So how much do you trust your GPS - version 2

[john_morris_uk]

On our recent sailing trip to the CI's and N Brittany a couple of incidents made me ask some questions about GPS and how much we have come to rely on it.

The first incident is a fairly well known and common phenomena; we were anchored in the Treguier River and I plotted our GPS position on the chart to find that we were well inland and over a cliff behind a house that we could see. A quick check showed that the chart hadn't been reprinted to the relevant datum and by adding and subtracting a few digits, the plot made sense. Now I know that all the admiralty charts are slowly being released to the standard GPS datum, but this one wasn't and I hadn't noticed. We came up the river in good vis and with half an eye on the buoys, half on the transits and half an eye on the GPS to check where we were. In poor vis, it would have been much more challenging, and I could have got it wrong by not remembering to check the datum.

The second incident was when we were leaving. We had engine problems (a rubber end cap on the heat exchanger was split and its another story) and we were in company with another boat with some friends on board. He didn't have the large scale chart of the area and when we did the passage plan together over dinner the night before he said he would follow me out of a fairly restricted channel. The tide was only just high enough for us to clear some rocks so staying in the channel was critical. Fortunately, the leading marks are fairly clear and we found them and made good progress (with son topping up the engine with coolant every five minutes!) I radioed my friend and suggested he note the transits - he said that he'd rather rely on his GPS. As someone once said, "With a transit I can tell which side of the cockpit I am siting on; GPS is good, but its not that good!"

Finally, some people I know made their first channel crossing a few days ago at the top of the last spring tides. They made a slow crossing - and explained it was because they were sailing on spring tides. "We were down to 1.5 knots over the ground at one point" they said. What they meant was that they sailed the rhumb line from Bembridge to Cherbourg and made sure that they never had any XTE. If they had done some planning and allowed themselves to be swept by the tide (perhaps five or ten miles) off the rhumb line one way and then the other they would have been in HOURS earlier.

I hate sailing without the GPS; its such a brilliant and usually very accurate bit of kit, and I write as one who grew up sailing with no nav aids except compass and log and echo-sounder and actually enjoy nav and chart work.

These incidents in the last week or so reminded me that the thing is an aid to nav - not the answer to all nav problems.

Hey ho...

 

[RIB_imposter]

Nothing wrong with GPS!

So none of those are faults with GPS - they are faults with the interpretation or use of the output data. The same factors apply to all navigation techniques.

 

[lw395]

On our recent sailing trip to the CI's and N Brittany a couple of incidents made me ask some questions about GPS and how much we have come to rely on it.


Finally, some people I know made their first channel crossing a few days ago at the top of the last spring tides. They made a slow crossing - and explained it was because they were sailing on spring tides. "We were down to 1.5 knots over the ground at one point" they said. What they meant was that they sailed the rhumb line from Bembridge to Cherbourg and made sure that they never had any XTE. If they had done some planning and allowed themselves to be swept by the tide (perhaps five or ten miles) off the rhumb line one way and then the other they would have been in HOURS earlier.


These incidents in the last week or so reminded me that the thing is an aid to nav - not the answer to all nav problems.

Hey ho...

I find the rhumb line story a bit depressing, it's not a GPS issue but speaks volumes of the way vectors and such like are badly taught at school and in YM theory classes.

The datum and precision issues are more real, your chart may also have errors, either mistakes or offsets. The GPS is only a postion indicator, whereas the sounder tells you here and now how much water there really is! Now that nearly all my charts are wgs84, I'm probably in danger of doing the same with the remaining older charts, and when I'm tired on a night watch, it's quite easy to put the correction in the wrong direction!

Had some fun with a TomTom in Cornwall recently, they are amazing toys for £100, and the mapping works brilliantly, but I found a complicated junction of lanes in some woodland that really confused it! The tree cover was enough for it to change its mind which way we were going about 8 times in a minute. A useful aid, but the signpost helped too!

 

[Pete7]

These incidents in the last week or so reminded me that the thing is an aid to nav - not the answer to all nav problems. Hey ho...

Trust GPS implicitly, all navigation and pilotage carried out at the wheel. Paper charts now used to mop up spills when using the chart table for food prep. Quite happy with twin chart plotters for all navigation with the occasional log entry, perhaps 3 for a channel crossing, start details, a plot half way and final entry with arrival details.

Pete

 

[stav]

I trust the gps completely provided the visuals add up. But clearly it has its limitations.

Datums are one but the sort of thing you only do once!

What was interesting this year going from Lezardruier to Treguier through the rocks was the CP followed the first transit exactly but going down the Passe De La Gain in to Treguier the CP was showing itself to be at least 100ft of the transit which would cause trouble on this route (especially at LW). Interestly I had the opportunity to use this passage twice more this year and the same was repeated! I suspect the limitations of the cartography are showing up.

