JP 3-50 National Search and Rescue Manual Vol I - US Navy

C. Visual sweep widths are determined by choosing an uncorrected
sweep width based on type of search object and SRU altitude and
correcting it for environmental conditions, speed, and fatigue. For
maritime SAR, sweep width corrected (W) = sweep width uncorrected (Wa) x
weather correction (fw) x fatigue correction (ff) x speed correction
(fy). For inland searches, see Chapter 8. Sweep width tables are
available in Appendix D and in Vol. II, Chapter 4. Factors affecting
sweep width are:
1. Target characteristics. The target size, shape, distance from
the SRU, color contrast and brightness contrast, movement, and
duration of exposure to an observer must be considered.
2. Meteorological Visibility. The maximum range at which large
unlighted objects such as land masses can be seen constitutes
meteorological visibility. Reduced visibility results in
reduced detectability and sweep width.
3. Terrain/Sea Conditions. Normally, the more level the terrain,
the more effective the search can be. Trees, rock
outcroppings, and other surface irregularities decrease search
effectiveness, as will whitecaps, wind streaks, foam streaks,
breaking seas, swell systems, salt spray, and sun reflections
over water. Overwater sweep width decreases as wind speed and
significant wave height increase.
4. Cloud Cover. Visual seep widths may be reduced 10 to 20
percent by cloud cover above the SRU due to reduced surface
illumination.
5. Search Altitude. As altitude decreases, the search target
passes more rapidly through the field of vision because of
angular acceleration. This effect is most pronounced at
altitudes below 500 feet. If a large number of targets are in
the area, a low altitude is preferable to reduce diversions
for identifying sightings. Also, low altitude favors target
identification because scanners are more familiar with objects
viewed from low angles. For maritime searches, the daylight
lower limit is 200 feet for all search conditions, and
altitude usually does not exceed 3000 feet even under good
search conditions. For land searches, scanner efficiency
decreases rapidly as altitude increases from 200 feet up to
2000 feet, and more slowly as it increases above 2000 feet.
See Table 5-5 for recommended search altitudes.