THE DIGITAL SOIL MAP OF WALLONIA (DSMW/CNSW)

Transcription

1 THE DIGITAL SOIL MAP OF WALLONIA (DSMW/CNSW) Ph. Veron, B. Bah, Ch. Bracke, Ph. Lejeune, J. Rondeux, L. Bock Gembloux Agricultural University Passage des Déportés 2, B-5030 Gembloux, Belgium, A.I. Mokadem Ministry of the Walloon Region General Direction of Agriculture Chaussée de Louvain 14, B-5000 Namur, Belgium, ABSTRACT Based on the Belgian soil map at 1/20,000, this digital soil map of Wallonia (DSMW) is composed of a cartographic database describing the soil characteristics of the region. Tool for a better soil management, this database will be integrated in a geographical information system where the soil layer could be used in relation with information concerning other aspects of land development. The different operations include : map drawing finalization of non edited sheets, creation of an image layer, vectorial capture of source objects, building up of the topology, characterization of the objects, geometrical corrections, creation of a continuous layer (edge-matching), creation of a homogeneous legend, integration in a geodatabase. The DSMW in figures corresponds to : 16,000 km 2 ; 600,000 soil polygons delimited by 15 million intermediate points (vertex); 6,000 pedological symbols; 700 MB. The advantages of this computerized cartographic database are continuity, legibility, accessibility and many possibilities of thematic analyses. SOIL AS A MAJOR COMPONENT OF OUR ENVIRONMENT Soil is one of the major components of our environment. It fulfils a function of support for the whole of human activities. This function is especially important for agricultural and forest production, but also for all the terrestrial ecosystems. Soil plays the role of filter in the restocking process of subterranean water tables. It also constitutes an important reservoir of biodiversity. THE DIGITAL SOIL MAP OF WALLONIA (DSMW), A TOOL FOR BETTER SOIL MANAGEMENT In application of the Environmental Plan for Sustainable Development (EPSD/PEDD), the regional Walloon government has decided to promote a project in view to obtain a tool offering a better knowledge and therefore, a more rational soil management; it is the DSMW. This project aims at building up and updating for the entire Walloon territory a geographical database describing the soil characteristics : texture, drainage,. It also has for goal to promote awareness of data collected in this way, by the development of decision making tools. This database will be integrated in a regional geographical information system where the soil layer will be used in relation with information relative to other aspects of land development (relief, land use, water, cadastral register, ). The Ministry of Agriculture and Rurality has entrusted a research team of the Gembloux Agricultural University (GAU/FUSAGx) with the production of the DSMW

2 TOWARDS DSMW From 1947 to 1991, the Institute for the promotion of Scientific Research in Industry and Agriculture (ISRIA/IRSIA) financed the realization of the Soil Map of Belgium. This map is the result of a 75 m. grid survey by augerings (± 6 million augerings for Belgium) and the limits of soil units were successively : drawn at field level on cadastral plan at 1/5,000 scale, called survey, transferred on topographical background at 1/10,000 scale, called minute, and finally, printed by the National Geographic Institute (NGI/IGN) in the form of colour sheets at 1/20,000 scale, each covering 8,000 ha (except at international borders) and having an explicative booklet. Approximately 80 % of the Walloon territory is covered by 200 edited sheets, the other 20 % or 57 sheets still being in the form of minutes. PHASES OF DSMW CREATION The first phase, called restitution phase, comprises four stages which are performed sheet by sheet (figure 1) : edited sheets at 1/20,000 are scanned to produce a geo-referenced image layer providing a reference background for the continuation of operations, soil unit limits are then digitalized in vectorial mode to produce the principal layer of the database made of polygons (soil units) and of noticeable lines (valley soil limits for example), topological relations between objects are then built, guaranteeing a coherent spatial structure, the last stage consists in a codification of each soil unit corresponding to its pedological characteristics. Each of these stages is approved by a control guaranteeing the quality of seized data and of the executed work. It is important to stress that one of the objectives of the restitution phase consists in the finalization of unedited sheets before their digitalization so as to obtain complete coverage of the Walloon territory.

