Let me throw this out to help clarify the concepts. I have a statistical major but after my accident some procedures are fuzzy to me and today most is done by computers anyway, so bear with me. I'll try too post a field technique for estimating an ellipse another time.
The strewn field is the physical distribution of the meteorites on the ground. It's true density, content and numbers are never known because you never know if you have recovered the very last meteorite. It tends to fit within an elliptical form, but when actually plotted can look like a Rorschach figure. It dosen't have a line drawn around it--not yet anyway.
The Distribution Ellipse is the geometric/mathmatical representation of the "predicted" boundary or"limit" or "edge" of the strewn field. The first reason for computing an ellipse is to focus search and recovery efforts. Its secondary use is to estimate angle of fall and altitude of fragmentation. Its tertiary use is to confuse novice meteorite hunters. When all the math and geometry is done you have an ellipse to overlay on your map board. The statistical significance of this a prediction is that 95%(2Standard Deveations-SD) or 99 %(3SD) of all the meteorites in this fall lie within the ellipse.
For actual use You duplicate the ellipse overlay on tracing paper and give it to each hunter team and send them off to cover each sector with an agreed upon search pattern( grid walk, etc.), adjusting assigned sectors based on the terrain and best judgement of the search director who may be playing every position if they are first at the scene. It maybe wise to search the centerline first.
As previously mentioned you need a minimum 4 points to start. Actually there can be a little cheating as to the number of points if one of the points includes what is clearly a large mass it is treated as two close points--i.e. The large mass is assumed to have fallen ON axis and/or you have additional clues as to the direction of fall. Remember this is a recovery tool which is refined as more finds trun up.
That said-- for an ellipse to be of search value you must have good sampling from the whole field which you are trying to estimate and that can be a catch-22 loop. For practical use, a laptop with a spread sheet template or statistical package is the only efficent way to get to the 95% confidence level and add additional locations. The inital plot will likely become skewed as more finds are plotted and the ellipse has to be adjusted and the new information shared with the searchers. It also assumes that you have a simple fall and you do not have multiple concentrations due to multiple fragmentations at altitude.
The eccentricity of an ellipse and the length of the two axies give a clue as to the angle of the fall and altitude of fragmentation. A narrow ellipse suggests a lower level fragmentation because the fragments have less time to disperse on the lateral axis before reaching the ground. An elongated ellipse suggests a low angle of fall (shallow) with fragments dropped along a path where the bulk of the mass carried forward momentum longer before reaching the ground. Conversely, a wider ellipse suggests a high altitude fragmentation. A short ellipse suggests a high overhead (steep) angle of fall.
Conventional wisdom says that larger masses are found at the far end of the direction of fall. And perhaps this holds true for a simple fall--one that has a single fragmentation episode. However Portales Valley had the larger fragments fall to the near end of the field. <http://www.lpi.usra.edu/meetings/LPSC99/pdf/1964.pdf>
An example of a complex distribution can be seen at <http://www.saharamet.com/meteorite/data/map/ellipse.html> . Note that there are several ellipses on the direction of fall suggesting several fragmentation episodes. This page shows the initial ellipse from the first expedition<http://www.saharamet.com/expedition/CO3/part1.html> See how the ellipse plot was expanded as new material was found? The actual strewn field recoveries are plotted both inside and outside the ellipses where they were recovered over two or more expeditions.
Elton
(Thank you, Dr. Jean Dyer,USARI, where ever you are)
mike miller wrote:
Hello, my question is about "strewnfields" in general, not how they relate to a dry lake bed. I started as a meteorite hunter and have evolved into a hunter/collector.
I think I understand the end of a strewnfield, the elliptical shape is created by most of the lighter material falling away to earth and the larger pieces continuing on a path down the middle of the strewnfield. They create a point that more or less follows the middle of the strewnfield. My first question is what is the reason for the point at the beginning of a strewnfield?
The second part of my question is are there other types of "strewnfields"? More to the point, explosions that occurred closer to the surface and that might be described as a shotgun pattern? I would be interested in any other types as the only types I have heard about are the classic elliptical and a few related pieces.
Thanks in advance for your thoughts and opinions.
Mike Miller
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