GPS - Tides

Author

Tides

Keith Sheppard

2007-06-01, 7:33 am

Strolling along the Cornish coast earlier this week, my nearest and dearest
asked if my GPS thingy could tell me when it would be high tide. It seemed
a perfectly reasonable question. The unit knows where I am; it knows the
phases of the sun and moon, surely it could work out when it would be high
tide. After a few minutes of optimistic menu searching I came to the
conclusion that it could not. Is this too hard a calculation for your
average GPS or are there factors affecting tide times that my GPS wots not
of?

The particular unit I was using was a Garmin eTrex Venture CX.

Keith

Dominic Sexton

2007-06-01, 10:33 am

In message <EuT7i.10192$I55.293@newsfe2-gui.ntli.net>, Keith Sheppard
<keith.sheppard@tesco.net> writes
>Strolling along the Cornish coast earlier this week, my nearest and dearest
>asked if my GPS thingy could tell me when it would be high tide. It seemed
>a perfectly reasonable question. The unit knows where I am; it knows the
>phases of the sun and moon, surely it could work out when it would be high
>tide. After a few minutes of optimistic menu searching I came to the
>conclusion that it could not. Is this too hard a calculation for your
>average GPS or are there factors affecting tide times that my GPS wots not
>of?

Tides are driven by the gravitational attraction of the moon and sun but
the shape of the sea bed and coast have a very large influence on them.
In order to predict them with any accuracy the actual tides at a
location must be analysed to determine the harmonic constants.

In the UK the Hydrographic Office have determined the harmonic constants
for the large ports and local to those there are tables of time offsets.
Sadly like the OS with maps the HO have onerous licensing requirements
so the official harmonic constants for the UK are not freely
available...
--
Dominic Sexton

Sam Wormley

2007-06-01, 12:33 pm

Keith Sheppard wrote:
> Strolling along the Cornish coast earlier this week, my nearest and dearest
> asked if my GPS thingy could tell me when it would be high tide. It seemed
> a perfectly reasonable question. The unit knows where I am; it knows the
> phases of the sun and moon, surely it could work out when it would be high
> tide. After a few minutes of optimistic menu searching I came to the
> conclusion that it could not. Is this too hard a calculation for your
> average GPS or are there factors affecting tide times that my GPS wots not
> of?
>
> The particular unit I was using was a Garmin eTrex Venture CX.
>
> Keith
>
>

There is no technical reason that tide prediction couldn't be included
but it would take a very large look up table for the location specific
data. Sun and Moon data is included in my Trimble ScoutMaster.

Our Restless Tides (NOAA) - A brief explanation of the basic astronomical
factors which produce tides and tidal currents.
http://co-ops.nos.noaa.gov/restles1.html

Keith Sheppard

2007-06-01, 12:33 pm

>>Sadly like the OS with maps the HO have onerous licensing requirements so[color=darkred]

Thanks Dominic. I might have known that it all comes down to money.

Keith

Keith Sheppard

2007-06-01, 12:33 pm

Thanks for the reference.

John Tyson

2007-06-01, 3:33 pm

"Keith Sheppard" <keith.sheppard@tesco.net> wrote in message
news:wQX7i.6233$lz3.1894@newsfe5-win.ntli.net...
> Thanks for the reference.
>
>
Garmin has tide tables which can be included as part of the map set you
download to the GPSr, but I don't think they have anything other than US
tide stations. I could be wrong, and you might be able to find something
for your area (third party of maybe even Garmin).

John

Dennis Pogson

2007-06-01, 3:33 pm

John Tyson wrote:
> "Keith Sheppard" <keith.sheppard@tesco.net> wrote in message
> news:wQX7i.6233$lz3.1894@newsfe5-win.ntli.net...
> Garmin has tide tables which can be included as part of the map set
> you download to the GPSr, but I don't think they have anything other
> than US tide stations. I could be wrong, and you might be able to
> find something for your area (third party of maybe even Garmin).
>
> John

Garmin BlueCharts will display tides for the area covered by the loaded
charts, which can be huge. I use BlueChart Atlantic and you simply click on
a symbol and select "predict tides" and a tidal graph and with times and
range details comes up immediately.

I have no reason to believe that every square mile of coastal area on the
chart CD is not covered, i.e the African Coast thru Northern Europe and
including parts of the Indian Ocean, but of course you can't load the whole
area into a Garmin instrument, although the newer ones which hold their data
on SD cards will hold a lot more data than my GPSMap60CS.

The tidal predictions are pretty accurate when checked against current
tables supplied by the national tidal data authorities, usually within 10-15
minutes or so.

Dennis.

