Map systems, covering larger regions or the whole surface of the Earth, are often consist of sheets, covering smaller parts of the target area. In this case, the sheet labels provide information about the location of the area, mapped in the respective sheet. Thus we can make a mosaic from them without following the projection or the latitude-longitude grids. Besides, there are map systems without any grid reference; in this case the geographic or the projected coordinates of the corners can be calculated from the sheet label.
The borders of the area, mapped in a sheet, are following parallels and meridians, or projection grid lines. In the first case, the shape of the sheet is an arc trapezoid, while in the second one it is a square or a rectangle. The sheet number exactly gives the coordinates of the corner points.
In Hungary, sheet border of the civilian topographic maps are following the grid lines of the EOV (the national grid), without any geographic coordinate indicated (Fig. 25). The sheets of the Gauss-Krüger type military map system or the old Stereographic system are bordered by parallels and meridian arcs (Fig. 26). If we have not a topographic but a derived map, whose sheet labeling system follows the one of the topographic maps, we can use the corners as control points, even if no coordinates are given in the map (Fig. 27).
The situation is similar at the sheets of the old (second) military survey sheets of the Habsburg Empire (Fig. 28). We have no coordinates indicated (Fig. 29) in the nice and detailed 1:28800 scale sheets of the map system. However, knowing the labeling system and the physical extents of the sheet area in the field, we can easily compute the grid coordinates of the corners in its native projection. Thus, the sheet corners can be used as control points, without seeking identical terrain points in the map.