13 FAA Chart Users’ Guide - VFR Terms and Symbols MEFs over land and open water areas are used in areas containing man-made obstacles such as oil rigs. In the determination of MEFs, the FAA uses extreme care to calculate the values based on the existing elevation data  shown on source material. Aeronautical Information Specialists use the following procedure to calculate MEFs: When a man-made obstacle is more than 200' above the highest terrain within the quadrant: MEF - Man-made Obstacle 1.  Determine the elevation of the top of the obstacle above MSL.  2.  Add the possible vertical error of the source material to the above figure (100’ or 1/2 contour interval when inter - val on source exceeds 200’. U.S. Geological Survey Quadrangle Maps with contour intervals as small as 10’ are  normally used).  3.  Round the resultant figure up to the next higher hundred-foot level. Elevation of obstacle top (MSL) 2649 Possible obstacle error +100 equals 2749 Raise to the following 100’ level 2800 Maximum Elevation Figure (MEF) Example: 