Keith Sheppard

2007-06-04, 4:33 am

>>Garmin BlueCharts will display tides for the area covered[color=darkre
d]

Unfortunately I've never owned a Garmin mapping product and it's not worth
my while buying one just to get the tide info, but thanks for the
information anyway.

Keith

Jack Erbes

2007-06-04, 10:33 am

Keith Sheppard wrote:
>
> Unfortunately I've never owned a Garmin mapping product and it's not worth
> my while buying one just to get the tide info, but thanks for the
> information anyway.

It is really hard to figure it out from the Garmin web pages but my 76Cx
comes with a database or supplemental basemap of marine navaids called
"Americas Marine Point Basemap,Jan 2001".

That is built into main memory, it cannot be removed or reloaded, it can
be turned on and off. If you click on the In the Box tab here:

https://buy.garmin.com/shop/shop.do?pID=350#

it has a link to the base map descriptions but those make no mention of
the marine navaids. If you do a compare on the handheld models it will
tell you which models have tide tables. I think that any model that has
tide tables will have the Americas Marine Point Basemap but I'm not sure
of that.

That basemap has the North American tide stations shown on it along with
an impressive collection of lighted, unlighted, fixed, and floating
marine navaids on it.

Selecting a tide station and pressing Enter gives me a very nice
graphical tide chart and I can change that to a simple table of the
times and levels of the highs and lows if I want.

Because I also use BlueCharts and am familiar with the waters, I
immediately realized that the Marine Point Basemap includes nearly every
navaid I ever use. Including the unlighted can and nun buoys that mark
some of the trickiest channels and passages.

If I wanted to navigate familiar waters without the soundings and
additional details from BlueChart, I could do it using the Marine Point
database. It would put the onus on me to keep the boat in safely
navigable waters and spot and avoid all the hazards that are not marked
with daymarks, buoys, and lights. But traveling by boat is that way
anyways, it just gets easier to do with better charting.

There are a lot of unmarked ledges, awash rocks, and nearly awash rocks
in the coastal waters up here so I want to have the BlueChart details
too whenever I can. There are a lot of places up here that look like
nice places to go with a boat until you look at the charts.

Jack

--
Jack Erbes in Ellsworth, Maine, USA - jackerbes at adelphia dot net
(also receiving email at jacker at midmaine.com)

Keith Sheppard

2007-06-04, 10:33 am

>>It is really hard to figure it out from the Garmin web pages but my 76Cx[color=darkred]

I'm UK based, so I have Atlantic base map. I'm using an eTrex Venture Cx
but I don't have it with me right now. I shall investigate the European
version of the marine point basemap but I'm not optimistic.
[color=darkred]

I'm not normally a boat user - just a walker. My interest in tides is
solely concerned with keeping my feet dry!

Thanks for the info though.

Keith

Dan Anderson

2007-06-04, 12:33 pm

Keith Sheppard wrote:
>
> I'm UK based, so I have Atlantic base map. I'm using an eTrex Venture Cx
> but I don't have it with me right now. I shall investigate the European
> version of the marine point basemap but I'm not optimistic.
> I'm not normally a boat user - just a walker. My interest in tides is
> solely concerned with keeping my feet dry!

Tide information is usually found in the GPS receivers sold as
marine units such as the 76 series. Seems to me I got tide
information on the GPS 76 with the basic points of interest
supplied. The points of interest supplied with the eTrex
Venture had more small towns than the one supplied with
the GPS 76 so I installed the POI's for the Venture to
the GPS 76 (I didn't care for the marine info). I could
probably go the other way and install the POI's for the
GPS 76 in the Venture and get tide information. These were
for the U.S. Maybe you can find some similar files for the
UK/Atlantic region. You need to look through the Garmin web
pages for files to restore the original POI's supplied with
the receivers.

--
Dan
(email change 2001 to 2004)
(www.gpsmap.net)

J. J. Lodder

2007-06-04, 12:33 pm

Sam Wormley <swormley1@mchsi.com> wrote:

> Keith Sheppard wrote:
>
> There is no technical reason that tide prediction couldn't be included
> but it would take a very large look up table for the location specific
> data. Sun and Moon data is included in my Trimble ScoutMaster.
>
> Our Restless Tides (NOAA) - A brief explanation of the basic astronomical
> factors which produce tides and tidal currents.
> http://co-ops.nos.noaa.gov/restles1.html

On the Cornish coast things are fairly simple.
The tidal wave comes in from the Atlantic,
and goes up-Channel from there.
For a given place there is a more or less constant delay
between high tide and full moon. (which you have to know)

Best,

Jan

Keith Sheppard

2007-06-05, 4:33 am

Thanks Dan

It might well be worth my while looking for the 76 series POI file. I'll
have a browse and see what I can find.

