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Archiver > DEED-MAPPER-USERS > 2010-12 > 1291469949


From: "Steve Stevens" <>
Subject: Re: [DMU] Additional Directional Info in Deeds?
Date: Sat, 4 Dec 2010 08:39:09 -0500
References: <-76626955680204385@unknownmsgid> <AANLkTimqnieGUr5QkqL8Fgq9WwaDHSw9x9tXQA0RTbid@mail.gmail.com><9A46DAF0A7E94EC69FDA7AE6132337F5@Roadrunner><4CFA3F9A.8090806@epix.net>
In-Reply-To: <4CFA3F9A.8090806@epix.net>


Purely an example of the use of back sight, which equally applies below or
above ground. Point is back sighting is used to more accurately perform a
survey. I believe the question was what does BS mean. As far as magnetic
anomalies I would suggest you Google some more.

http://en.citizendium.org/wiki/Compass

See paragraph titled:
"Magnetic fields of other objects and their affect on a compass"

While it is true that his distances are short, we do not know any of the
circumstance or location, so we can not assume anything.
Regards,

Steve Stevens

----- Original Message -----
From: "Jeff Owens" <>
To: "Steve Stevens" <>;
<>
Sent: Saturday, December 04, 2010 8:18 AM
Subject: Re: [DMU] Additional Directional Info in Deeds?


Because the longest distance in the survey example was only 550 ft, and
nothing was mentioned about being in a cave, mine, or lava tube it is
highly unlikely that the errors encountered are magnetic anomalies. The
errors likely result from other factors related to terrain encountered,
i.e. slopes and also theodolite inducements due to leveling inaccuracies.

Googling the subject of "backsight errors" will yield a number of
explanations, none of which mention magnetic variation changes for
typical property surveys. Over longer distances, such as mountain top
to mountain top at much longer ranges, magnetic differences may come
into account, but not as mentioned. There certainly would be nothing
like a "100 shot traverse" to measure 550 ft.

Acceptable error for optical methods and hand held compasses is stated
to be + or - 2 degrees with normal techniques for property survey.

Jeff Owens

Steve Stevens wrote:
> BS= Backsight
>
> "One has to assume that the survey we are looking at is the result of good
> survey practice. The normal sources of surveyor-induced anomalies (reading
> the compass with magnetic wire-framed glasses, helmet and light too close,
> etc.) have been avoided, so that the only magnetic anomalies are those of
> the stations themselves.
>
> Consider two stations, A and B, and a foresight from A to B, and a
> backsight
> from B to A. In the absence of magnetic anomalies you would expect that
> the
> foresight and backsight would differ by 180 degrees. However, if magnetic
> north is different at the two stations (not that unlikely in a lava tube),
> the foresight and backsight will differ, and no amount of remeasurement or
> checking will change that.
>
> What does this discrepancy mean? If, for example, the backsight is 5
> degrees
> counterclockwise from what would be expected from the foresight, then that
> implies (with a good, careful survey) that magnetic north at station B is
> rotated 5 degrees clockwise from the orientation of magnetic north at
> station A. The foresights and backsights on the next leg of the
> traverse1.3.
> can similarly be used to compute the direction of the deviation at the
> next
> station with respect to the first, and this can be continued for the
> entire
> connected survey. (Although I know of no cave surveying software that does
> anything other than average foresights and backsights.)
>
> Some folk might argue at this point that this means that the errors
> accumulate as one goes further and further down the survey. To some extent
> this is true, but one would expect that [with good survey practice] the
> errors are random and not systematic, which would mean that they, in the
> long haul, tend to cancel. Note that with any redundancy such drift can
> also
> be compensated for. I have noticed that such arguments (usually) falsely
> assume something like the following; "If the standard deviation of a shot
> direction is [for example] half a degree, then the standard deviation at
> the
> end of 100 shots is expected to be 50 degrees." (As elementatary survey
> books point out, the correct expectation for the 100 shot traverse just
> mentioned is closer to a standard deviation of 7 degrees.)"
>
>
> Regards,
>
> Steve Stevens
>
> ----- Original Message -----
> From: "Greg Matthews"<>
> To:<>
> Sent: Friday, December 03, 2010 2:25 PM
> Subject: Re: [DMU] Additional Directional Info in Deeds?
>
>
> Just a guess, but maybe "book says" since they the numbers are pretty
> close.
>
> On Fri, Dec 3, 2010 at 2:03 PM, John Hughes
> Sr.<
>
>> wrote:
>>
>
>> I have been running into deeds with additional directional information
>> within parenthesis and I haven’t figured out what it means. Below are
>> some
>> examples from one of the deeds. Note, that the additional directions do
>> not
>> appear on every line segment. Sometimes there are only on one or two of
>> these, sometimes many more. They generally always appear as follows:
>>
>>
>>
>> thence N. 1° 15' W. (B.S. N. 0° 30'E) 33.38 ch's to a rock corner
>>
>> thence S. 5° 20' W. (B.S. 4° 40') 18.37 ch's to a rock corner
>>
>> thence S. 89° W. (B.S. 86° 30') 16.49 ch's to a rock corner
>>
>> thence S. 2 1/2° W. (B.S. 7 3/4° W.) 21.37 ch's to a rock corner
>>
>> thence S. 35° 15' W. (B.S. 36°) 14.45 ch's to a rock corner
>>
>> thence N. 64° 20' W. (B.S. 66° 30') 11.32 ch's to a rock corner
>>
>>
>>
>> Can anyone tell me what these are, what does B.S. stand for (be nice
>> now),
>> and how do you deal with them in DeedMapper?
>>
>>
>>
>> John Hughes
>>
>>
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