Page 1 of 2 12 LastLast
Results 1 to 10 of 15
  1. #1
    Join Date
    Jul 2016
    Posts
    13

    Default Tide lower than LAT - how come?

    According to my regular tide app (Boatie) the low water tidal height at London Bridge tomorrow (Thursday 21 Feb 2019) is -0.3m. How can this be?

  2. #2
    Join Date
    Sep 2010
    Location
    Aberdeen
    Posts
    81

    Default Re: Tide lower than LAT - how come?

    High pressure ?

  3. #3
    Join Date
    Dec 2001
    Location
    Grenoble
    Posts
    28,690

    Default Re: Tide lower than LAT - how come?

    Super full moon.

  4. #4
    Join Date
    Nov 2007
    Location
    Boat Orwell - Me Norwich
    Posts
    7,471

    Default Re: Tide lower than LAT - how come?

    Quote Originally Posted by Fr J Hackett View Post
    Super full moon.
    But isn't that astronomically big/close?

  5. #5
    Join Date
    Jan 2006
    Location
    Bristol
    Posts
    678

    Default Re: Tide lower than LAT - how come?

    I asked the same question a few years ago. Got some sensible answers here
    http://www.ybw.com/forums/showthread...Lower-than-LAT

  6. #6
    Join Date
    Mar 2002
    Location
    Scotland
    Posts
    1,977

    Default Re: Tide lower than LAT - how come?

    Quote Originally Posted by Findhorn View Post
    High pressure ?
    I would doubt that the tidal prediction app predicts barometric pressure as well.

  7. #7
    Join Date
    Jul 2016
    Posts
    13

    Default Re: Tide lower than LAT - how come?

    Rob - your previous thread seems to have it well covered, thank you. To my mind now it goes something like: because chart datum is determined for only for a limited number of ports (presumably standard ports) tidal heights at other locations (secondary ports) can be lower than CD.

    For me this was only out of interest but it could have implications for moored vessels that don’t want to dry out and had done their calculations based on a LAT of CD? Presumably HAT is also similarly affected which could be more significant for craft on the Thames passing under all those bridges.

  8. #8
    Join Date
    Sep 2010
    Location
    Alness / Loch Ness Northern Scottish Highlands.
    Posts
    8,668

    Default Re: Tide lower than LAT - how come?

    This happened a few years ago. The conclusion is they got the chart datum wrong and 0 should really be a little bit lower than it is.

    I think this is only the second year since I have been a boat owner that this has happened.

    It also means high tide is higher than normal, today and tomorrow being very high. Some low lying areas will be worried and praying for still weather, i.e small waves.

  9. #9
    Join Date
    Jun 2017
    Posts
    722

    Default Re: Tide lower than LAT - how come?

    Quote Originally Posted by TheBomber View Post
    ...For me this was only out of interest but it could have implications for moored vessels that don’t want to dry out and had done their calculations based on a LAT of CD? Presumably HAT is also similarly affected which could be more significant for craft on the Thames passing under all those bridges.
    In theory, yes. But we're talking about a few tenths of a metre, less than a typical wave height. I hope you'd allow more than that as a safety margin?

  10. #10
    Join Date
    May 2007
    Location
    Cambridge, UK
    Posts
    5,945

    Default Re: Tide lower than LAT - how come?

    Chart Datums can only be established at locations where there is a permanent tide-gauge; the locations are available at https://www.bodc.ac.uk/data/hosted_d...e_network/#map, as are the data from which tidal constants can be derived.

    However, the important thing to recall is that a datum (either horizontal or vertical) is an arbitrary fixed reference frame. For convenience the vertical datum of charts is usually said to be LAT - but it can only be LAT at the site of a tide-gauge. Elsewhere, the lowest tide may well drop below the chart datum because of local effects - estuaries and embayments are particularly liable to these, as they can amplify the tidal amplitude, resulting in tidal extrema that are above or below the datum defined at a point elsewhere.

    It is not inconceivable (though unlikely) that two overlapping charts that happen to take their datum from different sets of tide-gauges COULD quite correctly show different charted depths. Of course, hydrographers will avoid this situation, and I certainly don't know of an example, but it is possible! I do know of maps in the Antarctic where the horizontal datum differs from place to place within a map; the map-makers (not us!) lost the errors in large glaciers!

Page 1 of 2 12 LastLast

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  

Latest YBW News

Find Boats For Sale

to
to