Pilotage in the United States is usually accomplished with the use of sectional charts, which show the ground with considerable accuracy, both for terrain levels and for man-made objects. The charts are marked with World Geographic Reference System or GEOREF grids, and at the center of each grid square a number shows the elevation (MSL) of the highest obstacle within that grid (the maximum elevation figure or MEF). Thus a pilot is alerted of how high he must fly while traversing that grid to assure clearance of all possible obstacles.