How to calculate critical searcher separation for a specific search area using the 'Northumberland Rain Dance', by Robert "Skip" Stoffel

Average distances from object in rain dance to determine Critical Separation

Critical Separation is a technique that is easy to teach, learn and put into practice. Perkins and Roberts (1989) orchestrated the Northumberland Rain Dance to determine the Critical Separation for any set of conditions that might exist in the local environment.

Searchers are said to be at Critical Separation when the visual horizon for each searcher falls at roughly the middle of the measured distance between any two searchers. (In other words, an individual searcher's visual horizon would neither overlap nor fall short of the visual horizon of the team member to his or her left or right.) The measurement is said to be elastic because it will vary with "purposeful wandering" and the difficulty of the terrain as the searchers pass through the segment.

Critical Separation is determined by a simple procedure called the Rain Dance. In the past decade, there has been a lot of speculation about just how this procedure is conducted. An object is placed on the ground that is similar in size and color to the object being sought. For many purposes a rucksack is a suitable, easily available object. This could be anything from a rucksack to a bag the size of a body or an actual person.

Eight to ten searchers are stationed around the object or person at a distance beyond where they can see the object or person being sought. Each searcher then walks slowly in a circle around the object for a few meters all going in the same direction. They then turn 90 degrees and walk toward the object until they can clearly see it. This procedure was designed to simulate that the searchers really didn't know where the object was.

At the point where the object is visible, each of the searchers then paces off the distance from "detection" to the object. All of the distances are then averaged. Twice the average distance is what is called Critical Separation. Commonly the configuration of the circle will be ellipsoidal and the developers of the tactic (Perkins and Roberts—1989) advocate that the differences between the maximum and minimum distances will be an indication of the amount of purposeful wandering that will be needed in a particular segment.

When searchers are spaced at a distance equal to Critical Separation (CS=1) the theoretical POD (Probability of Detection) was supposed to be right around 50%. However, during field trials of the procedure when trained teams are "purposefully wandering" through an area, PODS have proven to be consistently closer to 80%. Because of this, it has been postulated that a trained group searching with spacing equal to Critical Separation can be a very efficient use of search manpower.

An important responsibility of the overhead management team is to try, to the extent possible, to ensure that the terrain in any segment is as uniform as possible. This negates the requirement to be continually carrying out the rain dance procedure as visual conditions and terrain change.

Once the Critical Separation has been determined, the party starts to search. The members wander along in a purposeful manner, stopping regularly to look all around. Should a marked change in conditions occur or it is desired to search a small area for small clues, the Rain Dance may be easily redone.

Source: This article was adapted from The Textbook for Managing Land Search Operations, by Robert C. "Skip" Stoffel of Emergency Response International, Inc., in Cashmere, Washington. Copyright © 2006. All Rights Reserved. Excerpted with Expressed Written Permission.

To purchase a copy of this massive, 540-page, spiral-bound manual, or schedule search-and-rescue training or a survival course, point your Internet browser here.

Skip Stoffel is the President and founder of Emergency Response International, Inc.,
(ERI), formally known as Emergency Response Institute, Inc.

The company was formed in 1978 to provide information and courses in Search & Rescue Management, Survival Education, and Emergency Response to private, corporate, and government organizations.

Skip's experience includes:

• service as a USAF Rescue/Survival Technician during the Vietnam era,
• running a professional guide business for five years,
• working in the Washington State SAR Coordination Center for five years,
• and running a successful training and consulting business for over twenty-years.

Skip has authored three-dozen books on SAR Management and Survival Education, His most notable book—Survival Sense for Pilots and Passengers—has been used extensively in general and corporate aviation.

Skip is trained as a:

• USAF Survival Instructor,
• EMT,
• SCUBA diver,
• and glider pilot.

As the company's lead instructor and consultant, Skip continues to build innovative courses and support material for ERI's programs throughout he world. Learn more about ERI here.