Chart Datum - Lowest Astronomical Tide

In the UK at least, its the same thing I believe. Essentually, its the zero contour on a chart.

Edit: had to look at a chart to confirm, but it actually says so on the chart.
 
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Karouise said:
Yes know that thanks, am thinking about atmospheric pressure and what is the 'standard' etc

Might just be going too deep!
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Tidal height predictions are for 1013 mb

In theory a rise of 10mb will depress the tide level by 10cm but other factors can have a considerable effect.
 
Karouise said:
Having a mental block here! Are they the same thing?

Something is niggling in the back of my mind to suggest they are different but I can't prove or disprove to myself!
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The lowest the tide will ever be due to the gravitational effect of the heavenly bodies, obviously pressure wind etc. can be an additional influence and can cause the water level to be even lower than LAT or higher than HAT
 
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If the chart datum is says it is lowest astronomical tide, then that's what the chart datum is. Neither the chart nor the lowest astronomical tide is affected by pressure, wind or anything like that.

The actual height of the water will be affected by pressure & wind etc., even in the rare event of a lowest astronomical tide actually occurring. More usually the actual height of the water is far more affected by the astronomical height of the tide above its minimum than it is by weather, but bear in mind the latter when you're cutting it fine.
 
Chart datum is the level that soundings are taken from, and on current British charts usually approximates to LAT. However, there can be differences, noticeably in tidal reaches of rivers. If you are still using old fathom charts there were a range of datums used in the past, but usually a spring tide low water level. As the discussion of pressure effects etc. above point out sea level can fall below both chart datum and LAT. I once kept my boat drawing 2 metres in a marina dredged to 2 metres below CD. There was a groove in the sand under my boat where the keel settled when the water level fell below datum.

If you go into the Baltic Chart Datum is usually the mean sea level. The actual levels vary due to meteorological effects so you can have less water than is shown on the chart.

Local, unofficial, surveys may have different datums. An extreme example was a hydrograhic survey I made of a bay for civil engineers and (as was normal practice) used the same datum as that on the largest scale BA chart. The civil engineers then re-drew the survey to give the depths below Ordnance Datum, and passed this on to a company that was to bring a drilling vessel in to the bay. Knowing that I had made the initial survey the drilling company assumed the survey they were given was to chart datum, but were unable to get their vessel into some of the inshore locations. It was only when they called me out to see what the problem was that we found the datum discrepancy.
 
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Chart datum is not dependant of location

srm said:
If you go into the Baltic Chart Datum is usually the mean sea level.
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This is a dangerous assumption, Always check the chart datum
The International Hydrographic Organisation (IHO) has decided to use LAT for depth and HAT for hight clearance.
IHO:
http://en.wikipedia.org/wiki/International_Hydrographic_Organization
http://www.iho.int

Most hydrographical offises are converting from other datums to HAT/LAT.
Definition of LAT
LAT is defined as the lowest tide level which can be predicted to occur under average meteorological conditions and under any combination of astronomical conditions. It is recommended that LAT be calculated either over a minimum period of 19 years using harmonic constants derived from a minimum of one year’s observations or by other proven methods known to give reliable results. Tide levels should, if feasible, reflect the estimated error values obtained during the determination of these levels
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HAT is the oposite.

There are other things that can affect the water levels, the effects depend on the geographical conditions.
 
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