Horizontal sextant angles with beam converter?

[Adelie]

Originally Posted by Jdege

But what are you going to be taking bearings off of, at 15nm distance? All I can think of would be mountain peaks. And I'm not sure I'd be able to tell one mountain from another, at 15nm.

At about 15nm the Point No Point light in Puget sound became a dipping range for a 6' or so height of eye, as you lifted your head up and down the light would appear and disappear. Don't even need a sextant for that, just the appropriate visual range tables. Of course this is just like a transit, good for only 1 range from the light.

The PNP light is 30'. A light that is 60-75' tall, or any identifiable feature, angular height to top will give you the range. You don't need mountains until you get out to 25-40nm.

[NevisDog]

Originally Posted by Adelie

At about 15nm the Point No Point light in Puget sound became a dipping range for a 6' or so height of eye, as you lifted your head up and down the light would appear and disappear. ... A light that is 60-75' tall, or any identifiable feature, angular height to top will give you the range...

...but not without a view of the shoreline, and you just said the shore was 30' below the horizon at those distances (the height of the PNP light), so no measurements possible from the deck of a yacht at those distances?

[KiwiKen]

Originally Posted by Jdege

But what are you going to be taking bearings off of, at 15nm distance? All I can think of would be mountain peaks. And I'm not sure I'd be able to tell one mountain from another, at 15nm.

The requirement is for the landmarks to be identifiable both on the chart and on the ground, but assuming that is met it results in a very accurate position.


With vertical Sextant angles one must be able to see both the top and the bottom of the feature, which limits this technique to within the observers horizon, 3-4nm on a pleasure craft, much further from the bridge of a ship.


A position fix on the dipping range of a light coupled with a bearing is reasonable with an elipse of error in the order of 1/2 -1nm

[KiwiKen]

Originally Posted by Adelie

At about 15nm the Point No Point light in Puget sound became a dipping range for a 6' or so height of eye, as you lifted your head up and down the light would appear and disappear. Don't even need a sextant for that, just the appropriate visual range tables. Of course this is just like a transit, good for only 1 range from the light.

The PNP light is 30'. A light that is 60-75' tall, or any identifiable feature, angular height to top will give you the range. You don't need mountains until you get out to 25-40nm.

For a dipping Range on a light the luminous range needs to be greater than the dipping range, a good guide is that must be able to see the loom of the light after it dips.


Tiri Tiri Matangi Light in the Hauraki Gulf is an excellent example and very useful, it's luminous range is over 30nm.

[Adelie]

Originally Posted by NevisDog

...but not without a view of the shoreline, and you just said the shore was 30' below the horizon at those distances (the height of the PNP light), so no measurements possible from the deck of a yacht at those distances?

The radius of the earth is known so it is possible to measure the distance without seeing the base of the object, you measure the vertical angle between the top of the object and the horizon. This applies to anything you can identify that has a known height.

There are 3 tables to use to get distance from vertical angle.
1. Range from angle between top and waterline when waterline is seen and height is known.
2. Range from angle between top and horizon when base is over the horizon and height is known.
3. Range from angle beteen horizon and waterline when waterline is nearer than horizon and horizon is visible beyond.

For all of these height of eye must be known.

Note that for 3. You don’t need to know the height of the object. Because it requires you see the horizon beyond you can’t use it to determine range to a shoreline where the land rises much at all behind. But it works for isolated rocks and it works for other vessels you want to know the range to. The higher your eye the better the accuracy and range of this last method.

All 3 of these table are available in older versions of Bowditch.