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3.4.2 Identification of Projected Coordinate Systems

If a map or coordinate list is provided for which an Epicentre listed coordinate system is clearly identifiable, then its name or identifier will address the required parameters and their Coordinate_transformation_values within Epicentre. If the coordinate system is not listed it will be necessary to create a new coordinate system with its own coordinate transformation (parameter) values.

It may often happen that one is presented with a coordinate list or map for which the author or compiler has regrettably failed to provide any indication of parameter values or properties, - no projection name, no grid definition and no statement of spheroid or datum. On the map there may be no grid or graticule, or indeed neither.

In order to adequately relate the digital or displayed map data to other data it is necessary to establish the properties of the data or given map from what may be gleaned from their appearance and other information. Geographical coordinates without qualifying information do not allow identification of the coordinate system other than that it is a geographic one. Projected or grid coordinates may, by virtue of the actual and relative magnitudes of the Easting and Northing and knowledge of where in the world they relate to, provide clues as to the projection system. For example eastings between say 150000m and 850000m, allied with 6 or 7 figure northings correlated with latitude may indicate a UTM.

If the map bears neither grid nor graticule it will be useless unless one can identify a number of the point features shown for which one already has coordinate data. One may then be able to superimpose and fit a rectangular grid at appropriate scale from which other coordinate data may be read. If the map carries a grid then the numerical labelling of the grid lines, the assumption that it will be conformal or orthomorphic, and prior knowledge of approximately where in the world it covers may give some indication of the type of projection, but this may not be totally definitive. If the map bears a graticule the nature of the graticule lines will give some indication of the type of projection used in its compilation. For example straight meridians and concentric parallels would suggest a conical projection or, less frequently, a polar azimuthal. If the former, and assuming that it will be orthomorphic, then it will either be with one standard parallel or two and these will have been selected in relation to the latitudinal extent of the area, very possibly those in general use for that state's mapping. If the parallels are equally spaced it will be a simple equidistant conical projection. However for large scale mapping purposes the requirement that it is conformal will dictate that the parallels will not be equally spaced and it is more than likely that it will be some form of Lambert projection with either one or two standard parallels. Unfortunately there is no easy way of detecting which, nor the values of the standard parallels. The country it comes from and its national mapping system, if known, may suggest what these are. The Epicentre reference list will assist but is not exhaustive.

If both meridians and parallels of latitude are straight it will be a cylindrical projection but of the normal and not the more frequent transverse or oblique variety. Of the normal aspect cylindrical projections only the Mercator is conformal and it is not frequently used for mapping though it is invariably used for the production of marine navigation charts.

If both parallels of latitude and meridians are curved the projection has numerous possibilities but a form of transverse mercator may well be the most likely. One may attempt to identify the projection by computing the grid values of some of the graticule intersections for several possible projections in turn, plotting these to a rounded value for the estimated scale of the map e.g. 1:50000 or 1:100000, and attempting to fit the overlaid grid plot on the graticule. Repeating this for a number of potential projections for the area may be successful in obtaining a reasonable fit. But bear in mind that paper stretch may slightly distort scale from the nominal scale of the map, and the scale factor used in the graticule to grid conversions is another variable which may take only slightly different values e.g. a Gauss-Kruger takes a central meridian scale factor of unity while a UTM (like Gauss-Kruger, a Transverse Mercator) takes 0.9996.

Digital cartographic techniques make it relatively easy to plot grid and graticule for different projections with different parameters onto transparencies for "trial and error" overlays. The process can be time consuming so it is preferable to make maximum use of the clues which one may infer from the appearance of the map as initially presented - its origins, its national area, and the conventional projections used for that area.

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