're VMG, What's XTE?

[Vegable]

Following on from the VMG question, (http://www.ybw.com/forums/showthread.php?489814-Could-someone-explain-vmg-to-me exactly)
What is Cross Track Error? Is it just the angle you're drifting away from your waypoint/destination point whilst you're trying to tack up to it?
It's something I've never used or bothered with.....perhaps I should?
Mike

 

[RupertW]

It's the distance from your planned route - so shows whether you are covering more ground than you intended.

Very useful if you are in tightish waters with obstacles but no chart plotter, less useful if you are crossing a changing tidal stream and want to do the shortest distance through the water whilst describing a curve over the ground

 

[lw395]

XTE is how far you are away from a Great circle joining where your GPS thinks you started from and where it thinks you are going.

It can be useful. For instance....
If you are sailing from Portsmouth to Cherbourg, the XTE should show how the tide has taken you off track initially, or how much 'up tide' you are, as an allowance for the tide at the end of the journey. Or you might want to know you are a comfortable distance upwind of the straight line, in case the wind shifts.
Or you might want to stay within a certan XTE range to be clear of hazards.

 

[RupertW]

XTE is how far you are away from a Great circle joining where your GPS thinks you started from and where it thinks you are going.

.

Not a definition I've ever seen or experienced on any equipment. True if you only have two waypoints, start and finish, but in all other circumstances it's the shortest distance between you now and the route you plotted between your last waypoint and the next one.

 

[GHA]

Not a definition I've ever seen or experienced on any equipment. True if you only have two waypoints, start and finish, but in all other circumstances it's the shortest distance between you now and the route you plotted between your last waypoint and the next one.

Not necessarily great circle either, probably not.

As you say, most kit it will be the distance you are away from a line between the active waypoint and the previous one when following a route.

 

[macd]

I don't think it's particularly helpful to talk in terms of 'last waypoint'. There may not even have been one. XTE is your horizontal distance from the track created when the next waypoint was selected as a 'go to'. (Which may be part of a route, or not: that's irrelevant. Our plotter accepts 'go to cursor', which sets the cursor's then position as a volatile waypoint but does not save it beyond the duration of that particular instruction.) On most systems, the track would be a great circle, although on the scale of most navigation, that's scarcely significant.

 

[lw395]

Not necessarily great circle either, probably not.

As you say, most kit it will be the distance you are away from a line between the active waypoint and the previous one when following a route.

.....]

I dare say differences exist from one GPS to another.
I have a HH GPS I've used for many years, this does not require a route to be set in order to show XTE, merely a 'goto waypoint'.

It's a good point to understand how your particular navigator deals with XTE on a route.
Very often on a route, you will not exactly get to a waypoint. So if you pass half a mile abeam of the waypoint and switch to the next one, does the xte start at 0 or 0.5mi as you start to GOTO the next waypoint?
ISTR my old Magellan could do Great Circle.

 

[duncan99210]

Cross track error can only be displayed when your navigation kit has been told to go to a point (either a standalone waypoint or a waypoint as part of a route). When your plotter/gps has been told to go to a waypoint, it will assume that you're going to steer a course in a straight line between where you were when you gave it the instruction and the waypoint. As far as the plotter/gps is concerned, that's the "track" you should be following to get to your destination.
If you don't stick to that "track" there will an error in your course (as far as the machine is concerned): the distance away you are at that instant is your cross track error (displayed as a distance left or right of the track). You can reduce the cross track error by making a turn towards your destination. Or (for most plotters/gps) you can press a button and restart the track, reducing the cross track error at that instant to zero.
I use it mainly as a tool for tacking when making long passages: allow the cross track error to build to say a few miles, the tack and wait until the error rebuilds to a similar value but on the opposite side of the track, the tack again and so on....

 

[johnalison]

I don't use the function much, but, as Duncan says, it is just the distance from the track that you have set to your boat's position. I only normally check it when wanting to engage the 'track' function on my autopilot, which only works at < 0.1 nm. Occasionally, I put it on when doing a cross-tide trip such as the Channel or North Sea, to see how far off-track we have drifted.

 

[Uricanejack]

It's a good one. Terminology I never heard when learning navigation. New or just new to me. I don't know. I quite like it though. It sound like what it is.
Context is the important part. An error is no longer a problem when you know what it is
Useful. Sometimes
Other time not so much.
I find when sailing not so much. I tend to be going back and forth across my track to much

 

[Seajet]

Cross Track Error display is very useful, mostly just for checking but as mentioned can be vital in narrow passages.

