Portable GPS plotter dinghy/boat

Roberto

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Sometimes we sail in uncharted waters (mainly rivers o river entrances), that means that at times I have to anchor the boat, take the dinghy with the lead sounder to find a path where the 2m draught could pass, then back to the boat and there we go trying our luck.
Sometimes one finds useful conspicuous points, more often all the trees look the same, no indents on the shoreline, etc etc so all relies on memory.

I once brought on the dinghy our little handheld GPS, an old MLR with a 3x6cm very basic screen, recorded the track while on the dinghy, but once back to the boat the screen was too tiny and with too low a number of pixels to give any useful precision, at whatever zoom level, and if I put it in "track mode" then it would record the new track being made over the first one, which then creates too much confusion on the way back.

So, I'd need a handheld GPS (that is with disposable/rechargeable batteries), watertight/weathertight, capable of recording a track, displaying the same track again while *not* recording the second track, with the best possible level of precision in the repetition, the highest "density" of pixels.

Another option might be to have the possibility of putting a sequence of "marks", a bit like waypoints but I need with a lot quicker method (no time to insert lat long while in the dinghy); a sort of repeated "MOB" instruction: one goes here and there with the dinghy, recording instantly coordinates of suitable waypoints. MOB1, MOB2, etc, then back on the boat one follows the succession. Again, it should be possible to zoom in at the highest possible level.

Absolutely no need to have charts (most of the times they do not exist or show us sailing in the middle of the countryside), just the very best display of the track, at high zoom.
I suppose if one records a track at time t, and uses it again at time t+10minutes with the same instrument, the satellites might be in a very similar position so hopefully the error would be minimized (?).

It might also be possible to transfer the portable gps track/wp to the PC (we have OpenCpn with a USB GPS), but I fear unsing a different GPS might introduce additional errors (?).



Any suggestions (apart from 'remain where you have charts') about possible handheld GPS models which might be suitable ?







A moment of saudade, here we were on a friends' cat, on our blog there is a satellite picture of the place plus tracks shown on "charts"
http://sybrancaleone.blogspot.com/2011/06/cascata-di-tremembe.html

ah and no, buying a cat is not something we would do :)
 

scottie

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I think that most modern hand held gps will have a function to add way points and will automatically number them with a default number001-99 etc
I know my old unit does this when laying marks to check they do not move
You can build a route as required
 

doug748

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The old black and white Garmin Map 76 will do all that stuff, I think it is now marketed as the Map72 but without the built in World map of the earlier model. The 72 is about £100, plus your data cable, and parhaps 12v lead. These are excellent products with a great range of features, and flexible screen displays. Just the job for a small boat. I still miss mine even after getting a six hundred quid plotter. They can only store ten tracks but each can be pretty big, one of mine covers Guernsey to Brest
The Map78 has a colour display and can hold up to 100 tracks etc, etc but costs c£250
 

thomshap

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I had a Garmin CX60, I upgraded to an Oregon and hated it so went back to my CX60, great unit and perfect for everything I need.
 

Roberto

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***reply to scottie, I had not seen the other messages in between***

thanks for the answer


yes I suppose most HH GPS offer the WP1 WP2... function, what I'd need to know is which specific gps gives the level of accuracy I need (if a handheld like that exists)

Suppose you have a 4x7cm, or 5x10cm display:
1. If the WPoints are marked with a 1 square cm icon, the gps would be useless to my purpose, the visual of the track would be unsufficient
2. If when you arrive at 100m from a waypoint the gps automatically cancels the track and says "you have arrived at your wp do you want to continue to the next one blabla", that would be unsuitable; in some models the distance can be set by the user, but in my old HH for example is fixed set to 0.1 MILES
3. Ideally, a display centered on the boat would be much better (it should show a "moving track"), pushing buttons to move a highly zoomed track often leads to losing it :mad:
4. If at the highest GPS zoom the single pixel represents say 20-50m, again it would be useless

Now I don't know what modern handheld gps are capable of, if all of them offer these features then I'll buy the first one I see :)
otherwise if anyone knows of a specific suitable model or models...

thanks
r
 

lenseman

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. . . . Any suggestions (apart from 'remain where you have charts') about possible handheld GPS models which might be suitable ? . . . . . .

