How to work out my height above sea level?

MissFitz

MissFitz

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I’ve finally got round to doing my astro nav during lockdown. Very fortunately, I live in a south-facing flat on Brighton seafront so I don’t have to go outside to take sun sights. What I can’t figure out, though, is how to work out my height above sea level/chart datum. Can anyone help? (My building is on the same level as the promenade, I’m on the second floor.)
 
Take your sextant to the waters edge, measure the angle between the balcony and the high water mark. Measure the distance from where you are to the balcony by triangulating your position from a couple of objects ashore. Then work out the height above the high water mark your balcony is. Adjust on a daily basis for the tide height when you’re taking your sights.
 
Between, say, 15.1m and 21.4m the dip only changes by 1.2. So a guess inbetween them may well be close enough.

Thats my guess for second floor!!!
 
Height of promenade above averaged sea-level + height of your eye above promenade ...

On Ship for sights - the HoE of the bridge-wings are stated based on construction, (freeboard at the time) and average height of person. So apply similar for your situation ... ignore tidal ranges etc.
 
Use a ordanance survey map for ground height, Add the stairs. Height achieved..
Use a handheld gps, set zero by the sea then trot up to your flat?
 
Altimeter - 30' per millibar.

Look at OS map of your flats then drop a lead line from the balcony. (or use laser measure at night.)

Ask a fisherman to take a vertical sighting of your balcony from a known point offshore, or do so yourself from HWM somewhere to the west ... (That's what I would do.)
 
capnsensible said:
How about tie a bit of string to 'an object' thats not a pet, lower it from balcony to ground and measure string?
Click to expand...
The 'object' should classically be a barometer, no doubt a sextent would work just as well.

One could also throw the sextant into the sea, and time the splash.
You'd need to know the elevation at which you launch the sextant.

I would stick with using whatever height makes your sights look reasonable.
 
lw395 said:
The 'object' should classically be a barometer, no doubt a sextent would work just as well.

One could also throw the sextant into the sea, and time the splash.
You'd need to know the elevation at which you launch the sextant.

I would stick with using whatever height makes your sights look reasonable.
Click to expand...
How about using a piece of lead on the end of the line. You could call it a 'lead line'. :D
 
Here is a suggestion which I think will work quite well.

Wait until around noon, and hope it'll be sunny! The reason is not for 'noon sights' but simply to have a few tens of seconds when the altitude of the sun hardly changes. Have handy a bowl of water and arrange it such that you can see the sun both reflected off the surface of the water and directly. Measure the angle 'A' which exactly superimposes the two images. Then as soon afterwards as possible measure the sun's altitude relative to the horizon, 'B'. There are thus two measures which do not depend on looking up altitude.

Sextant reading A should be corrected for sextant error, but not for height of eye or half-angle. Divide it by two and then apply correction for refraction and parallax to get TZD. This is independent of height above horizon.

Sextant B reading you treat as normal, adjusting for a (guessed) height of eye and all other normal corrections, including sun's half-angle to get a second value for TZD. Adjust the estimate of height until the two means give the same answer. You should be able to measure angles to within 1 minute from your stationary 'deck' so it'll be good enough to practice astro.
 
Same principal as:
Outstretch your arm and hold the pencil so that you can measure the height of the tree on the pencil with your thumb. Then turn the pencil at the base of the tree by 90 degrees. Note where the distance measured by thumb hits the earth and measure the distance from this point to the tree. This is the height of the tree.
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