Page 35
Copyright Wet Paper Publications 2014
Tidal
flows
Anchorage
Reef
Sand
Soundings
Figure 35.3 Tidal terms of reference to a chart datum
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Figure 35.4 Research I
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Bearings
Anchorages and island topography
A safe anchorage is necessary for all work in a research site.
Figure 35,1 shows that Carsile and Brampton Islands are
protected from the wind and waves on any part of the island.
Internet maps
Some of the features that need to be considered when selecting
a offshore research site are facilities such as fresh water,
camping, chart features, anchorages, tides, currents.
Internet maps will also show some features on a chart. For
example, look at the south eastern tip of Brampton Island.
Goggle maps will show areas of sand, reef hills and rocks as
well as some information on water depth.
Compare these with the chart features,
Admiralty chart booklets have the definitions of many more
chart features.
Soundings and tidal datum
This is the depth of water calculated frommean lowwatermark.
• Locate. Devereux Rock South East of Carlisle Is. Notice at
mean low water it dries at tide height of 4.5 metres.
• Notice that aroundCarlisle Is, there are reefs, rocks and sandy
beaches and that the depth of water varies around the island
• Near Edgell rock there is information about the speed of the
current caused by a tidal flow.
Chart tidal datum
Chart datum is the level of water that charted depths on a
nautical chart are measured from.
Figure 35.2 shows that the datum is derived from the lowest
point of mean low waster springs.
Chart block
• At the top of the chart is the chart block which identifies the
chart features. Notice
- Depth of water is marked by soundings and these are in
metres and these are reduced to mean low water spring.
Note: Many chart blocks use lowest astronomical tide as
the datum.
- Places are located by Latitude and Longitude.
- Positions are related to world geodetic system 1984 datum
(WGS 84)
Sand
Reef
Rocks
Reef
Rocks
Hill 457
Sand
Figure 35.1 Chart features and soundings as seen on chart
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Figure 35.2 Chart features and soundings as seen on internet
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Brampton Island
1,2,3,4,5,6,7,8 9