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height of light above mark??????


harmie
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The way that I uderstand this is that the Theodilite,or older ones, transit, was used to see the light(sighted At)most of these markers were above the tree levels for sighting because the clearing of the lines(of sight) would be expensive. That is also why they chose High points to observe from, the could see(target) the outlying areas.this I beleive is called the Azmithal system, which is now obsolete.They are now all adjusted to the geoid 99 and defl 99 models.which from what I have learned is only in millimeters change, some up to a second.but still very accurate!

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quote:
Originally posted by harmie:

at the bottom of the ngs data sheets some of them give the term height of light above mark can anyone tell me what height of light above mark means or is and how it can be used to help find benchmarks


 

There are several records marked this way in my area, and so far as I have been able to tell they do not have any structures that have lights on them.

 

An example is PP2722 which has an indication of a light above mark of 1.5 meters.

 

I may be wrong, and Survey Tech could probably give abetter explanation, but I believe this indicator is for doing survey work from this site. When a surveyor sets up, he will place a tripod over the mark, and mount a "light" on the tripod. The light may be a physical light for working at night, but will more likely be a laser target that the surveyor will aim a laser rangefinder at to find the distance from the mark he is working at.

 

The added 1.5 meters has to be taken into consideration when attempting to determine the vertical change between the benchmark and the point being surveyed. For most measurements this will be accounted for by the fact that the rangefinder will also usually be on a tripod 1.5 meters off the surface, however if there is a reason that this hight will be different, say an obstruction that the surveyr has to work above or below, changing the hight at the survyors location, he may have to adjust. If he has to adjust, he is no longer working with a parallogram to come up with the correct location, he is working with a trapazoid. That trapazoid has at least three known side lengths, and two known angles, which means that with a little triganometry you can get the correct measurement, even for an obstructed location.

 

For benchmark hunters, this type of a light notice is not very useful, unless you find some indication that the light above the mark is part of the structure the benchmark is also part of.

 

That's just my opinion, I could be wrong.

 

-Rusty

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A light tripod to which is attached a plumb line may be centered over the station mark. If the stations were to be extensivly used an iron pipe was set verticaly in the ground. Where a tall mast is necessary for visibility it was supported by 3 guy wires. where a more permanent signal was required a tripod or tower was set up. such signals were constructed of either round poles or of sawed timber.with the vertical mast projecting upward from the junction with the legs. The signal would be solidly built and firmly anchored, with the vertical mast centered accurately over the station. done by the U.S. Geological Survey and the U.S. Geodetic Survey. from, Surveying Theory and Practice, by, Davis, Foote & Kelly. (IBID)ref. 4 & 7Birdseye,C.H., Topographic Instructions of the U.S. Geological Survey, U.S. Geological survey, Bull. 788, Goverment printing office, Washington D.C., 1928. ref. 7, Hodgson,C.V., Manual of second and third order Triangulationand traverse, U.S. Coast and Geodetic Survey, spec. pub 145, government printing office, Washington D.C. 1935

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You are right Rusty, the light is simply a temporary target that was set up directly above the point and observed when the station was viewed from some distant location at night. The reference to it on the data sheet serves to tell future surveyors that they may not be able to see the point from the ground without constucting some type of elevated target, at approximately the height indicated. I agree that it really is of no help to someone looking for the station.

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OK,after a bit of web research, I find that a such heights are/were acheived by surveyors using a Bilby Tower.

 

http://www.photolib.noaa.gov/historic/c&gs/theb2546.htm ('back up' this url to c&gs to see a bunch of pics)

 

A Bilby Tower is a tower-within-a-tower invented by a surveyor in 1926. It could be built to a height of over 100 feet (the tallest ever was 156 feet) in a day by 5 men and dismantled in half that time, according to NOAA history. http://www.history.noaa.gov/stories/geodetic4.html

 

This is interesting stuff to learn about!

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