Why am I sailing on the road

sailaboutvic

sailaboutvic

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Question for the Tec Guys
Why is it a GPS with road maps will show you at what corner you are at to the spot and a chart plotter GPS at time will show you that you on land or crossing a beach or at one time ,going up a river shown we was going along the road .
Talking to other people it seen we not the only ones who have this problem.
Yes it is set at WGS48

www.blueweatersailorcroatia.webs.com
 
sailaboutvic said:
Question for the Tec Guys
Why is it a GPS with road maps will show you at what corner you are at to the spot and a chart plotter GPS at time will show you that you on land or crossing a beach or at one time ,going up a river shown we was going along the road .
Talking to other people it seen we not the only ones who have this problem.
Yes it is set at WGS48

www.blueweatersailorcroatia.webs.com
Click to expand...

Road satnavs are programmed to "stick" you to roads up to a certain amount of error in position, whether the error be in road mapping or in GPS location. Chartplotters just show you where you are on a theoretical globe that fairly closely matches the real earth surface. If you are shown as on a beach or the middle of an island (I've seen that) it is the charting that is wrong. And it is wrong in quite a few places where big ships do not often go.

I presume you mean WGS84...
 
Well the Ordnance Survey still shows the Prehistoric Wadeway from the mainland to Hayling Island as a complete link, while nautical charts show the West Cut dredged in the 1800's; we watched with interest - no way of yelling a warning so we relaxed to observe - as a Landrover leading a convoy from the ' All Wheel Drive Club ' with a big Union Jack on the whip aerial charged along and discovered the dredged bit with a mighty splash...
 
This explains one similar problem.

GPS and chart plotters monitor positions at fixed time intervals which can sometimes be adjusted. Your course is assumed to be a straight line between these positions (a bit like mini waypoints). So if your actual course is curved (say around land) then the GPS will show you crossing the land. To minimise this effect change the time interval of position monitor to the minimum.
 
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Good replies guys , but I still lose , if I took the lat and long from a GPS and plotted on a chart I might be a few mts out , so I understand and please tell me if I am wrong , the chart plotter does the same , take a fix and place it on the chart , so why is there so much different when the charts are the same ?
Yes j Wilson I meant WGS84 , sorry mate .

www.bluewatersailorcroatia.webs.com
 
Marine GPS is not to be used as sole item for navigation
Most models tell you that every time you turn them on
Even some chart plotters remind one that they are dodgy bits of kit
The fact that they are sold as such is irrelevant - or so it would appear
 
theoldsalt said:
This explains one similar problem.

GPS and chart plotters monitor positions at fixed time intervals which can sometimes be adjusted. Your course is assumed to be a straight line between these positions (a bit like mini waypoints). So if your actual course is curved (say around land) then the GPS will show you crossing the land. To minimise this effect change the time interval of position monitor to the minimum.
Click to expand...

Not strictly true. They monitor positions around every second; they may plot positions at user-changable intervals, thus giving a trace will sometimes be only a crude approximation of actual positions and may cross land. In real time, the indicated position will be highly accurate (but may not appear so on a digital chart, for reasons stated). Apologies if this is what you intended to get across.
 
macd said:
Not strictly true. They monitor positions around every second; they may plot positions at user-changable intervals, thus giving a trace will sometimes be only a crude approximation of actual positions and may cross land. In real time, the indicated position will be highly accurate (but may not appear so on a digital chart, for reasons stated). Apologies if this is what you intended to get across.
Click to expand...

No apology necessary, life is a constant learning curve - shame electronics aren't the same.
 
sailaboutvic said:
Good replies guys , but I still lose , if I took the lat and long from a GPS and plotted on a chart I might be a few mts out , so I understand and please tell me if I am wrong , the chart plotter does the same , take a fix and place it on the chart , so why is there so much different when the charts are the same ?
Yes j Wilson I meant WGS84 , sorry mate .

www.bluewatersailorcroatia.webs.com
Click to expand...

When your chartplotter is telling you you are on the beach try carefully noting the lat/long and plotting it on your paper chart. You will probably also find your pencil is on the beach. Charts are sometimes wrong, the further from big ship/military places you are the more likely that is.

You may however just have found a location where the paper chart is not the one on which the electronic chartmaker based his mapping: this can happen if your chart is one made by country X and the chartplotter mapping is based on charts made by country Y. This is however less likely.

I don't think there are many chartplotters that refresh their position display at long enough intervals to give a seriously misleading track - typically the display updates reasonably quickly. Display changes are deliberately slowed down and speeds averaged to avoid position and speed displays jumping around irritatingly, but this takes place over a few seconds rather than minutes.
 
sailaboutvic said:
Question for the Tec Guys
Why is it a GPS with road maps will show you at what corner you are at to the spot and a chart plotter GPS at time will show you that you on land or crossing a beach or at one time ,going up a river shown we was going along the road .
Talking to other people it seen we not the only ones who have this problem.
Yes it is set at WGS48

www.blueweatersailorcroatia.webs.com
Click to expand...
Car Satnav usually ties to the nearest road, so the mapping (which is often actually quite crude) always LOOKS 100% accurate. I had one that showed the actual position without actually follwoing the roads drawn. It was surprising how often we found ourselves driving parallel to a road, and on more than one occasion we were 100 yards or more offshore. The mapping simply wasn't as accurate as the actual GPS. Same applies to marine GPS, The charts tend to be more accurate, but if the GPS is not showing you where you would expect, the fault is with the mapping. This is why its so important NOT to use it for primary nav unless you KNOW from previous visits that it is right.
 
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Don't you worry old Harry I never have or would ever have use it as a Primary Nav ,I have been preaching your words since they first come on the market , I still use the good old chart and rule and a good pencil when ever I need to , you cant beat it .
I still rely on the only true way to navigate when the need be .
one cant beat a three way fix when in any tricky situation even a two way fix is better then relying on a picture .and a good depth sounder
the reason that I asked the question was ,at times I have at time taken a GPS lat and long and found myself to be nowhere near where the plotter have put me , so I always been puzzled why that would be the case as I have already said , I understand that all the plotter is doing , regards the time different ,that people keep on about , I cant see where that comes into account as I am not talking about one fix for that time but over quite a few mins , example five years ago going up the river Guadiana , the plotter showed me going all the way up along the road , it even went around the bends in the road ,
maybe mine as road maps plug into it , :).
oh well one thing for sure it not reliable full stop .

www.bluewatersailorcroatia.webs.com
 
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