As for reliablity (it is a standard harizon CP180) it has been very good but during a very wet passage (most of them this summer) it failed due to water ingress. Thank crumbs for paper charts which I use with a yeoman plotter linked to a fruno GP32. After taking it apart and drying out the few spots of water it is working again except for the on/off switch so just plug in the pwr lead as necessary.

Additionally, when we entered Plymouth no charts showed up on the chart plotter???? Again paper charts to the rescue.

In conclusion love CPs (especially when single handed) but would not go to sea with out some paper back up even if not fully updated. And obviously I am bias but very happy with my set up as it lets me do things I would not have attempted using traditional skills.

I do my planning on paper charts but recently bought the PC p[lanner for the laptop and have used that a few times and seems very good, just need to amend my habits but still concerned about power consumption on my old sailing boat.

 

[kingfisher]

Blindly. Litteraly. I've had to do two forced night sailing trips with zero visibility:
- anchorage in Turkey that turned nasty when the locals decided to move in on the girls aggresively and their partners responded similarly, resulting in some shoving and pushing. They threatened to return with the police (lol: half the crew had diplomatic pasports), but the owner of the taverna said it was best to leave as they might cause more trouble in the night. So we moved to a bay accros the other side of Fethiye gulf in pitch dark. I only saw the shoreline when we were about 40 meters away.

- this summer we wanted to take a mooring in a marked but unlit side channel on the Oosterscheld, as it started to drizzle, and the wind started pickin up with spray. Then when we arrived at the moorings, all three were taken, and both them and the other two boats were rolling heavily. We quickly decided to turn tails and head on for another two hours, arriving in Zierikzee at 0200. All was done in low visibility conditions (drizzle + spray) purely on GPS, with the odd lit bouy here and there for reference.

Without the GPS, I would have
1) set up a watch detail
2) not even considered going into the unlit side channel after 2100hrs

But I am aware that the maps on my plotter are 5 years old, that the plotter doesnt show other boats, etc etc, but sometimes its all you have.

 

[Refueler]

This is the crux of the matter IMHO .... we are all doing things with our boats that not so long ago we would have shied away from.

Lets say you are crossing a bay or bit of sea ... thick fog descends as you approach entrance to destination. Before GPS, if you couldn't see and were unsure of the place - maybe first time there .... many would have anchored or hung off till viz improved. Today ? Most would use GPS to go in.

Thoughts of distances and other ports / harbours. Before GPS - many would have restricted their passage making if they didn;t have Decca or other aid. Today GPS is so common place it's unusual to find a boat without some form of GPS, and the cruising grounds extended because of its assistance.

I can honestly say that the only 'reverse' matter is boat size, where small boats in past travelled - today people tend to look on them as "you DID THAT !" .... I had people say that to me after crossing baltic in heavy 25ft motor-sailer ....

 

[john_morris_uk]

I get the idea that some might have missed the point that I was trying to make. GPS is fantastic. Its reliable and usually very accurate, but its only as accurate as the chart data that you have in your plotter or on your paper chart (and the datum that you work to) and its not a panacea for slapdash nav.

Once upon a time most yachtsmen and women wouldn't have dreamed of doing some of the things that we all do nowadays - its easy to get blase and forget that some common sense and some healthy navigators skepticism needs to be applied to what we do at all times.

It can also make us lazy and produce poor planning and slow passages.

 

[lw395]

It can also make us lazy and produce poor planning and slow passages.

Without it your friends who did not understand how to shape a course across a tide may never have got there. So it probably produces slow passages instead of failed passages. Before GPS, such people would probably end up turning around and giving up.

It does encourage us to be lazy, but it allows us to do a lot more. It is no substitute for pilotage or understanding tides, vectors and planning though.

 

[Refueler]

I don't consider I missed your point in fact I was adding to it ... and your later seems to agree with my view ..

 

[Sailfree]

GPS

A few years ago I sailed up the river to Pontrieux - a bit worrying scrapping our keel in the silt most of the way (2.2m draught) and knowing the short window to make the lock.

Did detailed passage planning as also had to get under bridge earlier on (triegier?) What worried me was had latest platinum navionics chip in chartplotter and we consistently sailed about 500m inland in the granite cliffs.

Made me very unhappy about relying on GPS especially remembering relying on it exclusively rounding the SE corner of Jersey a few years before with rocks either side of channel.

I now like visual checks as well.

