NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Tidal constants
From: Frank Reed CT
Date: 2006 Sep 1, 16:06 -0500
Geoff you wrote:
"I see that US port constants are available free on the internet, but it
wouldn't be a surprise in our 'Thatcherised' society that such information
for the UK has to be paid for: the same philosophy as for access to weather
forecasts."
"The" US constants are only examples of possible constants. There's no
perfect set. For example, the constants included with implementations of XTide
(which is the engine underlying most Internet-based tide calculators) are
distinctly different from the constants you will find on the NOAA/NOS site.
Generally, the 'thatcherization' you refer to seeks to get a piece of the
pie from commercial ventures, which is not unreasonable. It is, however, a
nuisance for hobbyists. But you will usually find that you can get this
information for private non-commercial use if you ask around... Another approach would
be to generate your own tidal constants from a long run of predictions from
some software that has licensed the official tidal harmonics (note that this
sort of reverse-engineering does violate the UK copyrights but as long as you
don't publish the results it's not a problem). This is similar to generating
tidal constants from observed tide gauge data but since there's no noise in
the inputs, the calculation will converge very rapidly on the correct
constants. About one year of hourly data is sufficient to get values for the major
tidal harmonics and most of the minor ones, too.
And:
"I have written, for interest really, a programme to calculate tide heights
from the Meeus(?) algorithms which works accurately enough for most
practical purposes."
Meeus tidal algorithms? Where did you get them?
And:
"Times of high and low water are pretty good - who can
spot the precise moments of these anyway? "
How many tidal harmonics have you included?
And:
"but tide heights (for the nicer
places to visit!) depend on the topographical shifts which are reflected in
the most recent tidal constants."
What sort of topographical shifts? With rare exceptions, you could use
hundred-year-old tidal constants (pretending that they exist and are calculated
correctly) and they should yield very good present-day tide predictions.
-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
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From: Frank Reed CT
Date: 2006 Sep 1, 16:06 -0500
Geoff you wrote:
"I see that US port constants are available free on the internet, but it
wouldn't be a surprise in our 'Thatcherised' society that such information
for the UK has to be paid for: the same philosophy as for access to weather
forecasts."
"The" US constants are only examples of possible constants. There's no
perfect set. For example, the constants included with implementations of XTide
(which is the engine underlying most Internet-based tide calculators) are
distinctly different from the constants you will find on the NOAA/NOS site.
Generally, the 'thatcherization' you refer to seeks to get a piece of the
pie from commercial ventures, which is not unreasonable. It is, however, a
nuisance for hobbyists. But you will usually find that you can get this
information for private non-commercial use if you ask around... Another approach would
be to generate your own tidal constants from a long run of predictions from
some software that has licensed the official tidal harmonics (note that this
sort of reverse-engineering does violate the UK copyrights but as long as you
don't publish the results it's not a problem). This is similar to generating
tidal constants from observed tide gauge data but since there's no noise in
the inputs, the calculation will converge very rapidly on the correct
constants. About one year of hourly data is sufficient to get values for the major
tidal harmonics and most of the minor ones, too.
And:
"I have written, for interest really, a programme to calculate tide heights
from the Meeus(?) algorithms which works accurately enough for most
practical purposes."
Meeus tidal algorithms? Where did you get them?
And:
"Times of high and low water are pretty good - who can
spot the precise moments of these anyway? "
How many tidal harmonics have you included?
And:
"but tide heights (for the nicer
places to visit!) depend on the topographical shifts which are reflected in
the most recent tidal constants."
What sort of topographical shifts? With rare exceptions, you could use
hundred-year-old tidal constants (pretending that they exist and are calculated
correctly) and they should yield very good present-day tide predictions.
-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---