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Double stereographic projection as an alternative to UTM for large scale maps
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Madjid Abbasi *   , Reza Dousti , Hamed Afsharnia |
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Abstract: (5 Views) |
The UTM projection is practically the standard map projection used in Iran. For medium and small-scale maps, there is not much problem in using UTM, but in large-scale maps where the high accuracy quantities are extracted, the inherent distortions caused by projecting the geometric features from the Earth’s surface to the plane are inevitable. It is easy to show that with this projection system, one kilometer distance on the Earth’s surface may differ up to tens of centimeters with its corresponding length on the map. On the other hand, the coordinate system changes from one UTM zone to another, which may confuse map users as well as some surveyors in transition areas. Although this problem is not specific to UTM, the problem can be reduced by choosing planar projections. Most of the map users prefer to avoid the coordinate system change and the distortions due to the map projection. As a result, most surveyors in Iran use a kind of local coordinate system with the contradictory name of "local UTM". Assuming a spherical earth with smooth topographical features up to a radius of less than 30km, this localization procedure is an "azimuthal" planar projection. In this case, the distances in the radial direction relative to a central point are undistorted. In rough topography, however, this projection system is neither conformal nor equidistant. In this research, the problems caused by this localization approach are discussed in detail, and the use of a double stereographic projection is proposed as a solution. The advantage of this projection is that it is conformal and equidistant up to about 30 km from a central point. The scale factor is practically equal to unity; thus, it does not apply significant distortion to the projected geometric features. After the theoretical analysis, some numerical examples of the behavior of height, grid and combined scale factors, as well as the discussed map projections are presented.
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Article number: 1 |
| Keywords: large-scale maps, UTM projection, Equidistant Azimuthal projection, Double Stereographic projection |
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Full-Text [PDF 3186 kb]
(3 Downloads)
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Type of Study: Research |
Subject:
Geo&Hydro
Received: 2024/07/21
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