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Digital Cartographic standards for 1:50,000 Geologic map: some important considerations
Descriptions of geologic features, relevant examples, cartographic specifications and notes on how to use the symbols should accompany a map symbolization standard. The standard should aim to be scale dependent, implying their usability with maps compiled or published at any scale. It should be applicable to both print and web media. While developing such a standard the following issues have to be taken into consideration beforehand.
- Interpretive nature of Geologic map
- Complexity of Indian geological set up
- Scale of original survey
- Basemap content
- Presentation of structural features
- Adherence to international conventions
- Media of presentation
In the following sections each of these issues are discussed in detail.
Interpretive nature of geologic map:
In bedrock geologic map, the extent of exposure and the character of the map units (Formations in case of 1:50,000 map) control the accuracy of the map. Often it is difficult to find natural rock units of suitable thickness for mapping at the intended scale and composed of an easily identifiable rock type. This implies that the identification of map units are not always objective and is fraught with uncertainty. Typically large portions of the map area reveal no outcrops (exposed bedrocks or surficial materials on the land surface). Consequently outcrop information must be extrapolated to cover the entire map. This extrapolation is based on understanding of the structure, and stratigraphy of the area (Spencer, 1993; Maltman, 1998). Mapping thus proceeds by observation, formulating hypothesis, testing further observation and so on. The final product is a mixture of recorded observations and the hypothesis that the field geologist considers to best connect those observations. The observations almost always reflect the present state of geologic knowledge. This is the major reason that advances in geologic knowledge commonly necessitates remapping of regions. Proper understanding of the stratigraphy of the area is needed so that the field lithology units can be translated into map units. Standard symbology for map unit display can only be prepared when standard definitions for the geologic features have been specified. A unified stratigraphic legend is a primary requisite to achieve this purpose. The intended standard document should specify the characteristics (lithological, structural, etc.) by which the field units can be assigned to respective map units.
Complexity of Indian Geology:
India is one of the rare subcontinents where, rock units ranging the entire time scale, from Archaean to Quaternary are exposed. Correlation between the diverse spatio-temporal geological sequences is an enormous problem. The complexity further aggravates due to the fact that evolution and disposition of the different geological units are not similar throughout the region. Some headway, however, could be made by considering the three physiographic domains, namely, the
- Peninsular shield,
- Extra-peninsular shield and
- Indo-gangetic alluvial tract.
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