I use the Garmin Oregon 300 (colour) which accepts the marine charts, Garmin Bluechart Vision. Totally waterproof and slightly bigger than a mobile phone and about the size of a small wallet. My wallet is small 'cos I am poor. :(
 

Stu Jackson

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What you need is a unit that will turn tracks on and off. The Garmin GPS Map 76Cx does that and the prices have been plummeting in the last year from $350 US down to well less than $200 US.

You're using way old technology.

The old Garmin 72 may do this, but I don't know for sure.

You can change the default waypoint symbol from a flag to many other symbols. I agree that the unit can automatically assign simple numbers as you go.

Your issue with "being there" is because you are using routes. Cancel the route function and just go wp to wp. You also need to turn off the proximity alarm function or feature.

If you overzoom the centered boat will move off the screen, but if you simply cycle between screens the boat in the middle returns. The 76Cx allows you to decide how many different screens you have and also what appears on the screen.

Input of wps is very easy: just push the MARK button.

The equipment exists to do what you want to do.

Try the Garmin website, I think the manual is posted online. Try this: https://buy.garmin.com/support/manuals/manuals.htm?partNo=010-00468-00

If it doesn't work, go to Garmin, search the unit, and find the manual to download.
 
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Signed Out

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Maybe not perfect, but I think the "outdoor pusuits" type GPS are possibly more oriented towards what you're after. Cycle GPS units, such as the Garmin Edge models, and Bryton Ride (?) are very compact and have basic mapping, but cyclists like tracks recorded easily so they can ride them again and again to improve times.

Likewise walking GPS units are good too, but expensive and typically have lots of land mappy type stuff.
 

Fantasie 19

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I think that most modern hand held gps will have a function to add way points and will automatically number them with a default number001-99 etc
I know my old unit does this when laying marks to check they do not move
You can build a route as required

+1 I have a Garmin GPS 72 that will do this...

As you voyage along your route put in plenty of way points - when you get back to home delete the track but leave the waypoints - all you then need to do is "join the dots" the next time you go out...
 

Amulet

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+1 I have a Garmin GPS 72 that will do this...

As you voyage along your route put in plenty of way points - when you get back to home delete the track but leave the waypoints - all you then need to do is "join the dots" the next time you go out...
I'm a Garmin GPS72 man too. The GPS72 TrackBack feature allows you to navigate a stored track (either forward or in reverse) by automatically converting it to a route. I'm fairly sure you cannot switch off the display of the current track on the map screen, but if you use trackback to retrace your steps you'd be best to switch to the "compass" or "rolling-road" page.

There are many ways to do much of what you say, but it's never going to be easy to input complicated data on a piddling wee handheld. The 72 is a very capable machine, but I manipulate all my stored routes on the computer and download them to the handheld.

I suppose if one records a track at time t, and uses it again at time t+10minutes with the same instrument, the satellites might be in a very similar position so hopefully the error would be minimized (?).
I don't understand what you're getting at here at all. The core data of a track is just a series of points expressed as Lat&Long. (It can have all sorts of other stuff associated with it - e.g., time, speed - but the locations are all you need to retrace it.) When navigating the return route the system doesn't give a monkey's when it was recorded.
It might also be possible to transfer the portable gps track/wp to the PC (we have OpenCpn with a USB GPS), but I fear unsing a different GPS might introduce additional errors (?).
Can't see why this would be a problem assuming both GPS's are communicating using the same protocol (probably NMEA). It will cause a hiccup if you unplug one and connect another while the system is running, but the software will have some way of reconnecting.

NB:
1. While many of the users talk of GPS72, the current model is GPS72H which is the same thing but with a USB connection for your computer (much better). Around £100.
2. NMEA is the "standard" protocol for communicating navigational info' between instruments - in your case computer and GPS. Many pieces of software allow you to use the Garmin proprietary protocol and give more functionality using it - e.g., transfer of routes. It will be necessary to select the protocol both on the software and on the instrument.
 

Monique

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The Garmin GPS 76Csx has a unique "where you have been" feature and leaves a dot on the screen every few feet/seconds.

