6-6. Overlay Construction
. In the construction
of the radiological contamination overlay, proper
consideration must be given to factors that locally affect the contamination pattern. This is particularly
true between points in an aerial survey. These factors include, among others, the presence of terrain
features such as bluffs or cuts, heavily built up or wooded areas, and bodies of water. For example, a
large river will carry away the fallout that lands in it, leaving a path relatively free of contamination.
Similarly, the contamination hazard near a lake will be lower than expected, since the fallout particles will
sink to the bottom and the water will provide shielding. In wooded or built-up areas, a measure of the
reduction of dose rate can be obtained by using the transmission factors for these areas.
6-7. Dose Rate Contour Lines. Dose rate contour lines depicting
the contamination hazard
in an area
can be drawn when all the dose rate information in the area is posted. To accomplish this process
a. Determine the H + 1 dose rate contour lines to be plotted (for example, 20, 30, 100, 300, or 1000
b. Determine the points along the various survey routes and course legs and near monitoring
locations where the desired dose rates are located, interpolating linearly between dose rates if necessary.
To interpolate means to estimate the value between two known values (Figure 6-3).
= Interpolated points
Dose rates at H + 1
Figure 6-3. Interpolating Between Two Known Values
c. Connect (with a smooth line) all points having the same dose rates. Use all plotted monitoring
data as additional guides in constructing these contour lines. Figure 6-4 shows a completed construction
of a fallout pattern.