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Height Of Light Above Station

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Back in the day, these stations were surveyed at night with battery powered lights. Many had towers erected over the station mark. This was done to increase the distance they could see.


When this station was surveyed, the time you are referring to, the light, or instrument was that high above ground level. We are talking the top of the tower. Meters to Feet is Meters divided by 0.3048.





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If you do a search here on "Height of Light", and select "any date", you will find several old topics on this in the benchmark forum.


Also, here is an interesting pictoral history site with lots of pictures of bilby towers used for surveying to get enough height of light. In this page, scroll down to the section called "How Far Can You See. There's lots of other surveying pictures from the old days that you can reach from this page.


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.


The history of Bibly towers is explained here in the section titled "Then of Steel" in NOAA's history section.


Some stations didn't require a bilby tower so a temporary tower of just a few feet was built to augment the usability of a survey station.

Edited by Black Dog Trackers
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I just can't figure out what they are doing!


The guy on top with binoculars is aiming the lights at other tower + likely spotting them also. They had to tip them up/down and right/left to get a good sight as the best sight was when the light looked like a dot to the instrumentman. Normal tower spacing was a much as 20-25 miles apart. They used morse code (flash the light) to communicate with each of the other towers sites before portable radios came on the seen. The guy Intrumentman) looking up at him it likely telling him what to tell the other distance light keepers about their aim.


The 2 guys on the platform are the notekeeper (sitting ) and the instrumentman.

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Actually the caption is incorrect. The tower type pictured is a Bilby Tower named after the inverntor Jasper Bibly. Mr. Bilby was a crew member with the USC&GS and he invented the portable steel tower structure. I worked in these towers for 9 years while surveying at Cape Canaveral Fl. The 6 inch lights that are in the process of alignment appear to be pointed to two individual targets. This was common place to both show a light and observe the recip angle. Each of these men have a specific job on the tower, lightkeeper, instrument man and recorder. Sorry to sound like Cliify.

Edited by Dan Vull
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Actually the caption is incorrect


Actually it is correct IMO.


The Caption is saying the picture was taken from a 100 ft Fire Tower which happened be adjacent to the CGS Tower, not thjs a picture of the tower you see. In many areas there were existing state fire towers (many were here in Michigan forests). How else could the have gotten that photo?

Edited by Z15
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Some time back there was a story in either Professional Surveyor or some similar magazine. It was a reprint of story circa 1940's or 50's by a local newspaper about a C&GS triangulation party at work in there area. I have not been able to find that magazine since. It was about 2000 +- or there about.


Very interesting reading with many old photos. I recall one comment about a crew building a Bibly Tower and local observers could not beleive these were government workers because of how fast there were working.


Wish I could find that magazine. I saved but where did it go???

Edited by Z15
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I use to enjoy reading prof surv until it became an infomercial for trimble and leica. I have a copy of Special Publication no. 234 in my office (Signal Building by Jasper S. Bilby) along with a couple of other USC&GS manuals from days gone by. Exciting reading like; Manual of Reconnaissance for Triangulation, Bilby Steel Tower for Triangulation, Manual of Geodetic Leveling along with a complete set of the manuals we used at DMA. I save too much crap.

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We had that stuff in our office also. I asked a NGS mark maint. man for a copy of the Tri_ang. Manual when we helped him move a station back in 1977 and a few weeks later all these CGS pubs came in the mail.


Back in the 1970's my dept (MDOT) provided help of a NGS project in this state. They wanted lightkeepers but I would not climb a 100' tower so I did not get to go. When they were done, we suddenly had a bunch of equipment like the lights, NGS logo orange unbrella's, big stash of metal witness posts and signs (1000?), and many other stuff (opps maybe I was not supposed to reveal that??). We did some large traverse projects in the UP (US-2) for about 10 yrs in the late 1970's and early 1980's that were blue booked. But managerment (desing Engs) decided it was not worth the cost to the projects.

Edited by Z15
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