You can use geographical, Gauß-Krüger, Swiss or UTM coordinates. They can use each point of intersection of the coordinate grid (grid cross) as passpoint. The number is unimportant but it should be at least four, if possible one in each corner of the map. Start the calibration with the input of the coordinate system and the map datum. You get this information usually from the map legend. The coordinates are printed on the edge of the map. Thus all standard maps with a printed coordinate grid can be calibrated. The following two screenshots for geographical, Swiss and UTM coordinates demonstrate the presentation of the coordinates in the maps:
Section of a geographical map of Switzerland. The topographical map with a scale 1:25.000 shows the geographical coordinate system and the specific Swiss coordinates. Both systems have the map datum "CH-1903". The coordinate grid on the map stands for the Swiss coordinates. Passpoint no. 1 has the right value 743 km and the height value 241 km. The other passpoints are not within the section. The average accuracy of the passpoints is at 15 m or 0,6 mm with regard to the map.
Section of a cycle map of the national park "Hohe Mark" with a scale 1:100.000 within a UTM coordinate grid and map datum WGS 84. The passpoint has got the right value 350 km and the height value of 5730 km. The UTM-zone is "32" (legend of the map). The other passpoints are not within the section. The average accuracy of the passpoints is at 22,50 m or 0,2 mm with regard to the map.
Fix four prominent points on the scanned map which will serve as passpoints (if possible one in each corner of the map). You should use rectangular crossings the coordinates of which you get with the software “Google-Earth” as follows. The following screenshot shows a city map of Rome (taken from a “VistaPoint” travel guide) without scale or coordinates which will be calibrated in spite of this:
Section of a city map of Rome. As we do not have any scale the input of a scale of 1:10.000 is the first step for the calibration. The correct scale is afterwards calculated (1:13.200) by the calibration. The tour (red) shown in the city map is described with all sight seeings in the travel guide. You only have to click on all route points one after the other in order to follow the route with your GPS receiver. The crossing of the streets "Via del Corso" and "Via del Tritone" serves as passpoint no. 1. The other passpoints are not within the section. The following 3 screenshots reveal the work-out of the coordinates of this crossing:
After download and installation of Google-Earth you should first of all adjust the coordinate grid to UTM. This is done in the menue "tools > options > 3D" in the category "LAT/LON" (see screenshot). The coordinates are now shown in "meter".
You are zoomed to the adequate street. For better orientation the street names can be activated by clicking on the left menue in front of "streets". The both above-mentioned streets are underlined in red. Now you go on the desired crossing with your mouse here marked by a red point. At the bottom you always find the coordinates of the mouse which enable you to use the crossing as passpoint.
When the calibration is finished „GROUPI“ shows the exact position of each pass point. These results should be at 2 mm or less (see following screenshot). Furthermore you can control the calibration online with the transfer software “EasyGPS”. More concerning this subject see here.
In this table you find all pass points which were produced for the calibration of a map. The before last line indicates the deviation of the points with regard to the calculated result (in m). The last line (red) indicates the same result which has been changed (in mm) according to the map scale. The results in the red line show very precisely the position of the pass points in the map. In this example it is 0,5 mm or less.