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  1. #61
    Join Date
    Mar 2007
    Location
    Atlantic
    Posts
    19,708

    Default Re: What did PhillM do wrong?

    The whole point of stop and anchor is that it's easy. Works for me.....

  2. #62
    Join Date
    Sep 2010
    Location
    Where life is good
    Posts
    13,367

    Default Re: What did PhillM do wrong?

    ..
    Ain't necessarily so..
    On the East Coast it is always shallow so your anchor can do something.

    On the West Coast of is often very deep or too deep to drop the hook ... then hard and dry in a sharp hurry.
    And given that context, I can understand why PhillM wasn't looking at the echo sounder all the time.
    Life is too short to drink bad wine.

  3. #63
    Join Date
    May 2003
    Location
    SPAIN,Galicia
    Posts
    12,450

    Default Re: What did PhillM do wrong?

    Was off the Galician coast with no engine ,no wind no e ho sounder in thick fog,so I anchored using all the rope I had attached to the second anchour.........morning and the fog cleared and we where about a mile off the Cies islands,anchoring works!

  4. #64
    Join Date
    Sep 2010
    Location
    Where life is good
    Posts
    13,367

    Default Re: What did PhillM do wrong?

    Fair point..

    But consider how long it takes to deploy every rope on the boat? Time and more to go bump whilst sorting them out perchance?
    Life is too short to drink bad wine.

  5. #65
    Join Date
    Mar 2010
    Location
    Ammersee, Bavaria
    Posts
    4,677

    Default Re: What did PhillM do wrong?

    Quote Originally Posted by lw395 View Post
    ....

    GPS is not 100% reliable, just the other day I was driving along a straight road in fine conditions and it started telling me I was 200m away on a parallel farm track.
    GPS in car navigation systems don't work like they do on boats. Vehicle GPS improves accuracy by using a map-matching algorithm. As it's a car it assumes you will be on a road, so it starts from a GPS position, collects track information from the vehicle odometry (gyroscopes, wheel speed etc.) to create a track and then goes to the point on the map indicated by the GPS and tries to match the recorded track to a segment of digitised road. This is then weighted to ensure you don't skip backward and forwards from one road to a parallel road but it does still on occasion happen if the fix becomes less accurate. What you will probably find is that the GPS 'track' was out by a few metres but was consistent, but the track from the vehicle odometry matched the parallel road geometry much better so the icon jumped.

    Another problem with vehicle GPS compared to boats is reflected signals from buildings which degrades the GPS accuracy in heavily built up areas.

    The vehicle odometry and map-matching is the reason why an OEM navigation system will track you accurately in a tunnel whereas a TomTom, Garmin or Bolt-On navi will not be as good.
    The best argument against democracy is a five-minute conversation with the average voter.

  6. #66
    Join Date
    May 2003
    Location
    SPAIN,Galicia
    Posts
    12,450

    Default Re: What did PhillM do wrong?

    Quote Originally Posted by Tomahawk View Post
    Fair point..

    But consider how long it takes to deploy every rope on the boat? Time and more to go bump whilst sorting them out perchance?
    Some where on reels and others hanked,oI kept a tight ship!,

  7. #67
    Join Date
    Nov 2009
    Location
    Home UK Midlands / Boat Croatia
    Posts
    18,880

    Default Re: What did PhillM do wrong?

    Quote Originally Posted by Baggywrinkle View Post
    GPS in car navigation systems don't work like they do on boats. Vehicle GPS improves accuracy by using a map-matching algorithm. As it's a car it assumes you will be on a road, so it starts from a GPS position, collects track information from the vehicle odometry (gyroscopes, wheel speed etc.) to create a track and then goes to the point on the map indicated by the GPS and tries to match the recorded track to a segment of digitised road. This is then weighted to ensure you don't skip backward and forwards from one road to a parallel road but it does still on occasion happen if the fix becomes less accurate. What you will probably find is that the GPS 'track' was out by a few metres but was consistent, but the track from the vehicle odometry matched the parallel road geometry much better so the icon jumped.

    Another problem with vehicle GPS compared to boats is reflected signals from buildings which degrades the GPS accuracy in heavily built up areas.