I guess it might even be possible to merge the two.

Keith

Keith Sheppard

2007-06-05, 4:33 am

>>For a given place there is a more or less constant delay[color=darkred]

Well I, for one, don't know the delay factor. Can anyone enlighten me?

Keith

dold@43.usenet.us.com

2007-06-05, 10:33 am

In sci.geo.satellite-nav Sam Wormley <swormley1@mchsi.com> wrote:[color=darkred
]
> Keith Sheppard wrote:

For the US, there is a mapsource set of tide tables.
http://www8.garmin.com/software/ Re...desSoftware.exe
This is a supplemental download for the GPSMap168, but it works well in my
60CS. There might be an equivalent for Europe, but I can't locate it.

There is also a tide table POI for some models, but it displaces all of
the mapsource that you've loaded, leaving you with just tide tables.

--
Clarence A Dold - Hidden Valley Lake, CA, USA GPS: 38.8,-122.5

Darren Dunham

2007-06-05, 12:33 pm

In sci.geo.satellite-nav Keith Sheppard <keith.sheppard@tesco.net> wrote:
[color=darkred]
> Well I, for one, don't know the delay factor. Can anyone enlighten
> me?

As he said, it's basically constant, but only for a given location.
Local topography is a huge factor. Without incorporating it, a
simplistic calculation based on lat/long and time would be almost
useless. So you have to do measurements at that location to figure it
out, or ask someone that has done those measurements.

From the XTide FAQ:

Q: Can you predict the tide and/or current if I give you the latitude
and longitude?

A: The short answer is no. XTide cannot predict tides unless you
provide harmonic constants (see What to do if your location isn't
listed).

From what I'm told, the tide models that were built from
TOPEX/Poseidon data work on a global scale, but they are inaccurate on
continental shelves. Some organizations have constructed models that
function in coastal waters in localized regions. For example, NIWA
has a model for New Zealand's coastal waters, and NOAA has a model of
currents in San Francisco Bay. Although XTide could make use of
harmonic constants generated from these models, XTide does not
implement any such models.

The XTide page contains a lot of information about the collection, use,
(and modification) of tidal data, and it also has a file that can be
used by other programs to display tidal data (with some restrictions,
read the license if applicable). http://www.flaterco.com/xtide/

One of the programs that makes use of the data is the Palm platform's
"Tide Tool". It don't have the convenience of using it in a GPS and
doing an autolookup, but it works well for me.
http://www.toolworks.com/bilofsky/tidetool/

--
Darren Dunham ddunham@taos.com
Senior Technical Consultant TAOS http://www.taos.com/
Got some Dr Pepper? San Francisco, CA bay area
< This line left intentionally blank to confuse you. >

Mike Coon

2007-06-05, 10:33 pm

Keith Sheppard wrote:
>
> Well I, for one, don't know the delay factor. Can anyone enlighten
> me?
> Keith

You only have to think about the double tides around the Isle of Wight to
realise that nothing short of some extensive tables is going to give an
answer. Plus the height of the tide influences the timing, as you'd expect
from a highly non-simple-harmonic motion. And the heights of course depend
largely on the phase of the moon. So no surprise that in the UK such tables
are proprietary and I think not available, except maybe in simplified form,
for free.

Mike.
--
If reply address = connectfee, add an r because it is free not fee.

J. J. Lodder

2007-06-06, 4:33 am

"Mike Coon" <mjcoon@@connectfee.co.uk> wrote:

> Keith Sheppard wrote:
>
> You only have to think about the double tides around the Isle of Wight to
> realise that nothing short of some extensive tables is going to give an
> answer. Plus the height of the tide influences the timing, as you'd expect
> from a highly non-simple-harmonic motion. And the heights of course depend
> largely on the phase of the moon. So no surprise that in the UK such tables
> are proprietary and I think not available, except maybe in simplified form,
> for free.

Somewhere, long ago, forgotten where, in a science museum,
I saw an early (19th century) example of a Fourier synthesizer.
It used gears to generate about 12 harmonics, the explanation said.
Unfortunately is was in a glass cage and no longer functional.

It was used originally (you guessed it) to predict the tides.

Best,

Jan

J. J. Lodder

2007-06-06, 4:33 am

Darren Dunham <ddunham@redwood.taos.com> wrote:

> In sci.geo.satellite-nav Keith Sheppard <keith.sheppard@tesco.net> wrote:
>
>
> As he said, it's basically constant, but only for a given location.
> Local topography is a huge factor. Without incorporating it, a
> simplistic calculation based on lat/long and time would be almost
> useless. So you have to do measurements at that location to figure it
> out, or ask someone that has done those measurements.

