GPS:A stupid question

[rgleason]

A GPS position used as a waypoint to goto.
Is it using a great circle route?
Depends on device/ program?
Opencpn?
Chartplotter?
Gps handheld?

[bdbcat]

Rick...

When you "Navigate to" a waypoint from OpenCPN, he does the following:

1. Create a temporary route from current position to selected point. The route has only two points, start and end.
2. Activate that route.
3. If a suitable output connection is available, he sends the following NMEA messages periodically to that port.
- ECRMB
- ECRMC
- ECAPB
- ECXTE.

4. The Cross-track error (XTE) output by OCPN is calculated as for Rhumb Line Sailing, i.e. straight line on Mercator chart.

If an autopilot is steering the boat, it may choose to follow GC route if it wants to. The current GPS position and the target waypoint position are available in the messages sent above.

If the A/P uses only XTE, then it is sailing Rhumb Line.
Alternatively, if it ignores XTE, and computes a running GC course based on the target waypoint location and current ownship position, then it is doing GC Sailing.

If the A/P is doing GC Sailing, then you would see the XTE reported by OpenCPN diverging from zero, as expected.

Of course, other chart plotters may do this differently from OpenCPN.

Personally, I wonder why we care. Rare indeed would be the case in which a two-point route long enough to benefit from GC Sailing would be traveled without need to change course for other tactical reasons (weather, traffic, etc.). Could happen with a large motor vessel, I suppose...

Hope this helps
Dave

[rgleason]

That's really good answer. Thank you.I expect other chartplotters and handhelds do rhumb line mercator too.

[Snowpetrel]

The old (very old...like the 12's) garmin units only did great circle. Can be dodgy over long e-w courses if you aren't aware of it. Most of the later units seem to default to rhumb line, and the more expensive ones simetimes give you an option.

Easy to check. Run two waypoints at say 45 S on different longitudes more than a few degrees apart. If the course between them is exactly 090t or 270t it is Rhumb line. If it is more than 090 or less than 270 it is calculating in great circle.

[Tupaia]

Originally Posted by Snowpetrel

The old (very old...like the 12's) garmin units only did great circle. Can be dodgy over long e-w courses if you aren't aware of it. Most of the later units seem to default to rhumb line, and the more expensive ones simetimes give you an option.

The Garmin web site Glossary indicates they still operate on GC.

https://www8.garmin.com/aboutGPS/glossary.html

See under "Distance"

[Snowpetrel]

Originally Posted by Tupaia

The Garmin web site Glossary indicates they still operate on GC.

https://www8.garmin.com/aboutGPS/glossary.html

See under "Distance"

Interesting. Can't say I have used a recent garmin unit. The Simrad I have been using gives me an option, which is nice. I really don't like using great circle.

[Tupaia]

Originally Posted by Snowpetrel

I really don't like using great circle.

What is your reasoning for disliking one over the other?

[StuM]

Originally Posted by Tupaia

What is your reasoning for disliking one over the other?

You have to keep changing heading

[Snowpetrel]

Originally Posted by Tupaia

What is your reasoning for disliking one over the other?

If I draw a route on any normal paper chart it is a rhumb line. It's nice if my plotter uses the same rhumb lines so everything matches, such as XTE, courses and distances.

And yeah, the constantly changing course is annoying.

[rgleason]

FAQ

GPS: A stupid question. When a GPS position is used as a waypoint to goto, is it using a great circle route?


Thanks guys.

[seandepagnier]

The autopilot route plugin replaces the builtin logic and uses great circles as well as many other "improvements"

It seems some people would prefer not to sail in the shortest path to follow a route to keep a constant heading, maybe it's worth having an option for it.

[rgleason]

Sean, we'll add that. Info for a more balance. Since some like getting there sooner.