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in topography, the grid of coordinate lines x = const and y = const on topographical maps made using the Gauss-Krüger projection. The lines of the reference grid are parallel to the central or axial meridian of the zone, which is taken as the x axis in the given coordinate zone, or they are parallel to the equator, which serves as the y axis for all analogous zones. These lines are inflected relative to the frames of the map at an angle equal to the convergence of the meridians. The numerical values of the lines of the reference grid are given at the points where they leave the frame of the map sheets. The values of x are counted from the equator to the poles, and the values of y are counted from the axial meridian of the zone, which is arbitrarily assigned a value of 500 km (for obtaining positive values of y within the boundaries of the entire zone). The reference grid is sometimes called the kilometer grid.