Digital Cartographic standards for 1:50,000 Geologic map: some important considerations  Asit Saha  Basab Mukhopadhyay  Niladri Hazra Dhruva Ganguly, Sujit Ranjan Sengupta Geological Survey of India, Kolkata Email: gsi_chq@vsnl.com Abstract A geologic map is the principal tool that geologists use to convey information about the structure and stratigraphy of the earth’s surface; the location and type of geologic hazards such as landslides and faults; and the location and type of earth resources such as sand and gravel, coal, oil, ore deposits, and ground water. The most powerful and useful aspect of geological maps (especially large-scale) is the correct representation of rocks in three dimensions (areal extent and height and depth information) with an added dimension of time (geological age). Taken this into consideration, a geological map can then be described as a four-dimensional information system on a horizontal reference plane, which is crucial in understanding not only where earth resources and characteristics are located, but also how and when these earth features were formed. The primary objective of geological map use is to reconstruct the geological evolution of the area. A geologist must be able to understand the geometries of the rock bodies and their lateral and vertical relationships from the map. Idea about the three dimensional nature of the of the rocks and their relative ages can be obtained from this information which in turn will help interpreting the geological history of the area. Having acquired this knowledge, the geologist can then apply his understanding in solving problems related to earth resources, hazard assessment, and land-use planning. It is evident that geological maps are extremely complex with many different types of information displayed. Additional complexity is introduced by the lack of consistency at different levels, starting from original surveying to design of GIS databases and in the products derived from them. Consistency from one map to another is not a primary concern when users are focused on local problems. As geologic maps are used for larger, regional studies, consistency of data representation becomes more important. Implementation of standards for data generation, database designing and presentation can facilitate the sharing of information within an organization and between organizations. GSI has been publishing hard copy geological maps at scales of 1:250,000, 1: 1,000,000 and 1: 5,000,000. While doing so GSI has been using international and national standards for map presentation. Field survey has always been done at a larger scale (1:50,000 and 1:25,000) than the published maps. Geological Survey of India is committed to participate as a contributor agency of the NSDI and has committed 1:50,000 geologic maps as the agency layer to NSDI. Accordingly it has become a prime necessity to develop national standards for geologic map presentation at a scale of 1:50,000. While developing the digital cartographic standards the following important issues have to be taken into consideration.
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