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Height Of Light


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Maybe this has been covered before..........I have researched everywhere and keep coming up empty handed........


In various BM descriptions you can find something like this:


"The Height of Light above the station was XX.X Meters"


What exactly does this mean and refer to?

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Spoo -


At the top of this page, click on Search.

In the keywords box, put Height of Light

In the Search Posts from... area, be sure to click on Older.

You will find several articles.


You could also look up "bilby tower" on the internet.


Basically, surveyors would build a high tower over the benchmark using a kit. They could put it up and take it down in a couple of days or less. They could climb it and set up a high observation and target at the top.

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You can look at this station Ambrose, I went to find it after SquareNail had recovered it. This is the only standing tower that I have ever found. Here is one picture of the tower:




One other stand that I found in less that perfect condition:




which can be found at the Montana Initial Point:GLO Post.


I hope these help you out.

Edited by CallawayMT
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I believe it refers to the old light line airway beacons. When I first started flying 53 years ago, beacons were located along flight paths between cities for visual nighttime flying. They were being phased out about then since radio A-N airways were being used followed by omni.


This is some interesting reading about a portion of a light line.


By 1933, the Federal Airway System operated by the Airways Division comprised 18,000 miles of lighted airways containing 1,550 rotating beacons and 236 intermediate landing fields. Air Mail pilots routinely navigated the skies during the night, following the “signposts” of the rotating beacons.


Picture of beacon.


Several edits to add web sites after getting excited about beacons.

Edited by Colorado Papa
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This gallery of survey towers can be reached through a link called "history" at the bottom of geocaching's benchmark FAQ.


These towers are not firetowers or airway beacons - they are temporarily made just for the purpose of surveying.


The 'height of light' is the height of the light on the tower that was used, probably at monumentation time. After that the tower would be dismantled and built again elsewhere.

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With these towers, I always wonder what came first. The mark or the tower.


Back in the late 70's we had a couple portable 25 ft survey towers we used at the DOT for traverse work. They were pulled around like a trailer, then set near the point and used attached hand winch to erect (tip up into place). We found it very difficult to setup over existing marks. Any mark we needed to set was done after tower was in place. I recall spending nearly a day trying to get one up a hill (no 4x4), then get it over a CGS mark on top of that hill. Had to winch it up in stages.

Edited by elcamino
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Thanks all for the responses. I went back to my books (HIGHER SURVEYING by Breed and Hosmer,1935 and SURVEYING by Davis and Foote,1940) and found the afore mentioned Bilby Towers complete with the pictures of their lights.


The phrase "height of the light" is never mentioned but I agree that that is what is refered to.


I am a pilot from 30 years ago and am not familiar with the light beacons mentioned outside of airports themselves.

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In the following discriptions there in no mention of a tower but the hight of light is in all three. MZ2042 HOL 4.8m, MZ2045 HOL 1.4m and MZ2085 HOL 3m. The hight of light is not very high so I don't think that it describes any kind of tower. MZ2040 gives no HOL but mentions a 27 foot signal. Other discriptions mention towers but no HOL's. As it is I still don't know what the hight of light means. I have asked a couple of survey engineers about this before and they had no answers for me.

Edited by ddnutzy
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Height of Light Above Station.



Station,At an Instrument Station it is desirable to have a signal of a type that will permit placing the instrument directtly over the station when angles are to be measured.

In a small triangulation system the triangle sides only a few hundred feet in length,and with few angles to be measured,a temporary signal which is readily moved may be all that is necessary.

A light tripod to which is attached a plumb line may be centered over the station mark.If the stations are to be used over a longer peroid,the stations may be marked by iron pipe set verticaly in the ground,in which pipe is placed a range pole or similar rod.

When the station is occupied,the pole is temporarily removed.

Where a tall mast is necessary for visibility,it may be supported in position by 3 guy wires attached to it near the top.

Provision is made for swinging the bottom of the mast to one side when it is desired to place an instrument at the station.

It is necessary that the guyed top be accurately centered over the station.

Where more permanent signal is required that does not need to be moved to provide for the setting of the instrument, a large tripod or tower is set up.

Such a signal may be constructed of round poles or sawed timber,with the vertical mast projecting upward from the junction with the legs.

Solidly built and firmly anchored,with the vertical mast centered accurately over the station and made as vertical as possible.

For High visibility,the cloth may be of the fluorescent type.


Field Instructions of the U.S. Geological Survey and the U.S. Coast and Geodetic Survey:




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If anyone is wondering what they used the lights for. Because it was the only thing you could see at night. They did not often observe during daylight because of atmospheric distortion of the line of sight by heat waves. Also they observed long lines, 25 miles or more in triangulation schemes and you just could ne see that far without using a light.


They continued to use them right up to the end of triangulation in the 80's. We also used them in our work at the DOT (our lights came from NGS when we worked with them in a big survey in SE Michigan in the late 70's), you could see a light much better than a normal target. In a mile a normal target was impossible to see but a light stood out like a sore thumb.

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