Abstract
Various software for Geographical Information Systems (GISs) have been developed and used in many different engineering projects. In GIS applications, map coverage is important in terms of performing reliable and meaningful queries. Map projections can be conformal, equal-area and equidistant. The goal of an application plays an important role in choosing one of those projections. Choosing the equal-area projection for an application in which area information is used (forestry, agriculture, ecosystem etc) reduces the amount of distortion on the area, but many users using GIS ignore this fact and continue to use applications with present map sheets no matter in what map projection it is. For example, extracting area information from data whose country system's map sheet is in conformal projection is relatively more distorted, compared to an equal-area projection one. The goal of this study is to make the best decision in choosing the most proper equal-area projection among the choices provided by ArcGIS 9.0, which is a popular GIS software package, and making a comparison on area errors when conformal projection is used. In this study, the area of parcels chosen in three different regions and geographic coordinates and whose sizes vary between 0.01 to 1,000,000 ha are calculated according to Transversal Mercator (TM, 3°), Universal Transversal Mercator (UTM, 6°) and 14 different equal-area projections existing in the ArcGIS 9.0 GIS software package. The parcel areas calculated with geographical coordinates are accepted as definite. The difference between the sizes calculated according to projection coordinates and real sizes of the parcels are determined. Consequently, the appropriate projections are decided for the areas smaller and equal than 1,000 ha and greater than 1,000 ha in the GIS software package.
Highlights
The number of Geographical Information Systems (GISs) applications on large regions has rapidly increased in recent years
The area of parcels chosen in three different regions and geographic coordinates and whose sizes vary between 0.01 to 1,000,000 ha are calculated according to Transversal Mercator (TM, 3 ), Universal Transversal Mercator (UTM, 6 ) and 14 different equal-area projections existing in the ArcGIS 9.0 GIS software package
This type of calculation does not give the definite size of area but it includes area distortions related to the chosen projection type
Summary
The number of GIS applications on large regions has rapidly increased in recent years. The coordinates on map projections are the transfer of ellipsoidal geographic coordinates onto the map or plane using a proper projection method Based on these coordinates, some values, such as length, angle, and area are calculated and the required data (statistical information) is acquired. Obtaining such data based on ellipsoidal geographic coordinates using a GIS software package is not possible It is only possible with map projection coordinates [1, 2]. Instead of ellipsoidal geographic coordinates, the calculation is performed based on projection coordinates This type of calculation does not give the definite size of area but it includes area distortions related to the chosen projection type. Since TM and UTM are used as map sheets in many countries, the conformal projections (TM, UTM) are compared with the equalarea projections
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