It also includes any feature that interferes with the crossing of
two or more streams of traffic.
Critical points include every bridge,
overpass, underpass, ferry, ford, road constriction, and sharp turn.
Railroad crossings of any kind are critical points.
Since these items
already have symbols of their own, there is no need to indicate them with an
additional special symbol on your overlay.
As you conduct your recon, you may find areas that you know are
important which do not have a specific map symbol. For example, an ambush
site or an area where there has been a rock slide. In order to indicate
these on the overlay, the symbol for a critical point is used. This is a
number inside a triangle. From the triangle, you extend a line to indicate
the specific location. In the legend area of the overlay, you explain what
the critical point is. Key terrain is always considered a critical point.
Curves with a radius of less than 25 meters are considered sharp
curves and are indicated as obstructions (OB). If a curve measures between
25.1 and 45 meters, it must be noted on the overlay, just as a sharp curve
is. It is not considered an obstruction. The wider curves are noted since
they are sharp enough to cause some traffic to have to slow down. In other
words, any curve with a radius of 45 meters (148 feet) or less is noted on
Curves are indicated on the overlay by a triangle.
(vertex) of the triangle is located at the curve. The radius of the curve
in meters is noted outside the triangle. When a series of curves occur in a
short distance, you may indicate it by drawing a second triangle inside the
The vertex of the triangle is placed on the first curve.
Immediately outside the symbol, you indicate the number of curves and the
radius of the sharpest one. For example, 3/24 would indicate 3 curves, the
sharpest of which as a radius of 24 meters.
The formula for determining the radius of
a curve is
R is the radius of a curve.
distance of the centerline of the curve from each outer extremity is
represented by C.
M is the perpendicular distance of the tape to the
centerline of the road.
See Figure 1-17.
If c is 15 meters and M is 2
meters, the formula becomes:
The result would then be that R, the radius of the curve,
is 15 meters. This is a sharp curve and must be indicated as in obstruction
(OB). It has the advantage over the other methods in that it is relatively