The data in the Digital Obstacle File
(DOF) is collected and disseminated as part of Aero-
Nav Services' responsibility for depicting the National
Source data on terrain and obstructions is
sometimes not complete or accurate enough for use
in aeronautical publications; for example, a reported
obstruction may be submitted with insufficient detail
for determining the obstruction's position and elevation.
Such cases are identified by AeroNav Services
and investigated by the FAA Flight Edit program.
The FAA Flight Edit crew conducts data veri-
fication missions, visually verifying cultural and topographic
features and reviewing all obstacle data.
Charts are generally flight-checked every three years.
This review includes checking for obstructions that
may have been constructed, altered, or dismantled
without proper notification.
Generally, only man-made structures extending
more than 200' above ground level (AGL) are
charted on Sectionals and TACs except within yellow
city tint. Objects 200' or less are charted only if they
are considered hazardous obstructions; for example,
an obstruction is much higher than the surrounding
terrain or very near an airport. Examples of features
considered hazardous obstacles to low level flight are
smokestacks, tanks, factories, lookout towers, and
antennas. On World Aeronautical Charts (WACs) only
obstacles 500' AGL and higher are charted.
Obstacles less than 1000' AGL are shown by
the symbol . Obstacles 1000' AGL and higher are
shown by the symbol. Man-made features which
are used by FAA Air Traffic Control as checkpoints
may be represented with pictorial symbols shown in
black with the required elevation data in blue.
|The elevation of the top of the
obstacle above mean sea level (MSL)
and the height of the structure AGL
are shown when known or when they
can be reliably determined by the
cartographer. The AGL height is
shown in parentheses below the MSL
elevation. In extremely congested
areas the AGL values may be omitted
to avoid confusion.
Obstacles are portrayed wherever possible.
Since legibility would be impaired if all obstacles within
city complexes or within high density groups of obstacles
were portrayed, only the highest obstacle in
an area is shown using , the group obstacle
Obstacles under construction are indicated by
the letters nearest to the obstacle type. If space is
available, the AGL height of the obstruction is shown
in parentheses; for example, . Obstacles with
high-intensity strobe lighting systems may operate
part-time or by proximity activation and are shown as:
5. The Maximum Elevation Figure (MEF) represents
the highest elevation, including terrain and
other vertical obstacles (towers, trees, etc.), within a
quadrant. A quadrant on Sectionals is the area bounded
by ticked lines dividing each 30 minutes of latitude
and each 30 minutes of longitude. MEF figures are
depicted to the nearest 100' value. The last two digits
of the number are not shown. In this example the
MEF represents 12,500': . MEFs are shown
over land masses as well as over open water areas
containing man-made obstacles such as oil rigs.
In the determination of MEFs, extreme care
is exercised to calculate the values based on the existing
elevation data shown on source material. Cartographers
use the following procedure to calculate
When a man-made obstacle is more than 200'
above the highest terrain within the quadrant:
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 interval 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.
Example: Elevation of obstacle top (MSL) = 2424
Possible vertical error +100
Raise to the following 100' level 2600
Maximum Elevation Figure
When a natural terrain feature or natural vertical
obstacle (e.g. a tree) is the highest feature within
1. Determine the elevation of the feature.
2. Add the possible vertical error of the source
to the above figure (100' or 1/2 the contour interval
when interval on source exceeds 200').
3. Add a 200' allowance for natural or man
made obstacles which are not portrayed because
they are below the minimum height at
which the chart specifications require their