 

[Oldhand]

Datum Error Avoidance

There is a way to avoid this problem when navigating on GPS:

1. Always have preplotted waypoints from the charts (papar) you are going to use
2. When plotting waypoints from a chart that is not WGS84 datum, include the datum in the waypoint name
3, When navigating towards a waypoint that is not WGS84, change the GPS datum according to the datum included in the waypoint name.

Method had never failed me.

 

[srm]

Trust GPS implicitly, all navigation and pilotage carried out at the wheel.

Just like a local fishing boat a few years back, then one night following his normal track through a narrow gap between unlit rocky skerry and steep too beach in the dark he came to a dead stop - on the beach . . . . .

 

[blackdogsailing]

I often get problems mid-atlantic with the gps positioning. It's always about 5 miles out from my sextant derived position. It's rarely exactly the same
Chris

 

[AntarcticPilot]

There is a way to avoid this problem when navigating on GPS:

1. Always have preplotted waypoints from the charts (papar) you are going to use
2. When plotting waypoints from a chart that is not WGS84 datum, include the datum in the waypoint name
3, When navigating towards a waypoint that is not WGS84, change the GPS datum according to the datum included in the waypoint name.

Method had never failed me.

Alternatively, convert all waypoint locations to WGS84. Within the bounds and accuracy of an inshore chart, it will be a constant offset. However, determining the correct offset may be fun! For charts on the old OS datum, which is likely for most users of this forum , there are tools on the OS web-site, I think (I have access to software to do that sort of thing, so I don't know for certain).

And I will repeat what others have said - don't assume your chart is as accurate as the GPS! It is a point I have to keep re-iterating in the context of my day job (mapping Antarctica...)

The mark one eyeball is still far and away the most useful navigational instrument in close quarters. And it is mass produced by unskilled labour!

 

[bilbobaggins]

.....And it is mass produced by unskilled labour!

"I can do that! Gissajob!"

 

[fishermantwo]

And I will repeat what others have said - don't assume your chart is as accurate as the GPS! It is a point I have to keep re-iterating in the context of my day job (mapping Antarctica...)

The mark one eyeball is still far and away the most useful navigational instrument in close quarters. And it is mass produced by unskilled labour!

The charts i use with my plotter are spot on at sea. However in rivers, ports etc they are often miles out. Most charts locally are updated version s of Captain Cooks and they never bothered much with rivers etc.

 

[LadyInBed]

Leaving Roscoff inner harbour I did note that the chart plotter was telling me that I was motoring along the breakwater
But as you say, Hey ho... it is an 'aid' to navigation

 

[srm]

And I will repeat what others have said - don't assume your chart is as accurate as the GPS!

Actually most charts are so accurate that you will not be able to measure the errors on the chart - provided that you understand that this is an internal acuracy within the chart. Coastal charts were produced by firstly setting up a carefully measured set of reference points on the land. Coastline and offshore detail was then fixed from the shore reference points.

The problem was then relating the shore grid and thus the chart to the lat and long grid. Lots of problems as different countries used slightly different grids, as was discovered when bomber command started flying on radio systems with transmitters in UK and targets in Germany, and again by the offshore industry when trying to decide where national median lines actually were in relation to the new gas and later oil fields in the North Sea.

GPS came along and uses yet another set of lat and long for reference or for the technical bods a different datum.

Which leads me to the point of this ramble - if there is a conflict between gps and your chart forget gps and navigate by measuring from the land. Thats the way the chart was surveyed.

 

[john_morris_uk]

Actually most charts are so accurate that you will not be able to measure the errors on the chart - provided that you understand that this is an internal acuracy within the chart. Coastal charts were produced by firstly setting up a carefully measured set of reference points on the land. Coastline and offshore detail was then fixed from the shore reference points.

The problem was then relating the shore grid and thus the chart to the lat and long grid. Lots of problems as different countries used slightly different grids, as was discovered when bomber command started flying on radio systems with transmitters in UK and targets in Germany, and again by the offshore industry when trying to decide where national median lines actually were in relation to the new gas and later oil fields in the North Sea.

GPS came along and uses yet another set of lat and long for reference or for the technical bods a different datum.

Which leads me to the point of this ramble - if there is a conflict between gps and your chart forget gps and navigate by measuring from the land. Thats the way the chart was surveyed.

I don't have a problem with why the inaccuracies and discrepancies exist. I used to work with the UKHO and I am always quick to defend the care with which charting is done. Absolute positioning always requires an established datum, and it wasn't until the advent of GPS and other modern fixing aids that many of the problems needed to be addressed. Each country had its own datum and we fudged the discrepancy when we moved from one country to another.

I think that the main conclusion that I have come to is that GPS and our reliance on it can produce some lazy navigators who don't produce effective and efficient passage plans. They also get lazy about applying basic pilotage skills. A GPS is not always the best pilotage aid.