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Gulf Stream
This is a summary of
relevant sections of the Expedition help file for the Bermuda race and
the Gulfstream.
If you have a bitmap
(.bmp) image of ocean temperatures or currents analysis, you can use
Expedition to create a grib file based on the information in the image.
This could be from Jenifer
Clark or another source. For example, but just save the image as a
bitmap (.bmp) file.
Tip: Select Extend
currents in time in the optimal routing settings.
Importing an image
Expedition allows the
import of Windows bitmap (.bmp) images. The image to be imported has
to have the following properties:
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be in bitmap
format (.bmp),
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use a north up
mercator or regular lat/lon projection,
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preferably using a
256 colour bitmap.
As part of the import
process, the image needs to be geo-referenced.
Open Expedition's
Chart management window and click on the import button.
When importing a .bmp
image, you will be prompted for first guess bounds (the north,
south, east and west limits of the image) and whether the image uses
a mercator or a regular latitude/longitude projection.
Be aware that most
recent Gulf Stream charts and images we have seen use a regular
latitude/longitude projection. This option is only available on
Expedition v5.5.
After the image is
imported, the geo-referencing can be enhanced by selecting
geo-reference from the right-click popup menu, which will allow you
to enter the exact position of the clicked on location. This is very
useful if the gridded area is smaller than the imported image or you
are using a very large image. Expedition requires two
geo-referencing points that should be as far apart as possible on
the chart for best results - for example in the top left and bottom
right or top right and bottom left corners. These user defined
geo-referencing points will be displayed as small blue x symbols on
the chart.
It is a good idea to
delete the 'first guess' geo-referencing points by right-clicking on
them.
At this point, your
image is ready to use. It may be opened later on from Chart
management or from the Right-click pop-up menu.
Gulf stream currents
and Grib files
Grib files of the Gulf
Stream can be obtained from various sources depending on preference,
including Ocens,
Saildocs
(select the RTOFS model) or direct from NOAA. Ocens and Saildocs are
easiest, but you can also get
RTOFS directly from NOAA - you want the files below.
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ofs_atl.t00z.n000.gs.grb.std.grib2
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ofs_atl.t00z.f024.gs.grb.std.grib2
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ofs_atl.t00z.f048.gs.grb.std.grib2
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ofs_atl.t00z.f072.gs.grb.std.grib2
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ofs_atl.t00z.f096.gs.grb.std.grib2
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ofs_atl.t00z.f120.gs.grb.std.grib2
These files are are an
analysis (n000) and 24 hourly forecasts. The forecasts can be very
useful for the Bermuda race as the meanders move so much. Obviously
these highly detailed grib files are too large to download at sea
for most users, but can be downloaded before the start.
Creating grib files in
Expedition
Expedition can be used
to manually create grib files of the Gulf stream currents. However,
with the model grib data discussed above, this is almost redundant.
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Select Create
currents mode from the Analysis menu. This will direct
Expedition to create a current vector wherever you click on the
chart with the left mouse button. It will ask for a value of
current drift. This should be entered in knots.
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Move the cursor
around a line of constant current drift, clicking regularly or
at important points. The current drift will be in the direction
you are moving the cursor, so start upstream.
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You can't
immediately click elsewhere on the chart to start another line
of current points as the first and last points will have
unintended sets. So, click on Current drift rate on the Analysis
menu or click on the Drift rate button on the toolbar and enter
the drift rate of the next series of points you intend to enter
(this could be the same rate) and then click on the chart as
before.
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When all creation
of current points is complete, select Create currents mode from
the Analysis menu to exit the current entry mode. Then, select
Convert created currents to grib from the Analysis menu.
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You will then be
asked for the bounds and resolution of the grib file you wish to
create. The suggested bounds north, south, east and west bounds
will be based on the data you entered earlier. The default
resolution is 0.5 degrees, but 0.1 or 1.0 degrees could be
selected for example. Also, choose whether to give the current
field a validity time (it could be a forecast for example) or
make it a climatologically mean current.
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Finally, select a
file name and location for the new grib file and click on OK.
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You can now clear
the manually entered currents by clicking on Clear created
currents from the Analysis menu and load the new grib file.
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It can also be a
good idea to enter zero knot currents along coastlines.
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