On the classic example of Portsmouth - Cherbourg the fastest crossing is to keep on the same planned course ( hopefully arriving at Cherbourg up-tide of the entrance ) - in reality the boat will describe a sine wave either side of the course line as the tides pull her one way then the other.

If one keeps eyes glued to the Cross Track Error and keeps adjusting steering angle to keep right on the overall course line, it will be a slightly longer / slower passage but may be handy if there are hazards to either side of the perfect line.

 

[Vegable]

Thanks for all your replies and thoughts about its usefulness, much appreciated.
Mike

 

[Dan Tribe]

Cross Track Error display is very useful, mostly just for checking but as mentioned can be vital in narrow passages.

On the classic example of Portsmouth - Cherbourg the fastest crossing is to keep on the same planned course ( hopefully arriving at Cherbourg up-tide of the entrance ) - in reality the boat will describe a sine wave either side of the course line as the tides pull her one way then the other.

If one keeps eyes glued to the Cross Track Error and keeps adjusting steering angle to keep right on the overall course line, it will be a slightly longer / slower passage but may be handy if there are hazards to either side of the perfect line.

I once helped bring a boat back from Denmark. The skipper was fixated by XTE.
I was steering a compass course and after 1/4 hr the skipper complained that I was off course. I explained that when the tide turned it would bring us back on course. He seemed satisfied with that but an hour later complained that I was even further off course.
So we spent the rest of the trip steering into the flood, then into the ebb. Must have added miles to the trip but the skipper was happy!

 

[johnalison]

Mention of narrow passages reminds me of one such place where XTE is useful. Approaching St Helier from one or two of the charted channels often requires the sailor to identify a transit and keep to it. Setting a track along the transit can be useful to confirm that the right marks have been chosen, and the XTE especially useful in poor visibilty to show that one is following the track, though some may prefer a rolling road. In good visibilty, it will usually be more accurate to steer to the visible marks.

 

[C08]

I have never understood why VMG is inextricably linked to wind direction (at least as far as my old plotter is concerned). The forward component of speed over the ground directly towards a waypoint is also a useful measure of real progress, regardless of anything to do with wind, unless there is some other measure I have not come across?

 

[johnalison]

I have never understood why VMG is inextricably linked to wind direction (at least as far as my old plotter is concerned). The forward component of speed over the ground directly towards a waypoint is also a useful measure of real progress, regardless of anything to do with wind, unless there is some other measure I have not come across?

I suggest that you go back and read some of the discussions about it, such as: http://www.ybw.com/forums/showthread.php?489814-Could-someone-explain-vmg-to-me

Basically, there are two VMGs which have appeared out of the mists.
1, the old one, around for over 40 years and so pre GPS. It merely combines water speed with wind data to produce speed towards the wind
2 the new interloper, invented by thickos at the electronics firms who have never been on a boat. It uses GPS data only to show speed towards the waypoint. The aforementioned idiots couldn't think of a new name for it so called it VMG as well, thus muddying the waters and creating havoc in sailing forums.

 

[Lucky Duck]

Bit like ground wind and true wind!

 

[GHA]

So saying VMC is velocity made good to a waypoint and VMG is velocity made good to windward - why bother with VMG?

Assuming you want to get there as quick as possible.

And where do you get VMG? Do the more clever wind instruments give you this?

 

[RupertW]

So saying VMC is velocity made good to a waypoint and VMG is velocity made good to windward - why bother with VMG?

Assuming you want to get there as quick as possible.

And where do you get VMG? Do the more clever wind instruments give you this?

Any instrument that gives apparent wind and water speed provides VMG to windward. I actually use GPS bases VMG to waypoint a lot more as that tells me my making tack and/or my time to go if not beating.

 

[ghostlymoron]

Is your letter 'n' not working?

Cross track error can only be displayed when your navigation kit has been told to go to a point (either a standalone waypoint or a waypoint as part of a route). When your plotter/gps has been told to go to a waypoint, it will assume that you're going to steer a course in a straight line between where you were when you gave it the instruction and the waypoint. As far as the plotter/gps is concerned, that's the "track" you should be following to get to your destination.
If you don't stick to that "track" there will an error in your course (as far as the machine is concerned): the distance away you are at that instant is your cross track error (displayed as a distance left or right of the track). You can reduce the cross track error by making a turn towards your destination. Or (for most plotters/gps) you can press a button and restart the track, reducing the cross track error at that instant to zero.
I use it mainly as a tool for tacking when making long passages: allow the cross track error to build to say a few miles, the tack and wait until the error rebuilds to a similar value but on the opposite side of the track, the tack again and so on....