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A collated digital, geological map database for the central Namaqua Province using geographical information system technology

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  • Qe Geology
  • Archaeology
  • Chemistry
  • Earth Science
  • Economics
  • Physics


The genlogy of the Namaqua Province is notoriously difficult to map and interpret due to polymetamorphic and multiple deformation events and limlted outcrop. Current maps of the Province reflect diverse interpretations of stratigraphy as a consequence of these difficulties. A Geographic Information System is essentially a digital database and a set of functions and procedures to capture, analyse and manipulate spatially related data. A GIS is therefore ideally suited to the study and analysis of maps. A digital map database was established, using modem GIS technology, to facilitate the collation of existing maps of an area in the Central Namaqua Province (CNP). This database is based on a lithological classification system similar to that used by Harris (1992), rather than on an interpretive stratigraphic model. In order to establish the database, existing geological maps were scanned into a GIS, and lines of outcrop and lithological contacts were digitised using a manual line following process, which is one of the functions native to a GIS. Attribute data were then attached to the resultant polygons. The attribute database consists of lithological, textural and mineralogical data, as well as stratigraphical classification data according to the South African Committee for Stratigraphy (SACS), correlative names assigned to units by the Precambrian Research Unit, the Geological Survey of South Africa, the Bushmanland Research group and the University of the Orange Free State. Other attribute data included in the database, are tectonic and absolute age information, and the terrane classification for the area. This database reflects the main objective of the project and also serves as a basis for further expansion of a geological GIS for the CNP. Cartographic and database capabilities of the GIS were employed to produce a collated lithological map of the CNP. A TNTmipsTM Spatial Manipulation Language routine was written to produce a database containing two fields linked to each polygon, one for lithology and one for a correlation probability factor. Correlation factors are calculated in this routine from three variables, namely the prominence a worker attached to a specific lithology within a unit or outcrop, the agreement amongst the various workers on the actual lithology present within an outcrop, and the correspondence between the source of the spatial element (mapped outcrop) and the source of the attribute data attached to it. Outcrops were displayed on the map according to the lithology with the highest correlation factor, providing a unique view of the spatial relationships and distribution patterns of lithological units in the CNP. A second map was produced indicating the correlation factors for lithologies within the CNP. Thematic maps are produced in a GIS by selecting spatial elements according to a set of criteria, usually based on the attribute database, and then displaying the elements as maps. Maps created by this process are known as customised maps, since users of the GIS can customise the selection and display of elements according to their needs. For instance, all outcrops of rock units containing particular lithologies of a given age occurring in a specific terrane can be displayed - either on screen or printed out as a map. The database also makes it possible to plot maps according to different stratigraphic classification systems. Areas where various workers disagree on the stratigraphic classification of units can be isolated, and displayed as separate maps in order to aid in the collation process. The database can assist SACS in identifying areas in the CNP where stratigraphic classification is still lacking or agreements on stratigraphic nomenclature have not yet been attained. More than one database can be attached to the spatial elements in a GIS, and the Namaqua-GIS can therefore be expanded to include geochemical, geophysical, economic, structural and geographical data. Other data on the area, such as more detailed maps, photographs and satellite images can be attached to the lithological map database in the correct spatial relationship. Another advantage of a GIS is the facility to continually update the database(s) as more information becomes available and/or as interpretation of the area is refined.

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