vertical sextant angles

[bluejuice]

Does anybody know?
When navgiating by non gps means or checking said instrument, the sextant can be used in it's vertical plane to tell the distance off an object of known height by arc angle when its top is viewed spilt with shore line. this is then reduced using a formula or tables to give the distance off. However on the chart the hieght of object is from MHWS therefore normally the water level will be below this thus makeing the non tide corrected reading think you are closer to the object.

The question:
is how much error in nautical miles will this create (understand the varbles of height and tidal range will make a diffrence)

Or do people when using this tend to correct for tide thus making a very complex calculation as i have tryed on occasion when i wanted to be acccrate for excises sake.

<hr width=100% size=1><P ID="edit"><FONT SIZE=-1>Edited by bluejuice on 27/01/2004 20:42 (server time).</FONT></P>

 

[AndrewB]

The charted height of an object is above mean high water springs, so the actual height above sea level is normally greater.

With a small vertical sextant angle, under 5 degrees, your error will be proportional to the height of the tide relative to the charted height of the object. If the charted height is X meters, and the actual height above sea level is Y meters, you will be further away than you think you are from your sextant reading, by a factor of Y over X.

E.g. if charted height is 50 meters and actual height is 55 meters then you are actually 55/50 = 1.10 = 10% further away than you think you are.

The difference in angle (in this example) is large enough to be distinguishable by a sextant at least up to 3 or so miles away, so a tidal correction might theoretically be made.

 

[bluejuice]

thank you very much, that clears that wondering up.
what your first pargraph/ sentence said was what i was trying to get at in my garble just was having a little trouble getting that all in to english!!!!

Thanks again Ed

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[bluejuice]

the fact i though it was closer when the height incresed with tide has just been traced back to something i read a long time ago in a very well know sailing intructional book. should have done the maths and worked out the real answer!!

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[philmarks]

One thing never mentioned in the books is that if your height of eye is too low relative to distance off, you can't meaure the "true" height angle, because the base of the L'house is below the horizon. (2.5 mls at 6' I think), so if my tired brain serves me correctly, you might actually be closer than you think you are.

<hr width=100% size=1>Rgds
Phil

 

[bluejuice]

okay been thinking about this in terms of the dipping, what you are saying is that at more than 2.5 - (3.2 my guess figure) miles off you can not get the true hieght of the object anyway so when useing this technique you must be within a defined distance from the object (2.5 - 3 mls) to get a reading that is non erronus.

hope that has not fryed anyone elses brain as much as mine!!


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[Guest]

The commercial way - as I was taught for Merchant Navy .... the error of tidal height etc. gives you a safety factor of placing you closer to the item than in reality, aming you stand off a little more.
It was accepted that you would work with charted \data and not correct for tidal levels etc., unless you were requiring super accuracy in surveying etc. - then you would use horizontal angles in preference anyway.

So take it easy and don't overcook the exercise !!!


<hr width=100% size=1>Nigel ...
So WHAT does the EU really stand for ????/forums/images/icons/cool.gif

 

[Guest]

Base of object below horizon .....

Sextant vertical angles can still be used - there is a formulae for this ....

As to tidal heights - all distance off tables do not correct for tidal height and use charted data etc.


<hr width=100% size=1>Nigel ...
So WHAT does the EU really stand for ????/forums/images/icons/cool.gif

 

[AndrewB]

Allowance for height of eye.

You are right that if you aren't high enough, the height may be underestimated. Distance to the horizon is 2.1 nm at a height of 1 meter, 3.0 nm at 2 meters, 3.7 nm at 3 meters.

But the amount is so small that typically it would be within the safe assumptions of the accuracy of a sextant reading from a yacht, and so there is little point in making an adjustment. For example, sitting at 2 meters high on deck and sighting a target 6 nm away over the horizon, the height will be 2 meters greater than apparent, equivalent to an error of about 1 minute of angle.

Theoretically refraction should also be taken into account, which is unpredictable close to the horizon and can be quite severe for narrow angles and long distances.

 

[bluejuice]

i defintily agree having done some calculations using the old 1.852 x ht in mtrs/ angle of arc in mins formula that taking the tide into account is problay begining to overcook the excise (in most cases) but is nice to know from andrewb the percentage error it can induce. as for the dipping i use the formula squ root of ht of eye (mtr) + squ root of ht of light (m) x 2.075 to give me distance of in n miles when an object appears. if this formula is taken for base land appering (object hieght taken as zero) with my height of eye 2.5m on average sextant strong postion i see base land at 3.2 Nm agreing prety much with privous post which from i assume the proper tables.
i think looking at information i have upto about 3.5 Nm with resonable hieght object refraction error should also not be too major, i am right or wrong.....

Also think i never should have got a sextant the whole thing appears to be getting slightly too intersting!!!!!!

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