Geo-referencing

“Geo-referencing” (also referred to as “geo-coding” and “positioning”) is the term used for allocating spatial reference information to a data set. In addition to pictorial data, a “geo-reference” file is compiled that establishes the reference to the co-ordinates of the Swiss reference system (national co-ordinates of upper left pixel, pixel size in both co-ordinate directions). Vectorial landscape models are geo-coded in that the individual vector points are recorded in the national co-ordinates system.

Thanks to geo-referencing to a uniform reference system it is possible for data sets from different periods to be superposed and directly compared with one another. However, it is important to bear in mind the fact that the degree of accuracy of the various series of Swiss maps (Dufour Map, Siegfried Map, National Map) varies as a result of the different surveying and reproduction methods used at the time of their production. Due to the varying degrees of accuracy in older maps, a local adjustment may be helpful, depending on the user’s particular requirements. In such cases it is recommended that the user carry out the geo-referencing directly based on suitable adjustment points with the aid of a geographic information system rather than relying on the swisstopo standard geo-referencing file based on the map’s key data.

Users who superpose older maps over modern ones and directly compare them should be aware of the fact that the accuracy of older maps is not equivalent to that of maps that are produced today. As a general rule for the National Map, a positioning accuracy (cartographic processing precision) of 0.1 to 0.3 millimetres may be assumed based on the scale used (thus for 1:25,000 maps the effective accuracy range is between 2.5 and 7.5 metres). In smaller-scale maps, local distortion effects can result in even lower levels of positioning accuracy.