Dem

- TIN from DGPS point source

- Grid from topographic map

FIGURE 10.3 Data layers created for use in a geographic information system (GIS) for the Management System Evaluation Area (MSEA) site in Ohio. (Courtesy of S.-S. Lin, pers. comm., 2006.)

original data. The errors that result from coordinate transformation must be considered to ensure the quality of each data layer. Most surveys performed on small areas are based on the assumption that the earth is a planar surface. For large-area surveys, it is necessary to consider the earth's curvature by computing an x,y position in terms of geodetic latitude and longitude. These computations can be quite complicated and lengthy, so a plane rectangular coordinate system is preferable.

Due to the constant improvement of geodetic surveys, the lack of agreement for a standard coordinate system among agencies, and the slow update of the older geodata layers, GIS analysts often encounter spatial data in a variety of projections and coordinate systems. For example, one may obtain a soils map in an Alber's equal area projection, Digital Elevation Model (DEM) data in a UTM coordinate system using the NAD 1927 datum, Global Positioning System (GPS) data of specific points located in a geographic coordinate system, a Landsat Thematic Mapper (TM) image in a UTM coordinate system using the WGS 1984 datum, or aerial photographs with a local coordinate system. Familiarity with the concepts of map projection and datums is necessary to convert data layers with different coordinate systems into a unified standard coordinate system.

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