Tuesday, August 20, 2013

"Bullet-proof" world wide navigation Using GE, GE2KAP, and OpenCPN

If you wish to sail the high seas, you need to know where you are going, so you need a map, or in landlubbers terms, a chart.

Firstly, the only thing that comes close to being "bullet-proof" when it comes to navigation are your eyes on a good lookout but the following process of navigating in a boat using readily available tools, is a pretty good.
 
Synopsis
Many expensive or free charts available to sailors are incorrect and even if you think your charts are accurate, we should not ASSUME that they are. Double check their accuracy. The only maps that seem to be right 100% of the time are Google maps available on GE - Google Earth.
Even if you have seemingly accurate charts, it is important to check them against GE before setting out.
The consequences of making a mistake in navigation can be life threatening so it is important that you get it right.

The acronyms GE, GE2KAP, and OpenCPN suggest a topic that is quite complicated and each have their own online explanations of use.
GE at http://www.google.com/earth/index.html
OpenCPN at http://opencpn.org/ocpn/
GE2KAP at http://www.gdayii.ca/

Example
Approach to Alice Town (North Bimini) in the Bahamas.
If you load the official, freely available charts from NOAA you will find the latitude and longitude measurements do not coincide with those of GE.
Which are correct? GE or NOAA? A check of http://www.noonsite.com confirms that the following waypoints specify the approach to Bimini
Outside: N 25:42.658, W 079:18.459
Turn: N 25:42.681, W 079:18.261
Inside: N 25:42.830, W 079:18.158
These appear to be correct on GE but not on NOAA. The third inside way-point is even on land for the NOAA charts.
So what to do? You cannot use GE as a chart plotter and the internet is not available in the middle of the ocean. The solution is to use OpenCPN and offset the incorrect chart using GE2KAP.

Steps to take
First overlay the bad chart into GE using GE2KAP creating a new chart.
Adjust the transparency slider setting and move the bad chart using the green cross-hairs to line up with the correct GE map.
Move the transparency slider back to the right and select OK to complete the process.
Once the process finishes, make a note of the offset that is displayed.
In OpenCPN, select offset by right clicking on the bad chart and entering the offset.

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