= This is the datum plane to which soundings are reduced in the course of a hydrographic survey and is therefore the datum used when compiling the Fair Chart or Final Tracing.
= It is the datum which the field surveyor will establish and adopt.
= Ideally SD should be the same as Chart Datum (CD), but may well differ particularly when the original survey is very old.
= One of these methods is to be used when establishing datum;
· Recover a previously established SD or CD in the survey area by leveling from benchmarks.
· Transfer datum from a place close to the survey area where CD or SD is already established, by taking simultaneous tidal observations over a period.
· Establish a new datum from observations.
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