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Author
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Topic: The elephant in the room - tide effect on cel nav.
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BasilBibi
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posted January 28, 2024 04:21 AM
A simple question that may have a simple answer.
How does tide affect cel nav sightings?
From: London UK
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David Burch
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posted January 28, 2024 09:22 AM
When underway takin sights on a boat, the answer is no. All the water you can see is going up and down with you, and thus there is no effect on dip.
It is just as if the radius of the earth gets a little bit bigger or smaller but remains a perfect sphere.
On the other hand, when standing on land, say a pier that is 4 ft above the water at the moment when the tide is 8 ft, and your eye is is 6 ft above your pier, then the HE is 10 ft for the sights.
But when the tide drops to 2 ft, the pier is now 10 ft above the water and your HE taking sights out over the water will now be 16 ft.
In short, it is more of a mouse than an elephant.
From: Starpath, Seattle, WA
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BasilBibi
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posted January 29, 2024 11:11 AM
Thanks for the response.
So on land how should I estimate the dip?
From: London UK
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David Burch
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posted January 29, 2024 02:47 PM
Best bet on land it is to just measure height above water if you can. Standing on a dock this is easy.
Standing farther up on the beach or a near by hill, measure it with a precision barometer.
We have a free one for phones (Marine Barometer) or more sophisticated version for $15 (Marine Barograph at the same link)... and we have our new high precision USB Baro that would take a laptop to read.
Our phone apps are easiest. Set to the app to high precision, read pressure at water level (for 10s or so) store it in the free app (the $15 stores automatically) then back to the measurement place, read it holding phone at eye level for 10s or so. Then subtract the two pressures, divide by 0.44 and multiply by 12 to get the HE
If you cannot get down to the water to make a baro measurement, then we have to do some chart work that takes longer to explain. See related articles at https://www.starpath.com/cyc/
From: Starpath, Seattle, WA
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BasilBibi
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posted February 02, 2024 10:08 PM
Ok many thanks David.
To summarise :
I can go down to the shore no matter what the height of tide and use my eye level above the water as dip and if I cannot estimate eye height above water level I can use some barometric means.
From: London UK
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David Burch
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posted February 03, 2024 07:25 AM
Yes. Exactly
...but If I might add, it is not just "some barometric means."
It is very specific and very simple, as noted above.
Pressure at the water level Pw (mb) and pressure at the eye level Pe (mb), then the height to be used for HE (ft) = (Pw-Pe)x12/0.44
(I will be making a short video demoing this process and will come back when ready and add a link here.)
From: Starpath, Seattle, WA
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BasilBibi
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posted February 06, 2024 02:09 AM
Fantastic. Thanks again David and looking forward to the vid.
From: London UK
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