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Old 01-02-2011, 08:20   #16
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Coastal Explorer has feature called range/bearing lines. One can place the end of line on the chart feature and plot a line from it. Of course one must use a reciprocal bearing to the one one gets from their compass. Plot a couple of these on your chart and you get an independent position from the one you get from your GPS.
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Old 01-02-2011, 08:33   #17
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Thanks for all your suggestions and help. What would have loved to see in a nav software package is what MaxSea apparently did in it's previous version (before Time Zero). Here is an excerpt from the manual:

Calculate fix by two bearings
Using the Bearing Tool or right-click any where on the chart and select the "Bearing tool" in the "Select tool" submenu.
If you have used a compass to take the bearing of two seamarks, drag the bearing line from the first seamark (click, drag and release) and then do the same to set the bearing line of the second seamark.
Select [Calculate fix] from the [Route] menu to obtain as before, the point of the bearing which is recorded in the active layer.
Note:
(1) - it is not necessary to lengthen bearing lines to make the intersection, MaxSea will perform this function.
(2) - instead of sliding the cursor over the chart you can click the seamark and then use the keyboard to enter the bearing in the dialog box which opens.
.
Calculate fix by three bearings
Using the Bearing Tool or right-click any where on the chart and select the "Bearing tool" in the "Select tool" submenu.
Proceed exactly as explained in the above paragraph to Calculate fix by two bearings , but trace three bearing lines instead of two.
After selecting [Calculate fix] from the [Route] menu, the software displays the point of theoretical calculation as well as the triangle of accuracy.
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Old 01-02-2011, 08:47   #18
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Quote:
Originally Posted by Bill Shuman View Post
Calculate fix by two bearings
Using the Bearing Tool or right-click any where on the chart and select the "Bearing tool" in the "Select tool" submenu.
If you have used a compass to take the bearing of two seamarks, drag....
That would be a fantastic tool to have in OpenCPN....
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Old 01-02-2011, 08:49   #19
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Here is a quick example.

Bearings are :
- 340 to the radio mast
- 270 to green flashing
- 60 to bridge
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Old 01-02-2011, 08:59   #20
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If anyone is interested, here is the full MaxSea manual section on DR tools. It's much more than just plotting a fix.
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Old 01-02-2011, 15:10   #21
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<< coefficients are periodically updated - the most recent update actually was done mid-2010 and covers the next 10 years. >>

According to the NGDC site, models need to be revised at least every 5 years. A new IGRF model is adopted every 5 years.

"new observations must continually be made and models generated to accurately represent the magnetic field as it is."

This is because the changes are not incremental and there may be local magnetic anomalies or disturbances.

So when the GPS dies, the best source of mag var is the local chart, provided that it is up to date, if not, it's a good idea to have a copy of Chart 42 (Magnetic Variation) for the current epoch. The errors of plotting with the wrong mag var can be surprisingly big.

Hopefully the alert navigator will pay attention to the local variation from the GPS before it dies, at the very least to see how much it differs from the chart.

Happy sailing
Pelican Pete
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