How to determine distance from an object of know height using a sextant?

[S/V Alchemy]

Originally Posted by SaltyMetals

Interesting but hang on a second. Measuring the height of an object like a lighthouse will give a different angle depending on the state of tide. Surely there needs to be a correction that takes tide into account?

In the video shown above, that correction is made.

[Cadence]

Originally Posted by SaltyMetals

Interesting but hang on a second. Measuring the height of an object like a lighthouse will give a different angle depending on the state of tide. Surely there needs to be a correction that takes tide into account?

Several assumptions are being made, you are at mean sea level the earth is flat and probably more. Doubt you would want to navigate by it but guessing when you might reach port? Even that at a shallow angle might be suspect since a slight rocking on deck could make a hell of a difference in angle.

I nice exercise in trig..

[tinkrman69]

I like to keep it simple. knowing the hight of my eye off the water and the hight of the object in question a simple formula of sq/root of hight x 1.5 My eye is 11 feet off the water=3.31 approx. x 1.5 =4.96 miles to the horizon. do the same formula for the hight of the object. if it is on the other side of the horizon add the two and there you go. no need to calculate tide, wave hight, and sextant error. there is error in judgment if it is a tower and you can see half of it above the horizon, using half its hight in the formula will get you close. if it's within five miles of you then use a handheld compass and triangulate off a few points.

this works well and is fast, can be done with just a simple calculator with square root. takes a minutes or so. I've never ran aground using this method, alto I have grounded in back waters not paying attention and only need assistance getting off once in forty years of boating, but never damaged. I keep it simple and the stress stays low and the fun stays high.

[OldFrog75]

You can use your thumb for an approximation. Your thumb is about 1" wide. Make a fist and straighten your arm with you thumb extended horizontally and compare the width of your thumb to the height of the object. 1 thumb width; distance = 30 X height of object. 1/2 thumb width; distance = 60 X height of object. 1/4 thumb width; distance = 120 X height of object. Etc.

Obviously not as accurate as using a sextant and the corresponding arithmetic but a pretty good estimate.

[boatman61]

Man.. you must like hard work...
Triangulation with a compass is so much easier.. 2 shore sights and where the lines cross is your position within 500yds.. allowing for travel while calculating.
Then a straight line from there to shore gives your distance.. much more accurate

[hamburking]

Originally Posted by sharkwatcher

Thanks guys, very helpful. I also after looking thru all my nav reference books found table 9 in Bowdich Which does all the math for you! Thanks again. It's good to know that there are still people that do this type of hands on, non electronic navigation.

I always use the chart in bowditch. Just too easy.

If you are looking for other easy/fun traditional nav methods, you might try "doubling the angle off the bow". Its another way to get distance off from just one point visible on land.

In short, take the angle off the bow of a known point on your chart(on land..like a chimney, tower, or small island). Next, record the distance you cover until the angle off the bow is double the original angle. At that point, the distance off is equal to the distance run (an isosceles triangle if you draw it on the chart). You can draw a distance circle from the point on your chart. Combine that with a bearing and depth, and you've got a fix from just one visible point.

Cool eh? If you don't think so, you are likely in the wrong thread.

[markpierce]

Except for bridge/overhead-wire clearances, none of my charts show the height of shore-side objects. Find radar an easier way to measure distance.

[hamburking]

Originally Posted by boatman61

Man.. you must like hard work...
Triangulation with a compass is so much easier.. 2 shore sights and where the lines cross is your position within 500yds.. allowing for travel while calculating.
Then a straight line from there to shore gives your distance.. much more accurate

I have to disagree.
A small error in the bearings...maybe deviation, unknown variation, or just rough water...can cause a huge error using two points as you describe boatman.

For greater accuracy (and simplicity) use 3 points. Record the angle between points 1 and 2, and points 2 and 3. Setup the angles on your STATION POINTERS, layout on the chart, and presto, a super accurate fix, independent of compass error.

[boatman61]

Originally Posted by hamburking

I have to disagree.
A small error in the bearings...maybe deviation, unknown variation, or just rough water...can cause a huge error using two points as you describe boatman.

For greater accuracy (and simplicity) use 3 points. Record the angle between points 1 and 2, and points 2 and 3. Setup the angles on your STATION POINTERS, layout on the chart, and presto, a super accurate fix, independent of compass error.

Hey.. that's cool..
In a classroom I would.. however real life solo and coastal 2 is more than adequate for me.. have not sunk any islands or continents yet..

[clownfishsydney]

Buy a cheap golfing rangefinder, works up to 500 metres.

[hamburking]

Originally Posted by clownfishsydney

Buy a cheap golfing rangefinder, works up to 500 metres.

Or just turn on your GPS (or chartplotter).
These skills, and associated tools, are great...but really, modern electronics have killed them all.

[Ungvar]

I'm thinking height of the object over Tan Angle would give you a basic distance, but I don't think many people navigate like that. I don't any way.

Sent from my XP7700 using Cruisers Sailing Forum mobile app

[Stu Jackson]

Originally Posted by hamburking

Or just turn on your GPS (or chartplotter).
These skills, and associated tools, are great...but really, modern electronics have killed them all.

That may be quite true, but not the original question.

I'm an old geezer.

I lament the lack of basic navigational skills for PILOTING.

Not sextants for offshore and the like, but near shore techniques, including the sextant.

Too many kids with their heads in electronic widgets.

But heck, they do get "home" most of the time.

I simply ENJOY using those techniques to confirm and have backup on positions when coastal cruising.

Duttons was and still is a fun read.

To each their own.

[hamburking]

Originally Posted by markpierce

Except for bridge/overhead-wire clearances, none of my charts show the height of shore-side objects.

Thats funny, my canadian charts show the height of every clock tower, flagpole, chimney, water tower, etc, etc:

Navionics Webapp

[UNCIVILIZED]

Originally Posted by hamburking

Or just turn on your GPS (or chartplotter).
These skills, and associated tools, are great...but really, modern electronics have killed them all.

When you don't learn (or care about) basic navigational skills, what does that say about the foundations of your nautical knowledge & competence?

I mean we put guys onto the moon using slide rules, literally, & yet we now have a couple of generations of people who cannot do basic mathematics. To me, that pretty much defines FUBAR.

On the last Mercury Space Flight, Gordo Cooper's navigational computers went down. So based on the positions of what stars he could see through the hatch (which the astronauts had DEMANDED that the capsules have), using a grease pencil on said hatch, & a slide rule; he correctly calculated his reentry trajectory angle.

Had he not been able to do that, or had he goofed, math wise; he would have either:
- Burned up on reentry
- Bounced off the Earth's atmosphere, & long ago, left the Solar System, via his spacecraft's inertia.

25yrs ago, as the Conning Officer (OOD) on a Destroyer, I was figuring CPA's on the Bridge's RADAR scope, using a grease pencil & a popsicle stick.
Something I could still do in my sleep. It ain't rocket science... or perhaps it is

Gotta' Know AND Practice the basics.