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Thursday, April 9, 2009

The Anatomy of a Grid Search

It seems like it should be a no-brainer: Searchers line up, side by side, and walk. And look. And, if it (or he or she) is out there, they'll find it, right? Uh ... maybe. Or maybe not.

Having been in SAR for about a year and a half now and put in hundreds of mission hours already, I've had my fair share of practice at grid (or line) searching, and it sure isn't as easy as it used to look to me on TV, where I'd often see a bunch of SAR members and other citizen volunteers lined up across a field, walking, unhindered, several feet apart. But I've yet to do a grid search like that. I'm usually crawling through, under and over thick brush, snagging my hair, clothing and skin on thorns and cacti, and struggling to keep in line with and in sight of my teammates to either side.

I've heard it said that grid searching is often the strategy of last resort, simply because of the manpower needed to adequately cover large areas. The fewer searchers there are, the more passes have to be made. And the smaller the object of the search--sometimes as small as a bullet casing or even tinier than that--the closer each person must be, meaning each pass will cover a narrower area. In SAR Field Search Methods by South Carolina's Rubicon Search & Rescue, the following example is given:

Noting that, as a rule of thumb, a trained grid search team requires about 3.5 hours to cover one mile, imagine you have one square mile to search. "Using a grid search, you have to decide how many searchers to put in the search area. Assume you assigned 25 people to search that one square mile. Now, you have to decide on spacing. If you space out the searchers with 20 feet between them, this 'line' will need to make eleven parallel passes through the area to cover it all and will take a total of 37 hours! That’s 924 MAN HOURS and 37 actual search hours, if the searchers are moving at a slow, deliberate pace as they are trained to do."

Putting 100 searchers out there in that same square mile, with the same 20 foot spacing, you’d still be talking about 924 man hours, but the search could be completed in about 3 passes, with only 9.2 hours in the field.

And when I say "field," I'm not necessarily referring to an actual field. Where I come from, it's usually ponderosa pine forest or canyon-filled, cactus-covered, pinion/juniper country, where 20-foot spacing would result in a fairly low probability of detection (POD). And imagine trying to locate something the size of a bullet casing in terrain like that? Not likely.

And I don't know about other teams, but we rarely have as many as 25 people turn up for a search. In high-profile cases I've followed in the news, often involving missing children, untrained members of the general public have usually come out in droves to assist. The article quoted above states that those non-SAR searchers are often assigned to perform grid searches in low probability areas to "give them something to do."

Effective grid searching requires paying attention to a number of details, all the while keeping your eyes peeled for clues (or an unresponsive subject). Maintaining spacing between each searcher, continuously keeping track of the person to your right or your left, is key. That spacing can be determined by placing an object the size of the smallest item you're looking for on the ground between two searchers and determining how far apart they can get and still see the object.

It is also important to keep on your direction of travel as much as possible rather than avoiding brush and other obstacles, because that's often where the clues end up, perhaps carried by the wind or animals or, in terms of people and especially children, they can get caught in the thick stuff if wandering around in the dark.

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Being a grid search leader is not an easy task. Depending on the number of searchers in the line, the leader may locate on one end or, particularly with larger groups, in the center, where they can more easily communicate up and down the line. On many of the grid searches I've participated with--usually lines of 5 to 10 people at most--the team leader has stayed at one end with his or her GPS on tracking mode, maintaining our course while taking terrain contours into account, and periodically checking to be sure we can all see the person to our left (or right). The leader checks, often by radio, that no one is falling behind or getting too far ahead. The leader also ensures we don't speed up too much, that we're taking the time necessary to look under bushes and even sometimes up into the trees. The faster the line goes, the lower the POD.

Once at the end of a pass, the line will then pivot. The person on the inside will stay in place, becoming the axis, while the others come around. Then that inner end will move over into the unsearched area the length of the pre-determined interval (spacing between searchers), and everyone else will spread out accordingly. Then the whole line moves in the opposite direction, making the next pass.

What our team usually does is have the searchers on both ends of the line keep their GPS's on track. That way, when the line pivots around, they can both maintain the contours of the pass in an effort to ensure that a gap doesn't form, meaning an area would not have been adequately covered.

You can read about one of our team's successful grid searches, where we helped to solve a two year-old missing person case, in my blog post, Cinder Hill Cindy.

1 comment:

Milord said...

I will be participating in a drill for SAR with a CERT tomorrow morning. This was a helpful refresher for me. Thanks.