# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Fwd: Lehmann's Rules and surveying
From: Peter Fogg
Date: 2007 Mar 17, 18:39 +1100
This message came directly to me, rather than to the group, which I assume was not intended, so have forwarded to the NavList.

Hope this is OK with you, Richard. It does not seem to be in the nature of a personal message.

---------- Forwarded message ----------
From: Richard <rmpisko1@telus.net>
Date: Mar 17, 2007 4:36 PM
Subject: Lehmann's Rules and surveying
To: peter.fogg@gmail.com

On Thu, 15 Mar 2007 23:30:01 -0700, P F <peter.fogg@gmail.com > wrote:

> Lehmann's Rules were used by surveyors, mainly for orienting the plane
> table
> (essentially a drawing board mounted on a tripod).
>
I'm glad that you mentioned that analogy, as I had been thinking that way
for some time.  However, no analogy is perfect.

Note that even in the standard three point resection problems there are
four cases:
1.  The true location is within the great triangle formed by the three
reference objects
2.  The true location is outside of the great triangle, but within the
great circle running through the three reference objects
3.  The true location is on the Great Circle (the indeterminate case)
4.  The true location is outside the great circle.

Only in case "1." will the true location be found inside of the little
triangle on paper (cocked hat) formed by sighting and drawing with a
misaligned table (systematic error).

Case "3." will always show as one of many perfect intersections on the
Great Circle no matter what the table orientation, so find some other
reference objects or change the plane table setup location.  You could try
the Italian method instead, which will show the c-B and a-B intersection
(x) to lie right on top of the b point on the paper.

Italian method, three steps:  From left to right on the ground A, B, C.
On the paper, a scale plot of the objects called a, b, c.  For the
indefinite case, assume you set up on P very close to the fourth corner of
a square A-B-C-P.   Place the alidade on "c" and "a", pivot the table so
c-a-A.   Lock the table.  Rotate the alidade only about the point "c", and
draw a line from "c" toward B.

Set the alidade in the opposite direction compared to step one, so as to
line up with "a" to "c".  In step two, pivot the table so the alidade will
line up a-c-C and lock the table.  Pivot the alidade about the point a on
the paper so it points to B on the ground and draw a line from c toward
B.  The two lines, c-->B and a--> B will intersect at a point "x" on the
paper.

Step three:  Draw a dotted line through x and b on the paper, set the
alidade along this line, release the table clamp and point the alidade
aligning x-b-->B (or perhaps b-x-->B) on the ground.  The table is now
properly oriented, so resecting from A-a->p and C-c->p will give your true
position "p" on the paper and "P" on the ground.  But, if you are on the
"Great Circle" along with A, B, and C; point "x" will land nearly on top
of "b", and it will be obvious that it is impossible to orient the table
with any accuracy in step three, and thus "p" cannot be found.

Generally, fairly good intersection angles can be obtained if you chose
reference objects and set up such that you are closest to the middle
object . . . but all this is for mapping, not navigation.  You (correctly,
I think) mentioned the stars could be thought of as being all at the same
distance.

--
Richard . . .
Using Opera's e-mail client since Dialog, "the Dog", died.

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