GIS as a Support to Soil Mapping in Southern Brazil
9.2 Material and Methods
The State of Rio Grande do Sul is located between latitudes 27◦00S and 33◦45S and longitudes ranging from 57◦40S to 49◦35W, and has borders with Argentina and Uruguay (See Figure 9.1). The study area is defined by a rectangle with a sur-face of 13,490 km2, situated at the Serra Ga´ucha region, NE portion of the State, between latitudes 28◦30S and 29◦30S and longitudes 50◦45W and 52◦W (See Fig. 9.1). The area covers 86 municipalities, 44 of them totally covered and 42 partially covered (20 with more than 50% of their territory and 22 with a lower proportion).
The material used consisted of navigation GPS receivers, Cartalnix (Clarklabs c) vector editing software, Idrisi (Clarklabs c) GIS software, Corel Draw (Corel Corporation c) desktop publishing software, and a cartographic base of the study area.
The cartographic database for the rectangle corresponding to the study area is composed by a set of 20 topographical map sheets from the Brazilian Systematic Mapping in scale 1:50,000 in UTM Projection, Zone 22. They were generated by the Brazilian Army (Diretoria de Servic¸o Geogr´afico – DSG) and are in the largest scale available as continuous mapping of the whole region. Despite their small scale, they represent a material of great relevance, since there is not any other better base available in larger scale.
The mapping process was conducted in a way to integrate the information about a semi-detailed soil survey and elements of the cartographic base for the generation of a GIS-structured digitized soil map. The soil survey was based on the methodology established by EMBRAPA (2006) for semi-detailed soil survey according to the
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Fig. 9.1 Location of the State of Rio Grande do Sul and the 20 map sheets in scale 1:50,000 of the Wine Zoning Project in Rio Grande do Sul
Brazilian Soil Classification System. Field work was carried out in order to make a description of the soil profiles and soil samples were collected for physical and chemical analyses with the support of GPS receivers and 1:50,000 scale topographic map sheets, laminated with a plastic film.
The use of GPS aimed to facilitate the association of the visited places with their correspondent sites in the topographic sheets. The use of laminated map sheets made the delimitation of the mapping units easier, giving the spatial basis for drawing the limits and providing support to make spatial adjustments and corrections. Moreover, laminated map sheets are more resistant and prevent the loss of information due to humidity, rain, tears, and other damages that may easily take place during the field work.
During the mapping stage, the main physiographic units of the study area were firstly delimited on the topographic charts. Then these physiographic units were traversed in the field from the lower parts to the highest point of the terrain, in order to visualize the sequence of soil distribution in the landscape and to establish a preliminary legend for soil types.
After that, the necessary routes for data collection were established based on detailed examination of the 1:50,000 topographic sheets and smaller scale maps of geology and soil types of the region. The soil survey along these routes was done through auger sampling and observation of trenches and road embankments.
During the field work, all the routes performed, the soil profiles described and ad-ditional points of interest were registered with GPS. Also at this stage, the necessary
9 GIS as a Support to Soil Mapping in Southern Brazil 107 adjustments and corrections to the preliminary legend of soil types were done in order to obtain a correct classification of the soil types found.
The distribution of the identified soil types, the knowledge about the relation soil-landscape acquired during the establishment of the preliminary legend and improved during the survey, the use of GPS and the equidistance of the contour lines in the topographic sheets made possible the identification of the points for observation and sample collection, as well as places of soil taxonomic class changing. For each taxonomic unit, a complete profile was described (Klamt et al., 2000) and, in some cases, a complementary one was done, based on Lemos and Santos (1996). This information was used to draw the limits of the mapping units on the topographic map sheets in scale 1:50,000.
The mapping process took into account the set of features that were potentially important in soil use. Among them, vegetation, relief, and the presence of gravels or rock outcrops were used to subdivide the units and used as indicators of water condi-tions, the susceptibility to erosion, and the possibility of mechanization. Some other elements used in separating the units were clay’s activity, saturation with bases, saturation with aluminum, the type of horizon A, texture and, for the less developed soils, the rock substratum. In some cases it was not possible to individualize soil types, either for the fact that some classes did not present geographic extension enough or because their intricate occurrence did not allow individual delimitation in the desired scale. In such cases, the mapping was done as soil associations.
Parallel to the field work for soil survey, a cartographic base was structured in a GIS through the digitizing of map sheets in scale 1:50,000. Paper maps were scanned using a large-sized scanner and were georeferenced based on an UTM grid.
The main information, like contour lines, hydrographic network, road system, and urban areas were then digitized on-screen. The respective layers were topologically structured and the objects were associated with a set of attributes in linked tables.
Lastly, a Digital Elevation Model (DEM) of the whole region was generated through interpolation based on the digitized contour lines (See Fig. 9.2).
After finishing the field work and the GIS cartographic base, the soil mapping units resulting of the conventional survey were digitized and edited. Following the same steps used for the cartographic base, the laminated map sheets with the soil
Fig. 9.2 DEM Hill shading of the 20 map sheets and 3D view of the region (SW to NE)
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mapping units drawn on the field were scanned and georeferenced based on an UTM grid. Then the limits of the mapping units were digitized manually on-screen, us-ing the georeferenced field map sheets as a basis, producus-ing vector line features (See Fig. 9.3).
Fig. 9.3 The topographic map sheet resulting from the field work and the mapping units after digitized (See also Plate 10 in the Colour Plate Section)
Vector lines extraction was performed in a continuous way, alternating the back-drops but capturing the limits of the mapping units in a unique layer without the map sheet’s divisions. After the topologic structuring of the limits of mapping units, soil polygons were built and associated with a set of attributes, such as their area (in hectares), order, suborder, group and subgroup, acronym of the mapping unit, among other features. The objective of this procedure was to guarantee the consis-tence of the attributes and the spatial contiguity of the polygons among contiguous sheets, in order to generate a vector polygon file of soil types with the continuous coverage of the 20 map sheets.
The last step involved the preparation of the material for printing. The area of each of the 20 map sheets was clipped and used to generate a printing layout with the desired layers, including soil layer, complementary layers (hydrographic network, road system, and urbanized areas) and ancillary information (legend, map grid, text layer, etc.). In order to keep a regional context, the complete legend of the whole region was used in each sheet legend, turning grey the ones absent at the respective map sheet. In this printing layout the soil mapping units were colored using the colors defined by EMBRAPA (2006). Finally, in order to include relief information with an easier perception and comprehension than with simple contour lines, the colored soil polygons were combined with a DEM analytical hill shading.