3 Edited sheets Non-edited sheets (map drawing) Map drawing finalization Creation of an image layer Vectorial capture of source objects (contour of soil polygons) Building up of the topology Characterization of the objects (pedological symbols) Figure 1 : Stages of the restitution phase

4 The image layer obtained at the end of the first stage constitutes an already usable product in many applications necessitating punctual determinations of pedological characteristics. The second phase, called integration phase, comprises three stages and concerns the whole vectorial layer produced during the restitution phase (figure 5) : the distortions observed between the limits of certain soil units with regard to the corresponding field figures (valley bottom for example) and to the hydrographical network of the reference NGI topographical background at 1/10,000 have to be attenuated (figure 2). Although, this stage concerns all the sheets, more attention is paid to the sheets edited on an old topographic background (BONNE projection), for which the distortion with regard to the present topographical background (LAMBERT72 projection) are the most important. Approximately 40 % of the sheets have been edited on this old topographical background. Figure 2 : Example of distortions observed between a soil unit of a sheet edited on old topographical background (chestnut colour) and the field figure corresponding to the reference topographical background (hydrographical network in blue). The method selected is based on the application of corrective factors defined locally, themselves based on the identification of control points dispersed within the zones to be corrected for a given sheet. These control points are located on the soil map (document to be corrected) as well as on the reference topographical background. A translation is then calculated for each digitised point on the soil map. It represents the weighted average of the shifts observed on the control points as a whole; the weighting being function of the square of the distance separating the point to be corrected from the different control points. The number and the position of the control points are function of several factors : - the precision required, - the importance of existing shifts between the two documents, - the number of noticeable points (crossroads, churches, confluents, rivers, etc ) identified without any ambiguity on the two maps. The major difficulty of the method lies in the identification of reliable field figures, i.e. those for which the deformations result only from the change of the projection system (BONNE to LAMBERT) and/or from the imprecision of the production methods of the old topographical background. The seams between neighbouring sheets have to be verified to correct possible discrepancies, whether they are of geometric or semantic order (figures 3 and 4). No automatic correction is executed (for example on the base of the polygon size) for the seams. The corrections applied, when geometric or semantic discontinuities are observed, are the result of a pedological reflection and the execution case by case by an operator.

5 The systematic integration of this pedological analysis of the discontinuity cases encountered ensures a far better quality of the corrections made compared to methods of automatic corrections, admittedly quicker but very imprecise from a pedological point of view. Figure 3: Example of geometric discrepancy between two pedological sheets Figure 4 : Example of semantic discrepancy between two pedological sheets Finally, a homogeneous legend for the whole collection of sheets is produced. During the realization of the ISRIA Soil Map of Belgium between 1947 and 1991, the legend evolved, for example due to the observation of new pedological characteristics or to the determination of new soil series. As a result, although the general principles of the legend remain identical for the whole collection of sheets, specific characteristics have been introduced for certain sheets. A unique and homogeneous legend for the entire Walloon territory is produced in such a way as to obtain a thematic continuity between the different sheets. The general principle of the unique and homogeneous legend is to associate to each pedological feature one and only one definition.

6 Geometrical corrections Creation of a continuous layer (edge matching) Creation of a homogeneous legend Figure 5 : Stages of the integration phase (source documents in French) Finally, the vectorial layer produced is integrated in a geodatabase (figure 6).

7 Integration in a Geodatabase Figure 6 : Creation of the geodatabase (source documents in French) PERSPECTIVES OF USE OF THE DSMW The soil map in its present version has many users : regional, inter-communal and communal administrations, universities, research institutes, consulting offices, private experts, farmers, foresters, naturalists,. The projects using the soil map are very diversified : nature conservation, forest development (site aptitude, species choice, forest zoning), agricultural monitoring (advice to farmers, experimental fields, ), land development (zoning plan revision, drawing up of communal structure schemes, communal development plans, ), land improvement (land reallocation, artificial drainage, layout of agricultural lanes, ), impact study,

8 The information extracted from the soil map is rarely used in its primary state. It is more often simplified or transformed under the form of thematic variables of different types : agricultural or forest aptitudes, sensitivity (compaction, erosion, ), soil capacity (infiltration, ), The advantages of a computerized cartographic database are numerous : continuity : suppression of the paper map divisions, better legibility : the map comprises an important number of different soils which are sometimes very difficult to distinguish on the paper map. In the database, the information associated to each soil unit is automatically identifiable and without error or risk of confusion, greater accessibility : the integration of the database in a server of cartographic data offers the possibility to access in real time the pedological information for any part of the regional territory, thematic analyses : the integration of the soil map into a geographical information system (SIG) offers many possibilities in terms of thematic analysis in combination with other layers of information (relief, hydrology, land use, vegetation, ). THE DSMW IN A FEW FIGURES area covered : 16,000 km 2, number of cartographic units : 600,000, delimited by 15 million intermediate points (vertex), number of pedological symbols : 6,000, size of the geodatabase : 700 MB.