I just installed OpenCPN
and am impressed with the work
you folks have done. I have used electronic charting software
since 1999 during tens of thousands of miles from Alaska
to southern Mexico
to the BVI
However, I am confused about Magnetic courses and bearings in OpenCPN
4.0.0 and WMM 1.0 (with the 2015-2020 wmm.cof installed). When I use the OpenCPN measure tool to calculate a Magnetic
bearing, heading, or course the bearing I see is incorrect and often dangerously wrong compared to using a paper chart and the Magnetic Compass
I tested this using Measured Nautical Mile courses on many charts
with a variety of magnetic variations. I also compared measured OpenCPN Magnetic bearings with those calculated by Visual Suite using the same chart.
My computer is NOT connected to a GPS
during this testing which is the normal case when I am doing route
I have read the entire WMM thread and find nothing about this issue. If this issue is known and has been discussed – please point me in the right direction.
When I display the extended WMM information I see that the BOAT
variation is INCORRECT (for the current
chart) but the cursor information is CORRECT (for the current
chart). However, the displayed bearings and course info is based, (apparently?), on the boat
The reason for the erroneous Magnetic
course, heading, or bearing is that my boat was not placed anywhere on a chart and WMM uses (apparently because I just installed it) 15° E as a default. In my first test case the variance should have been 11° 46’ E but the WMM icon showed 15.0.
When I move the boat onto the chart the variance is then measured and displayed correctly and Magnetic bearings, headings, courses are calculated correctly.
When the CURSOR (but not the boat) is moved to a chart with a different variance, then the measured course and bearing on EVERY chart is calculated with the variance where the boat is located – NOT based on the current chart!
This makes the software
less than useful for planning.
Why in the world would you knowingly use the wrong magnetic variance? The chart data contains the variance appropriate for the chart.
The WMM documentation
does not state that the boat must be on the current chart to display accurate magnetic information.
The OpenCPN documentation
says only to install WMM for magnetic information.
I think the requirement that the boat be on the chart in use should be documented in BOLD RED