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gnbrotz

"Height of light"

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Could someone explain to me what it means when I read:

quote:

HEIGHT OF LIGHT ABOVE STATION MARK - 18 METERS.


This is found in the definition of the azimuth mark for JV4200. I suspect it has something to do with a beam (laser?) projected from the aziumuth mark to the station mark, since I also read:

quote:

AZIMUTH MARK IS NOT VISIBLE FROM THE GROUND BUT IS VISIBLE FROM A POINT HALFWAY UP THE FIRE TOWER.


in the description, and this is confirmed by a crew in 1973:

quote:

THE AZIMUTH MARK WAS NOT FOUND. IT IS BELIEVED STILL IN POSITION BUT UNDER HEAVY BRUSH. THE AZIMUTH MARK WILL NOT SEE THE STATION FROM THE GROUND THEREFOR THE ANGLE COULD NOT BE USED TO LOCATE IT.


 

I'd like to understand this all better in hopes it will give me one more thing to add to my 'bag of tricks' when benchmark hunting.

 

Greg

N 39 54.705'

W 77 33.137'

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take a look at the responses in the thread opened by harmie with a substantially similar name.

 

The brief answer is that you are generally correct. A light is a device placed over the benchmark to allow surveyors to find the benchmark from a distance away where there may be ground obstructions.

 

The height generally ranges from 1.5 meters for a tripod mounted light or reflector to as high as 33 meters for towers that are assembled over the benchmark for the survey work to be done. These are not absolutes in that a light may be an existing light on a building that is less than 1.5 meters above the benchmark, or may be the beacon at the top of a 100 meter radio tower, otherwised used to warn off aircraft.

 

-Rusty

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Thats correct, the light is just an ideal target at night, when most high precision control work was done, in order to avoid the heat wave disortion that occurs during the day. This information really is no help in trying to locate a point, unless you happen to find remains of an old tower.

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Take a look at "Great Trigonometric Survey of India"

 

Extensive use of light towers in the survey that resulted in this map..

http://www.headmap.com/book/sphaeric/survey.jpg

 

A good starting place is..

http://www.frontlineonnet.com/fl1909/19090660.htm

 

Now that's benchmarking!

 

>Personally Responsible for the Recovery of .00217% of the Benchmark Database!<--watch this number!

 

[This message was edited by Raouljan on September 21, 2002 at 07:04 PM.]

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