Georeferencing of topographical maps using the software arcgis.

Abstract

The digital map must be built by using all resources available based on a rigorous analysis of their content and costs involved, having purpose of reaching necessary quality under conditions of maximum efficiency. Each data source implies existence of specialized software which will bring that data in a digital map format, starting with insuring necessary equipment, going through technological problems and data conversion and ending with proper training and preparation of personal. Before using spatial data in Geographical Information System it is necessary to define spatial reference system that can establish with maximum accuracy corresponding positions from earth. This can be done through a process called georeferencing process that can identify ellipsoids and cartographical projections simulating real space. The georeferencing process implies framing of a plan representation into a reference system called location. For georeferencing a map we need at least two pieces of information: a series of points easily identifiable on map whose location (latitude and longitude, rectangular coordinates) is precise (here principle is: the more, better) and projection system where original map on paper was made. The georeferencing process of topographical maps based on known coordinates represents most ordinary process of georeferencing a scanned map. This type of georeferencing process is used when we can accurately find positions of points on surface of map (usually a topographical map). During time in Romania there have been used more projection systems, and one used from 1971 until present is Stereographical Projection 1970 on unique secant plan based on Krasowsy ellipsoid. Before using georeferencing process, we must define Romanian National Projection system into a CAD / GIS software. The parameters of this projection which will be used to input in used software are: Code: Stereo 70; Description: Stereographical Projection 1970; Coordinate System Type: Geodetic; Datum: Pulkovo 1941 Russia (GIS and former USSR Teriritories); Oblique: Stereographic; Northing: 500000, Easting: 500000; Projection Parameters: Origin latitude: 46d , Origin longitude: 25d; Scale reduction: 0.9998.