Unfortunately, for some a GPS receiver is a tool
for avoiding the need of having to talk to the locals.

In the case of tides it fails,

Jan

Keith Sheppard

2007-06-06, 7:33 am

>>Unfortunately, for some a GPS receiver is a tool[color=darkred]
I don't think anyone would ever accuse me of that. I'm more than happy to
talk the proverbial hind legs off a donkey with anyone who has any
interesting local knowledge.

The tide question came up simply as a matter of interest during a walk, when
there were no locals about. Had it been important (eg. if the walk had
encompassed any areas in danger of being under water at high tide) then we
would have checked before we set out.

The answers here have confirmed my suspicions that it's a more complex area
than one might have thought, which explains why the information isn't
readily available from my GPS.

Keith

J. J. Lodder

2007-06-06, 7:33 am

Keith Sheppard <keith.sheppard@tesco.net> wrote:

> I don't think anyone would ever accuse me of that. I'm more than happy to
> talk the proverbial hind legs off a donkey with anyone who has any
> interesting local knowledge.
>
> The tide question came up simply as a matter of interest during a walk, when
> there were no locals about. Had it been important (eg. if the walk had
> encompassed any areas in danger of being under water at high tide) then we
> would have checked before we set out.

-Do- check before you set out for a walk towards the sea
from Mont Saint Michel,

Jan

J. J. Lodder

2007-06-06, 7:33 am

Dominic Sexton

2007-06-06, 7:33 am

In article <1hzabnm.us8atg1vf0vbyN@de-ster.xs4all.nl>, J. J. Lodder
<nospam@de-ster.demon.nl> writes
>"Mike Coon" <mjcoon@@connectfee.co.uk> wrote:
>
>
>They are not considered to be a military secret anymore?

No it is a money thing. They want organisations providing tide
information to license it from the UKHO.

However it seems that the XTide folk have analysed tide gauge data and
independently derived the harmonic constants for 44 locations around the
UK. These constants are now freely available.

See the last paragraph of this section:

http://www.flaterco.com/xtide/faq.html#60

--

Dominic Sexton

Jack Erbes

2007-06-06, 7:33 am

Dominic Sexton wrote:
<snip>
>
> No it is a money thing. They want organisations providing tide
> information to license it from the UKHO.

That's pretty greedy. You can usually get free tide book from most bait
and tackle stores in the U.S.

How about the times of sunrise and sunset? Do they want you to pay for
that information too? And do they charge extra for a beautiful sunset?

Jack

--
Jack Erbes in Ellsworth, Maine, USA - jackerbes at adelphia dot net
(also receiving email at jacker at midmaine.com)

Alan Murphy

2007-06-06, 10:33 am

On Tue, 5 Jun 2007 23:09:54 +0100, "Mike Coon"
<mjcoon@@connectfee.co.uk> wrote:

>Keith Sheppard wrote:
>
>You only have to think about the double tides around the Isle of Wight to
>realise that nothing short of some extensive tables is going to give an
>answer. Plus the height of the tide influences the timing, as you'd expect
>from a highly non-simple-harmonic motion. And the heights of course depend
>largely on the phase of the moon. So no surprise that in the UK such tables
>are proprietary and I think not available, except maybe in simplified form,
>for free.
>
>Mike.

It is an interesting exercise to plot the time of High Water for the
major ports/harbours along the south coast of England from say
Falmouth to Dover. For example go to the BBC website/tides:
http://www.bbc.co.uk/weather/coast/tides/south.shtml
and related pages. Plot the HW times against longitude (or distance).
You will find something interesting regarding the HW times between
Portland and Swanage.

Next time you take a bath slosh the water up and down and think about
the time of high water at each end of the bath and observe the time of
HW half way along the bath?

Next time you walk the coastal path Swanage/Kimmeridge/Mupe Bay think
about your bath.

Happy Walking.

Alan Murphy

Dominic Sexton

2007-06-06, 10:33 am

In message < 4666a890$0$9934$4c36
8faf@roadrunner.com>, Jack Erbes
<jackerbes@adelphia.net> writes
>Dominic Sexton wrote:
><snip>
>
>That's pretty greedy.

Indeed it is. It is the same story with mapping over here.

>You can usually get free tide book from most bait and tackle stores in
>the U.S.

The US is much more enlightened about access to government acquired
data.
>
>How about the times of sunrise and sunset? Do they want you to pay for
>that information too? And do they charge extra for a beautiful sunset?

I expect that they would if the thought they could get away with it!