    The vehicle odometry and map-matching is the reason why an OEM navigation system will track you accurately in a tunnel whereas a TomTom, Garmin or Bolt-On navi will not be as good.
    We notice when we're on the Eurotunnel train that sometimes the satnav puts the car icon bang on the railway track shown on the map for 5 or 10 seconds but then it suddenly decides that a car can't run on a railway track so it flips the icon over 100 yards to stick in on the nearest stretch of road. A few seconds later it decides that the road is too far away, so we're back on the railway etc etc.

    Richard

  8. #68
    Join Date
    Nov 2010
    Location
    Solent
    Posts
    3,080

    Default Re: What did PhillM do wrong?

    Thanks for the contiunued discussion here. All very useful. A few points:

    1) I have NOT been into Plymouth so did not know my way in. Next time I shall wait for daylight before entering a strange port.

    2) I had made a paper based pilotage plan. My Plotter was backup (and it failed). Also, by the looks of it did my manual pilotage plan. I have to say that its ages (at last a year) since I went into anywhere I didn't know. Perhaps I've got complacent. Certainly, in the past I would have had a set of waypoints in my handheld GPS as backup to my pen and paper exercise. Not sure why I didn't consider that necessary for this trip. Hmm just thinking about that statement, using the HH with Wpts was before I brought a plotter .... hmm

    3) I considered anchoring but, a) I had not prepared where in advance and b) there are a lot of restricted areas inside and outside Plymouth. Lesson learned about preparing an anchorage as a backup.

  9. #69
    Join Date
    May 2007
    Posts
    36,685

    Default Re: What did PhillM do wrong?

    Quote Originally Posted by Baggywrinkle View Post
    GPS in car navigation systems don't work like they do on boats. Vehicle GPS improves accuracy by using a map-matching algorithm. As it's a car it assumes you will be on a road, so it starts from a GPS position, collects track information from the vehicle odometry (gyroscopes, wheel speed etc.) to create a track and then goes to the point on the map indicated by the GPS and tries to match the recorded track to a segment of digitised road. This is then weighted to ensure you don't skip backward and forwards from one road to a parallel road but it does still on occasion happen if the fix becomes less accurate. What you will probably find is that the GPS 'track' was out by a few metres but was consistent, but the track from the vehicle odometry matched the parallel road geometry much better so the icon jumped.

    Another problem with vehicle GPS compared to boats is reflected signals from buildings which degrades the GPS accuracy in heavily built up areas.

    The vehicle odometry and map-matching is the reason why an OEM navigation system will track you accurately in a tunnel whereas a TomTom, Garmin or Bolt-On navi will not be as good.
    This was a TomTom, not a fixed system with any odometry.
    Yes they use map matching techniques and various rickery like putting you on the correct side of the road according to drive on the left or right countries, but the root of the mater is that the fix was somehow more than 100m adrift, so the wrong road was nearest.
    This happens with GPS. When you don't have a particularly good aerial, it happens more.
    I have designed products with GPS modules in, if you log their output for long enough you will find spurious results.

  10. #70
    Join Date
    Feb 2019
    Location
    The Known Universe
    Posts
    1,073

    Default Re: What did PhillM do wrong?

    Quote Originally Posted by PhillM View Post
    Thanks for the contiunued discussion here. All very useful. A few points:

    1) I have NOT been into Plymouth so did not know my way in. Next time I shall wait for daylight before entering a strange port.

    2) I had made a paper based pilotage plan. My Plotter was backup (and it failed). Also, by the looks of it did my manual pilotage plan. I have to say that its ages (at last a year) since I went into anywhere I didn't know. Perhaps I've got complacent. Certainly, in the past I would have had a set of waypoints in my handheld GPS as backup to my pen and paper exercise. Not sure why I didn't consider that necessary for this trip. Hmm just thinking about that statement, using the HH with Wpts was before I brought a plotter .... hmm

    3) I considered anchoring but, a) I had not prepared where in advance and b) there are a lot of restricted areas inside and outside Plymouth. Lesson learned about preparing an anchorage as a backup.

    Hindsight is a wonderful thing when we look back , we have all done it and we knew better when looking back , this is how we learn through our faillings , it is always hopeful that those failings are minimal .
    You have learned and will do thinks diffrently and by posting you have allowed us to share in your learing and some of us will take heed and adjust our own thinking
    So TY for posting it is not easy to come on here and show failings that we have all done but most never want tot own up to
    Flying birds have no master

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