Manual – OpenStreetMap
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.
The OpenStreetMap 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 here and here.
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 instead.
OpenStreetMap projection distortion
The map downloaded from OpenStreetMap uses the Mercator projection, which is 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 distortion cannot be ignored. Distances measured on a Mercator map are also greatly exaggerated near the poles.
Removing the distortion
To compensate for the distortions induced by the Mercator projection, MapAnalyst re-projects the control points before computing transformations, distortion grids, displacement vectors, isolines or other visualizations. 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.
Medium and small-scale maps
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 FAQ for more information on this.
Import and export of points of the OpenStreetMap
Points of the OpenStreetMap can be imported and exported in longitude/latitude format. Geographic coordinates are in decimal degrees using the standard web map datum with a spherical radius 6,378,137 m. See Points and Links for more information about import and export capabilities.
Misalignment of points created with MapAnalyst 1.3.0 to 1.3.23
MapAnalyst version 1.3.0 to 1.3.23 had a bug that resulted in wrong coordinates when exporting OpenStreetMap points. This bug was fixed in version 1.3.24, but OpenStreetMap points created with earlier versions of MapAnalyst will be offset in version 1.3.24 and later by a few kilometres when displayed in the OpenStreetMap in MapAnalyst. Version 1.3.24 contains a new menu command to correct for this offset. Select Maps > Correct OpenStreetMap Misalignment. Note: only apply this correction once to your points, then save the project or export the points to a new file.