essentially a grid cell (raster based), the size of the cell is the major factor. This was chosen to be 150m, giving a thematic database of 500 x 700 cells (pixels). 3.1. Thematic Maps The production of the thematic map database produced inputs related to the resitance of soil to erosion, namely the Geology and Soils of the region. Digitisation of soil and geology class boundaries was achieved using both a digitising tablet, and a process of video digitisation and boundary extraction. Both resulted in a class boundary vector database extracted in the pre-defined co-ordinate system. The boundary data from various mapped sheets was then aggregated and any map sheet edge problems resolved. The boundaries were then used in a graphical seed fill process, thus converting each pixel within a boundary to a pixel value indicative of class. The boundaries are then removed by a process of line replacement from adjacent pixels. In this type of raster based Spatial Information Systems, pixel brightness is related to a table describing class attributes. Thus to obtain the class of any pixel is a simple case of looking up the class table. The location and topology of each pixel is only known from its position in the grid. A number of other thematic databases where also created at this stage to assist in future work. These included Land Management Units, (official) Land Use and Stream Sub-catchments. 3.2. Topographic Maps Given that slope and slope length, aspect and elevation are important considerations in the model a digital elevation model (DEM) had to be constructed. In may parts of the developed world DEM are commercially available, for developing countries this information is often restricted. It is therefore necessary to have tools for generating a DEM from any available data. For the study area large scale topographic maps were not available, due to government restriction, and an alternative source of data was needed. Tactical Pilotage Maps are generally available worldwide and this was used as a source of all topographic information for the study. Individual section of the pilotage map were raster digitised at high resolution to facilitate the extraction of contour data. All relevant contour information was extracted by a manual process of on-screen digitisation - an extremely laborious task. Other topographic features such as roads, built areas and the drainage network were also extracted to assist in the geometric rectification of the remote sensing input. The elevation data consisting of contours and spot heights was then converted to a raster representation of height by an interpolation process. A number of methods were tested including various Least Squares (Jancaitis and Junkins, 1973) and Multiquadric (Hardy, 1971) procedures. Later work suggested the more efficient use of triangulation routines. Given a satisfactory raster representation of elevation, where pixel brightness is directly proportional to height, slope, aspect and other related measures can be derived. The algorithm used for generation of slope and aspect for the model was a modified version of the algorithm developed by Ritter (1987). 3.3. Remote Sensing The difference between land use detailed on available maps and actual land use was considered to be significant; particularly in upland areas where considerable de-forestation had occurred. Satellite remote sensing was seen as a means of providing up to date land use information at sufficient scale for use in the model. A Landsat MSS scene was acquired and subsequently used to produce a land use map using multi-spectral classification techniques. A 1536 x 1536 extract was taken from the full MSS scene. This was first used to define a number of ground control points using on-site experience and the topographic map. The extract was then corrected to the database co-ordinate system, primarily for use as a frontispiece in a report overlayed with topographic features and catchment boundaries. What is also extremely evident on this image is the siltation of the reservoir compared to the downstream irrigation channel. The extract was then used to define training areas for land use. Training statistics were produced and used in a Maximum Likelihood classification of the extract. The accuracy of the classification was visibly checked for a further set of areas for which ground truth was known. In light of the available alternative, and subsequent empirical processes involved in the model, the classification was considered to be sufficiently accurate and the classification was then corrected to the database co-ordinate system. 131

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