Datums (NAVD88 and Mean Low Tide) for the

Hydrographic Surveying
• Datums &Tides
• Why Multibeam?
Datums and Tides
Mean Low Tide versus Mean Lower Low Water
And the winner is……….!
The following slide is taken directly from the NOAA/TCOON web sites pages
that gives the official relationships for the various Sea Level Datums (Mean
Sea Level and it’s various statistical offsets) and the Land (Terrestrial)
Datums (NAVD88 and Mean Low Tide) for the Rainbow Bridge on the Neches
River Channel. This location has been chosen because it is one of the
(currently) few NOAA tide gauge sites for which data is available in real time
on the internet, and has published the relationship between the Sea and
Terrestrial datums.
One thing should be borne in mind. Mean Sea Level changes both by
location around the coastline of the U.S. and over long periods of time. The
relationships at the Rainbow Bridge do NOT hold true anywhere else although the differences MAY be minor.
The relationships shown here are
given reference to an arbitrary
“station datum”. Following slides
have changed this to make things
relative to Mean Low Low Water via
high level math (addition and
See slide #12 and #13 for definitions
of these datums.
NAVD 88 elevation updated 5/2013, stated accuracy
is 0.028m (about 1 inch). These datums are marked
preliminary until the 2013 levels are run.
Courtesy TCOON
Web Site
Confused? Lets Try an Example!
NOAA Tides & Currents Web Page
Take two aspirin and call
713-690-4626, ask for
Chris or Mike and have
your credit card ready!
Just joking about the credit card!
The following are provided for reference purposes - if you are
feeling bold.
Why Multibeam?
R2Sonic 2024
Reson 7125
Odom MB-1
And new
Edgetech 4600
Example #1
Bottom Elevation Contour Chart (Multibeam)
Bottom Elevation Contour Chart (Single Beam)
Dock Face
Survey Lines (Single Beam)
Channel Centerline
Bottom Elevation Surface Chart (Multibeam)
Example #2
• Multibeam Surveys are faster to execute, especially for larger
areas and in the case of busy docks.
• They give 100% bottom coverage (single beam surveys at 50
feet spacing give only 28%!)
• The data can still be sectioned and volumes calculated by the
average end area method but we can now also use a full
terrain modelling approach.
• New system can beam-steer and look up under docks,
moored barges or shallow draft vessels (depending on space
between the hull and bottom)
• Some systems also provide sidescan sonar type imagery from
the amplitude returns (not as good as a specialist sidescan
sonar but no co-location issues)
• More expensive than single beam (30 to 50%)
• Data takes longer to edit and produce final deliverables

similar documents