measuring height

[Ross]

I have the need to determine the height of some light fixtures inside a large building and have in mind to use my sextant.
I think that if I position myself twenty feet away from the vertical position of the light and measure the angular height of the light above the horizontal then the tangent of the angle times my distance from the light will give me the vertical height. Is this correct?

 

[Old Skool Neil]

Correct on the tangent function Ross. Wouldn't the floor need to be your horizon? Perhaps a horizon target could be set up the same height off the floor as the sextant, then add that height after the trig is done.

 

[Rich Stidger]

I agree with Old Skool Neil that the most straight-forward way would be to use a horizon target at the eye level of the sextant.

Just curious, why do you need the height of the lights?

 

[Bill Roosa]

doing trig

DITO on Neil's comments. Instead of doing the horizon thing you could just measure the distance from the sextant angle pivot (a protractor and bubble level would work fine too) when held to your eye and the floor. The adjacent leg of the triangle is actually parallel to the floor so if the floor is not level between the sighting location and directly under the light you will have to know how much the floor changes between the two locations.

Alternatly you could throw a ball up to the lights level and time the total flight time
x=1/2 of 1/2 * 32.2 * (time in seconds)^2 or 32.2/4*t^2 answer is in feet

Or you could measure off a 45 degree angle to the light and just measure the distance between where 45 falls and directly under the light (that is using the 45-45-90 triangle relationship) Same thing using the 30-60-90 triangle but the height is either twice the horizontal or half of it depending on which angle (30 or 60) you use.

I prefer to count bricks on a nearby wall if they are avalable as that requires no instrimentation and yeald accuracies to 4"

 

[kloudie1]

Yup, as Neil says put a spot/line on the far wall at sextant height and use that as your horizon. Add that height to the calculated height careful with conversion from degrees/min/sec to degrees/decimals for tangent calc.. That is very accurate..

 

[Ross]

When we need to relamp the fixture we need an extension ladder that will be long enough.

 

[Ross]

I bet this would work for mast height too.

 

[Bill Roosa]

did you know that

Using a sextant you can determine distance off by measuring from the top of a lighthouse to the high water mark. The height posted on the charts of a lighthouse is measured from the high water mark.
Distance off = lighthouse chart height / tan(angle between the top and high water mark)

and now you know why some lighthoused are much shorter than thier chart heights. They typically sit on top of cliffes where it is a long way down to the high water mark.

Similar amazing things can be done with horizontal angles. I recommend Bowditch

 

[Don12364]

Better luck to you than I had

I had this dead tree. Close by was our telephone line. No amount of eyeballing it would let me decide whether, when I cut the tree down, it would take the phone line with it - so I got out my sextant and my trig tables.
I took the angle between the base of the tree and its highest branch. Measured the distance I was standing from the base and used my degree level mathematics to calculate the arc it would fall through.
Great news, the tree would clear the phone line as it fell.

You already know what's coming.

Half an hour later I am in my neighbour's house ringing the phone company.
I said "We have just cut this tree down in our garden and it has taken out our phone".
The nice lady said "Its those tree guys again. They do it all the time. Never mind dear I'll get our gang out and we'll have you connected up in no time".
Duly humbled I thanked her and put the phone down.
Within the hour the gang arrived and put me in a new line.
No Charge Sir!

Ross, I am still convinced it works though.

 

[Old Skool Neil]

I took the angle between the base of the tree and its highest branch. Measured the distance I was standing from the base and used my degree level mathematics to calculate the arc it would fall through.

I bet it's that pesky 6 foot offset again.

 

[Ross]

The limbs flex as the wind from the fall picks up.

 

[Rich Stidger]

I said "We have just cut this tree down in our garden and it has taken out our phone".

Donalex,

Thanks so much for sharing. You made my day. I am always glad to learn that I am not the only one who is tortured by Mr. Murphy.

 

[Rick D]

I Have About 5' of New Deck

.... for the same reason. It was SO close...

 

[Benny17441]

The right tool for the job.



EM55 High quality ultra sonic measure tape .

Good quality with multiple function.

$29.95​

 

[Ross]

But I have a sextant.
I wonder if the devise can find a light fixture hanging 20 feet below the ceiling in a theater.

 

[Bill Roosa]

ladders

You know Ross that ladders only come in specific sizes. Your allowable error bars are pretty large. An eyeball estimate or protractor and line level or the throw the ball method will no doubt get you a sufficiently accurate estimate

 

[Ross]

Re: ladders

As an intellectual exercise this is a lot more fun.

 

[RAD]

Ross, I know the sextant sounds like its fun but I've taken 3/4 lengths of pvc conduit and used that as a measuring tool

 

[Ross]

Sextant is easier to carry. I have several lengths of 1/2 conduit.

 

[Ed A]

The laser meausre will give you an accurate reading by putting the lazer light on the fixture and the device on the floor. Of course all the other stuff can be done too but life is pretty short!

Borrow one and you dont have to spend anything!