GPS Terminology

[Little Dorrit]

I bought a second hand basic garmin GPS (76S) the use manual does not make it very clear how the Heading and Track are worked out.

I am a little confused as how the GPS can differentiate between the two; I assume that without any tidal information they would be the same all the time on my basic GPS.

Is this right?

 

[mcframe]

I bought a second hand basic garmin GPS (76S) the use manual does not make it very clear how the Heading and Track are worked out.

I am a little confused as how the GPS can differentiate between the two; I assume that without any tidal information they would be the same all the time on my basic GPS.

Is this right?

Correct.

Track is between where you've been and where you are (over the ground) ; heading is just track projected forwards (Assuming GPS with no fluxgate compass). Garmin went through a phase of confusing "Heading", "Track" & "Course" with "Bearing to waypoint".

On more marine-oriented GPS, getting your "Course Over Ground" (track) pretty close to "Bearing to waypoint" (bearing in some Garmin-speak) allows for leeway, but not tide.

Forgot to mention - on some Garmins (GPSMap 65) "Course" is always ( & stays at) "Bearing from start of nav to waypoint" which is fine if your orienteering, but not much use when you add leeway & tide. It bears *no* resemblance to how to get to where you want want to be from where you are *now*, IYSWIM.

 

[Little Dorrit]

Hi and thanks for clarifying this. I thought perhaps I was going a bit mad as the User Manual is very confusing.

I assume the track and heading are updated in real time on the last two positions?

 

[theoldsalt]

Not true if you are sailing as the boat will make leeway on many points of sailing equivalent to a tide. Some boats will make leeway even when not sailing (eg shallow draught motor vessel)

 

[johnalison]

On more marine-oriented GPS, getting your "Course Over Ground" (track) pretty close to "Bearing to waypoint" (bearing in some Garmin-speak) allows for leeway, but not tide.

I don't understand that. If your COG is the same as your Bearing to Waypoint, you will reach your waypoint, regardless of leeway or tide (though will not predict future changes of tide).

 

[alahol2]

If your COG is the same as your Bearing to Waypoint, you will reach your waypoint, regardless of leeway or tide (though will not predict future changes of tide).

Agreed.

 

[mcframe]

I don't understand that. If your COG is the same as your Bearing to Waypoint, you will reach your waypoint, regardless of leeway or tide (though will not predict future changes of tide).

Short term, yes - that's why I qualified it with "pretty close" - the Garmin abuse of terminology fair did my head a couple of years ago - then I got fed up moving GPS from car to boat and fitted an SH 'plotter.

Cog & XTE can be handy to ensure that you are upwind/uptide of your target.

q.v. sailing round Prince Consort buoy holding a constant heading.

 

[timbartlett]

I bought a second hand basic garmin GPS (76S) the use manual does not make it very clear how the Heading and Track are worked out.

I am a little confused as how the GPS can differentiate between the two; I assume that without any tidal information they would be the same all the time on my basic GPS.

Is this right?

Garmin are excellent products in many ways, but their use of navigational terminology has always been sloppy to the point of being misleading.
No "ordinary" (single antenna*) GPS is capable of calculating your heading or your course: all it can do is calculate your average track angle ("CoG")over some (sometimes unspecified) period of time.
In Garmin-speak the so-called "heading" is really your track angle. The so-called "course" is the "straight line" between two waypoints (they don't specify whether this is rhumb line or great circle, but I believe it is great circle)

* GPS compasses do exist. They really can measure heading to a very high degree of accuracy and reliablity, but they do it by using two or three antennae, and measuring the phase difference between the GPS carrier waves received at each antenna.