[meteorite-list] Strewnfield

From: MstrEman <mstreman_at_meteoritecentral.com>
Date: Sat, 17 Apr 2010 21:27:03 -0400
Message-ID: <p2q704f522a1004171827re1f6dca2r9bb315f5c0eda768_at_mail.gmail.com>

Good idea James but one as to have access some of not all of recovery
data. Meteorite hunters tend to form teams and don't share outside
their team.

The purpose for mapping the strewn field in early stages of the search
is to develop a statistical tool called a "distribution ellipse" to
aid in searching the most likely lands for recovery. The statistical
goal is a 95% probability that 95% of the material lies within the
elipse.* ( It is statistically "non-productive" to hunt outside the
ellipse/box). One needs a good statistical program to crank out the
ellipse unless they are a math whiz and don't mind staying up all
night to refine calculations.

Once an idea of the axis is firmed up, one would expect: in a shallow
fall, that the heavier stones would have fallen at the far end of the
axis and very close to the axis. In a steep fall, one might see a
near circular ellipse with the heavier fragments in the center and
smaller fragments on the outer edges. Either way, plotting an ellipse
is the best "early way" to estimate where to concentrate searching.
Just off the cuff, my initial impression for this fall is that the
strewn field is no where near 10s of miles in length that the news
agencies mentioned. The recoveries will be more concentrated based on
the steep angle revealed by Doppler returns. Time will tell.

As to accuracy and utility, just 3-4 closely spaced points are not
much better than random guessing. However, 10- 20 or more widely
spaced plots is probably very accurate in defining the ellipse.
Refinement eventually shows the axis of the fall very accurately. The
more points and the further away each point lies from others, the
more reliable the "predictive" value of early plotting. A distribution
ellipse is typically used by organized teams searching for crash
debris. for example bu,t I don't know anyone else that uses the
approach anymore. Meteorite hunters tend to not share locations early
enough to make an ellipse of any use. Its value comes in revisiting
the field after the initial chaos is over and one goes back to
re-search the ground. In earlier times one would send out teams to
search certain areas as daily finds are incorporated into the
database. There can be more complications to plotting such as with
multiple fragmentation. In this case one can see clusters of clusters
within the ellipse which may skew the graph. All else I can say is
that the plot is statistically more productive in recovering the most
material in the long run than skimming the cream and leaving areas
un-searched or over-searched.

Eventual mapping of the recoveries in wrap up back at the office in
order to graph the strewn field is an important part of the science of
recovery. With the advent of satellite detection and other sensor
measurements, the ellipse has fallen into disuse in favor of walking a
grid and get the most the quickest.

Map marks over dinner? Maybe but now days we would all enter the GPS
fixes onto Google Maps via our wireless laptops.

There is a FIreball working group on yahoo( FBWG) if anyone wants to
start working a database for this fall.

Elton

* one measures the distance from each find to a hypothetical axis
using root mean square calculations to find the true fall axis.

On Sat, Apr 17, 2010 at 5:55 PM, James Balister <balisterjames at att.net> wrote:
> We are all on the list here so the mapping wont be that public.?? It would be a simple thing to do.? Someone gets a map.? Then when everyone gets together for dinner they can mark an X on the map where they found their rock.? It can be a daily thing.?? By the end of the week everyone will have a good idea of what the field looks like.? So who wants to donate a map?? I know I would like to know where?the meteorites?are found in relation to the town.
>
Received on Sat 17 Apr 2010 09:27:03 PM PDT