--
Dominic Sexton

Mike Coon

2007-06-06, 10:33 pm

J. J. Lodder wrote:
> Somewhere, long ago, forgotten where, in a science museum,
> I saw an early (19th century) example of a Fourier synthesizer.
> It used gears to generate about 12 harmonics, the explanation said.
> Unfortunately is was in a glass cage and no longer functional.
>
> It was used originally (you guessed it) to predict the tides.

Ooh, that rings a bell! I think there may have been (appropriately) a
hydraulic version too. Must look it up...

Mike.
--
If reply address = connectfee, add an r because it is free not fee.

J. J. Lodder

2007-06-07, 4:33 am

"Mike Coon" <mjcoon@@connectfee.co.uk> wrote:

> J. J. Lodder wrote:
>
> Ooh, that rings a bell! I think there may have been (appropriately) a
> hydraulic version too. Must look it up...
>
> Mike.

What we are talking about should be related to:
=====
William Thomson (a.k.a. Lord Kelvin) was a prime mover in the
exploration of the possibilities. Using a ball-and-disk integrator
invented by his brother, James Thomson, Kelvin's 1886 Harmonic Analyser
linked an array of such integrators to extract Fourier coefficients from
waveform data. (Kelvin saw that such integrator arrays could solve
differential equations, but was unable to overcome the problem of torque
loss between stages). The Analyser provided data for another analogue
machine, the Harmonic Synthesiser, whose practical application was to
compute tide tables. Once the Fourier coefficients had been computed
from measured data for a port, the process could be reversed. The
Synthesiser contained a row of rotating wheels representing the
amplitude and phase of the harmonics of solar and lunar components, a
wire wrapped over the whole array producing the total.
====

What I saw sometime, somewhere, must have been a surviving model.
All I know for sure is that it was not in a major science museum.

Best,

Jan

Keith Sheppard

2007-06-07, 4:33 am

>>>You can usually get free tide book from most bait and tackle stores in[color=darkred]
The tide times for the current day were on the noticeboard at the hotel (had
we thought to check before setting out) and are usually freely displayed
outside the local tourist information office (probably coast guard stations
too). It seems like the information is fairly freely and prominently
displayed for the current day, at least. I wonder who is paying for that?

It's just the more general ability to calculate for any given date that
seems harder to come by.

As I said elsewhere, this wasn't a life or death situation otherwise we'd
have checked. We were on the coast path which is generally up on the cliff
tops. The question came up when we were sat observing the cars parked on
the beach at Crackington Haven. We were mildly amused by the mass exodus in
advance of the approaching tide - which is when I was asked if I knew what
time it would be fully in.

Keith

Dave Martindale

2007-06-10, 4:33 am

jjlxa31@xs4all.nl (J. J. Lodder) writes:

>Somewhere, long ago, forgotten where, in a science museum,
>I saw an early (19th century) example of a Fourier synthesizer.
>It used gears to generate about 12 harmonics, the explanation said.
>Unfortunately is was in a glass cage and no longer functional.

>It was used originally (you guessed it) to predict the tides.

There's one of these in the Science Museum in London, I think. The one
I saw looks something like this:

http://cs.nyu.edu/courses/spring00/...ory/kelvin.html

Although you can't see it in the photo, there was some sort of thin
cable or cord anchored in one place on the frame. The cord was then
threaded around a series of cylinders mounted on the shafts you see
facing the camera. The cord went vertically down, half-wrapped around
one cylinder, then ran vertically up to the next cylinder, wrapped a
half-turn around it, then back down to the next lower cylinder, and so
on until it had gone around each cylinder in sequence. The end of the
cord is attached to a pen that writes on the strip chart paper at the
bottom.

Now, as I understand it, the cylindrical drums are mounted on a
mechanism that allows the centre of the cylinder to be offset by a
variable amount from the axis of the shaft that drives it. With the
cylinder off-centre like this, as the shaft turns the cylinder
alternately requires extra cord length to go around it and then less
cord length. This causes the recording pen to go up and down in a
sinusoidal motion. The period of the sinusoid is set by the gearing and
is different for each shaft. The amplitude of the sinusoid is
determined by how far the drum is off centre, while the phase of the
sinusoid is determined by the angular position of the offset on the
axle. Since the cord goes around all of the drums, the pen motion is
actually the sum of all the sinusoids being generated at the same time.

So, once you have the Fourier coefficients for the tides in a particular
place, you program them into the machine by setting an offset distance
and angular position for each one of the shafts, then turn the crank.

Kelvin also had a machine called a harmonic analyzer to calculate the
coefficients in the first place. It used 11 mechanical analog
integrators.

Dave

LinkBot