Geodetic grid
A geodetic grid is understood to be a system of intersecting lines that form squares and that is attached to a map as a Cartesian coordinate system .
A geodetic grid is based on the circles of longitude and latitude of the earth's body, but only coincides with these in a few places, since the circles of longitude and latitude are curved due to the spherical shape of the earth, but the geodetic grid represents a flat (two-dimensional) coordinate system. A uniform geodetic grid can only be constructed for a limited section of the earth's surface if the accuracy deviations due to the earth's curvature are to remain within narrow limits. Correspondingly, geodetic coordinates only make sense on larger- scale maps , and in contrast to geographic coordinates , they are usually given in “right” and “north” values. In order to nevertheless be able to cover the entire earth's surface with a geodetic coordinate system, strips running along circles of length are defined, whose code numbers or other unique identifiers are placed in front of the geodetic coordinates. Well-known geodetic grids are, for example, the “ Gauß-Krüger coordinate system ” (mainly in Germany) and the “ UTM coordinate system ” (all NATO countries and many others).
The angular difference between the vertical grid lines and a meridian (degree of longitude) is called meridian convergence and must always be taken into account when vertical grid lines are assumed to be north on the map. The meridian convergence is indicated on many maps for the center of the map face.
Sometimes there is also or in addition the “ needle deviation ” which is calculated as the difference between declination (magnetic declination ) and meridian convergence. Since the calculated declination is subject to annual changes, the annual change is usually noted on the map.
With the needle deviation as course feed, the vertical grid line can be used as a north reference.
Taking the map scale into account, the edge length of the grid squares usually corresponds to 1 km in nature; in special cases and for reasons of clarity, the edge length can also be a multiple of 1 km.