Isotidal maps...

[Externet]

Greetings.

 Do isotidal maps exist ?  Like Isobaric, isothermal do.

[studiot]

20 minutes ago, Externet said:

Greetings.

 Do isotidal maps exist ?  Like Isobaric, isothermal do.

Not as such no.

Temperature and pressure are scalars like elevation and isomaps are countour maps.

 

Tides have two charateristics that make them unsuitable for such maps.

1) Tides are vectorial. They have a height, a direction and a velocity,   all of which vary with.

2) Tides vary with time.

 

Incidentally marine maps are called charts.

Tidal maps can be obtained in tidal atlases.

Here is the

 

 

 

front and one extract from my local area

 

The complete set comprises 13 charts with arrows and stream rates (in knots) from different times of the tidal cycle.

[Externet]

Thanks, studiot, as helpful as always. 

Trouble with my poor grammar as always; I was trying to express something like 'contour lines' for a given moment / time.  Sort of 'isoelevation'  / 'isolevels' 🙄  if there is a name for such.  As in topography but for sea surface depending on astral positions in function of time.  { Not as tidal current vectors }

 

[studiot]

I'm sorry I didn't explain myself very well but I was just packing up for bed when you originally posted so I was rushed.

 

OK so iso means 'the same'

The same what ?

And also the same under what circumstances.

First the simplest case.  The same at two or more different places at the same time.

Here is a map of part of North Eastern New Mexico showing typical looped or circular contours depicting elevations ie hills and mountains.

 

These hills haven't been going anywhere for tens of millions of years so time is not a factor of interest.

The pattern remains the same for very long periods of time.

So we are just interested in place.

 

Next consider weather maps and isobars.

These change over time but recognisable structures move across the map so we are now interested in both time and place.

 

Once again we have recognisable structures in the isobars but this time they do not stay put but move across the map.

 

Now look at this tidal cycle for Narransett Bay

 

As can be seen the shape of the figures varies over the tidal cycle from springes to neaps.

This means that not only do the maximum and minimum tidal heights (and all others inbetween) vary from one day to the next but also the shape of the curve varies as we progress through the cycle.

Added to this other places on the map have the matching points in their cycles at different times.

Between these two variations they introduce a phase difference over both time and space.

 

On 9/28/2024 at 2:54 AM, Externet said:

  { Not as tidal current vectors }

This phase difference is what I was referring to, rather obliquely, when I mentioned vectors.

 

The point is that you cannot pick either two places or two times to directly compare.

To summarise

Contours compare values at two places at the same time.

Isobars compare values at two places at the same time but a tracking series compares two times at the same place.

 

Does this help

[Externet]

Thank you.

1 hour ago, studiot said:

OK so iso means 'the same'

The same what ?

-Same water surface level contour-

Mapping (charting) the 'isotidal' image above, would be a line across that river, one side of the surface being a guessy example of a level 0.25m? higher.  And changing (advancing) depending of the astral time of day.

===============================================================================

This 'isotidal' above would show the similar curved line on a chart. Also, in function of time.

=======================================================================

An 'isotidal' chart would show for the Mediterranean sea, lines from coast-to coast at Gibraltar; lines advancing eastward to Cairo denoting zero at lowest mean tide, stepping at any chosen graph interval (0.25m, 0.5m...) always in function of time.

Closer together 'isotidal' lines would denote higher speed water currents (slope).  Did I invent the isotidal word ? 🙄   If tide escalars level data exist for every port, an isotidal line would connect those points with same level at a given moment.

Well... do these charts exist, as 'isoelevation' 🙄 in topography ?

 

 

 

[studiot]

23 minutes ago, Externet said:

Well... do these charts exist, as 'isoelevation' 🙄 in topography ?

Not to my knowledge.

What would be the point ?

When you say isoelevation, elevation above what ?

Land elevation has a meaning as all elevations on Earth are related to some simple and recognisable datum that does not change appreciably with time.

Quote

Publisher Summary

https://www.sciencedirect.com/science/article/abs/pii/S0422989408707797

 

It is general practice to choose the average physical water surface as the reference level not only for all measurements of altitude on land, but also for all depth determinations at sea. Nevertheless, every oceanographer is aware that the mean sea level in the oceans and seas varies considerably and that deviations from the average may not be neglected even for practical purposes. In order to give an example, it may be mentioned that owing to the fact that the water density deviates highly between the Atlantic Ocean and the Pacific Ocean, the latter ocean stands, on the whole, higher than the former ocean. A number of other contributing factors must also be taken into consideration. Although the effect of the wind-caused piling-up of water cannot be proved to occur in the deep open parts of the oceans, special attention must always be paid to this effect when comparing the mean sea level at two places situated at some distance from each other along the coast, especially if the orientation of the two places is different. It must also be remembered that the effect of piling-up increases in shallow water and land-locked basins.

 

 

Reid (1961b) estimated that with respect to the 4,000-decibar surface the mean sea level in the Pacific Ocean is about 68cm higher than in the Atlantic Ocean, a result which is in close agreement with the values in Table XXXII.

 

[swansont]

40 minutes ago, Externet said:

-Same water surface level contour-

That's going to be a continuous value unless there's a sharp change in the solid surface underneath (a waterfall, for example) and you can get a continuous contour line in a 2-D surface. But a coastline is not a 2-D surface, so there's not going to be a contour of constant value unless you have the trivial case where it's constant everywhere.

[Externet]

5 hours ago, studiot said:

When you say isoelevation, elevation above what ?

Above minimum sea level, or, above/below  ¨mean sea level¨   Seen both used as reference.

The point would be to find the location of fastest (or slowest) sea currents at a given moment. (spacing between 'isotidal' contour lines)

Edited September 29 by Externet

[studiot]

26 minutes ago, Externet said:

The point would be to find the location of fastest (or slowest) sea currents at a given moment. (spacing between 'isotidal' contour lines)

I can't imagine a worse predictor of the speed of tidal streams than this.

What equation would relate stream speed to difference in height above either msl or minsl ?

[Externet]

Well, I thought that velocity of sea currents would obey the slopes, same as isobars affect wind speed...  Am wrong.

[studiot]

47 minutes ago, Externet said:

Well, I thought that velocity of sea currents would obey the slopes, same as isobars affect wind speed...  Am wrong.

Pressure is not the same as height.

 

Pressure difference can result from height difference.

 

It even says pressure in your article.