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ACRO E-mail Archive Thread: Alternative Box Markers

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ACRO E-mail Archive Thread: Alternative Box Markers


Thread: Alternative Box Markers

Message: Re: Alternative Box Markers

Follow-Up To: ACRO Email list (for List Members only)

From: bwana at ('Bwana' Bob Buckley)

Date: Tue, 15 Apr 1997 03:30:34 UTC


> Having been a past contest director and the person who was out in the
> field determining corner/center locations, I offer this idea.  Use gps to
> locate the points.  This can be done by averaging the dither displayed on
> a handheld gps unit and locate a point without the use of traditional
> surveying equipment.  Once the points are located, use helium balloons of
> 3-4 feet in diameter staked/tethered to the ground  (color optional).  I
> don't know of a source for the balloons, but I have seen enough of the
> kind I am thinking of floating above car dealerships to know that they are
> readily available.  Simple corkscrew anchors would suffice.  Once the
> points are located, a five pound bag of lime poured into a hole dug into
> the dirt will allow convient relocation in the future.  Comments and
> critisisms welcome.

I think I'll stay clear of the balloon bit ... However,

Earlier this spring I posed a question to director Tom Myers concerning
use of GPS' for laying out boxes. I was aware that it has been done, and
is being done with more frequency. Toms reply included this:

   "without using some form of selective availability correction, you
    will have about 100 meters of noise in your box. a differential gps
    setup reduces that to about 1 meter, which is plenty good enough for
    a box.".

As far as I'm aware, the US government is still employing selective
availability (i.e., intentional signal degradation) in its GPS
satellites. Hence, the only way to consistently layout a correct box
is to either ensure that the GPS that is being used is receiving and
applying a differential signal from a second transmitter located at a
known location, or place the GPS receiver at a known location to
determine the current error then applying a manual correction at the
corners (within a finite time frame).

I forwarded this information to Pres. Rihn and asked about his exposure
to the level of Chapters laying out boxes via things like hand held
GPS'. Who else has done it this way? I'm aware the Clyde did it for
at least two of ours in Denver.

So, the moral of the story is, beware of the unknowns with this
technology. I am in no way an expert, but I've done alot of cursory
reading and research on it.  It's possible to layout a corner that is
~425' off, possibly more! If it's layed out over a large time frame you
could have a pretty trashed trapezoid! This could really be critical
given pilots in some boxes traditionally use references such as known
roads, tree lines, etc., for positioning, which would then be completey
wrong, given the new layout.

In the case of laying out a box, I think people need to be aware of the
limitations of GPS and the correct way to use them to get an accurate
box. And if laying out the box results in quite a bit of inaccuracy,
pilots and judges need to know to be able to have the correct perspective
for flying and judging.

P.S. Keep the Box. It's a critical ingredient to Competition!

'Bwana' Bob Buckley      |                     |   Hughes Training, Inc.
Visual Engineering       | "Let the good times | P.O. Box 6171, M.S. 402
(817) 619-8727           |     'Snap' Roll"    |    Arlington, TX  76005
bwana at  |                     |          (817) 619-2000


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