Here's how it's done:
Maximum Elevation Figures (MEF) are computed by the National Imagery and Mapping Agency (NIMA) and take into account both terrain and obstacles.
If the highest feature within a latitude/longitude grid is terrain, NIMA adds 200 feet to elevation, plus the vertical accuracy of the identified point. This value is then rounded to the next 100-foot value.
If the highest vertical obstruction (man-made) is higher than the highest terrain elevation plus 200 feet, then the MEF is computed using the elevation of the obstruction plus its vertical accuracy, then rounded to the next higher 100-foot value.
What is critical to keep in mind is that the MEF may give over 200 feet or less than 50 feet of clearance, and does not ensure absolute clearance of terrain or obstacles.
for example: 3^1 means the highest feature within that grid is between 2901 and 3000 feet.