The OpenStreetMap is well suited for analyzing maps at large scales. If the analyzed map is not very accurate, medium scale maps can also be analyzed using OpenStreetMap. If the east-west extension of the old map exceeds approximately 10 degrees, a different reference map with a projection approximating the projection of the old map should be used. is a collaborative project to create a free editable map of the world. It covers large part of Europe, North America and other parts of the world. The maps are created using data from portable GPS devices, aerial photography and other free sources. The quality of the data is comparable to other maps, as shown MapAnalyst downloads a map from an OpenStreetMap server. Hence, to use OpenStreetMap, a reasonably fast Internet connection is required. If the server cannot be accessed by MapAnalyst, a regular icon pattern is displayed. The map downloaded from OpenStreetMap uses the Mercator projection, which is quite an unfortunate choice for the distortion analysis of old maps, because the Mercator projection adds considerable areal distortion. Areal distortion increases dramatically near the poles, for example the size of Greenland is more than ten times too large on a Mercator map. But also at latitudes closer to the equator, this additional source of distortion cannot be ignored. Distances measured on a Mercator map are also greatly exaggerated near the poles. To compensate for the distortions induced by the Mercator projection, MapAnalyst re-projects the control points before computing or other visualization. The following procedure is automatically applied by MapAnalyst to remove the distortion introduced by the Mercator projection: The control points are projected from the Mercator projection to geographical coordinates using the WGS84 ellipsoid (this is a so-called inverse projection). The points in geographical coordinates are then projected to a Transverse Cylindrical Equal Area projection centered on the mean longitude of all control points. The points in the Transverse Cylindrical Equal Area projection are then used to compute a transformation between the old and the new map, including the global scale factor and the global rotation angle. The same coordinates are used to compute the different visualizations. The visualizations for the new reference map (e.g. the distortion grid) are finally projected to the Mercator projection of OpenStreetMap and displayed. The Transverse Cylindrical Equal Area projection is much better suited than the Mercator projection for most map analyses, as it introduces much smaller distortions. However, the Transverse Cylindrical Equal Area is not perfect either. If old maps at small scales are analyzed showing entire continents or even the complete globe, the Transverse Cylindrical Equal Area projection is not well suited either. For such small-scale analyses, a reference map using a projection that approximates the projection of the old map should be used. See also the Source.

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