In your case, you can investigate your route using this feature, return to your boat staying well clear of the inbound course, not to add a confusing set of dots, then bring your yacht in on the "dots"... you don't even need a waypoint.

In my opinion, the best use of this kit is in a MOB situation, Place your vessel in irons, drop sails, motor back in your exact reverse course by following the dots. Exact over ground track.

Almost certain to find a lost crew member unless there is a lot of surface drift.

I use the GPS76 Csx preferably to my Raymarine chart plotter; yep, it is that good!!!
 

Roberto

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I don't understand what you're getting at here at all. The core data of a track is just a series of points expressed as Lat&Long. (It can have all sorts of other stuff associated with it - e.g., time, speed - but the locations are all you need to retrace it.) When navigating the return route the system doesn't give a monkey's when it was recorded.

...

Can't see why this would be a problem assuming both GPS's are communicating using the same protocol (probably NMEA). It will cause a hiccup if you unplug one and connect another while the system is running, but the software will have some way of reconnecting.


Just to explain my concern, actually a doubt:
my old handheld MLR-something gives a position which is slightly different from the one given by the fixed gps. Boat moving or attached to the ground.
Now I do not know where this may come from: does the old HH use a different algorithm ? is the data smoothing factor different ? Does "old" mean "less accurate" ? Have they a different sensitivity to satellite position or whatever ? DO they round or truncate decimals in a different way ?
I have no idea.

The coordinates given by the handheld obviously are not "enormously different" from the others, but a difference exists indeed and in several cases it was significant.

I agree of course that a track is a sequence of lat/long, but if two gps sets give slightly different positions, then while following a track I would feel more confident using the same gps that recorded the track as a positioning device in the first place.


Maybe newer gps all offer exactly the same position to the nth decimal ? I don't know, if so so much the better :)
 

Amulet

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Just to explain my concern, actually a doubt:
my old handheld MLR-something gives a position which is slightly different from the one given by the fixed gps. Boat moving or attached to the ground.
Maybe newer gps all offer exactly the same position to the nth decimal ? I don't know, if so so much the better :)
I'd be pretty surprised if there is any significant difference in position. Forgive me if I'm teaching you to suck eggs - but are they both set to the same datum? There are different datums which place the lat and long slightly out of kilter. Mostly charts ar now on the WGS 84 datum, which has become the marine standard. Old fashioned UK maps are on OSGB36. I believe the difference is at most a couple of hundred yards. My GPS 36, and even a GPS 12 which I had ten years ago allowed you to set the datum from a list of choices. You have to have them both set to the same datum - WGS84 normally.

However, do check out how different the positions are - i.e. calculate it in feet - to see if it's so small as to be in the noise.
1.0000 minutes latitude = 1 mile
0.1000 minutes latitude = 600 ft (1 cable)
0.0100 minutes latitude = 60 ft
0.0010 minutes latitude = 6 ft (1 fathom)
0.0001 minutes latitude = 7 inches

This means that if you are agreeing to the second decimal place you are within 60 ft (north south). I'd expect you to agree to that figure and then be close but not exact on the third figure with normal GPS accuracy.


-- don't read this bit unless you are geekily interested --
-- if you do read it - correct me! --
This may sound like madness - but it makes historic sense. Up until the days of GPS we didn't think about intercontinental distances with great precision. We all know that the earth is not a perfect sphere - but it is not even a perfect ellipsoid. All "pure" shapes used for navigational calculations are approximations. Before GPS each country chose the shape (geoid) which was most accurate for them locally, rather than a compromise for the whole earth. Now we are standardising, as it has come to matter on a global scale. Having chosen the geoid, you then have to choose the exact position and orientation of the lat long grid - with historic minor differences in opinion. Pre GPS you'd have thought things were fine if you could locate yourself to a few hundred metres - even in much coastal navigation.
 

Roberto

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I'd be pretty surprised if there is any significant difference in position. Forgive me if I'm teaching you to suck eggs - but are they both set to the same datum? There are different datums which place the lat and long slightly out of kilter. Mostly charts ar now on the WGS 84 datum, which has become the marine standard. Old fashioned UK maps are on OSGB36. I believe the difference is at most a couple of hundred yards. My GPS 36, and even a GPS 12 which I had ten years ago allowed you to set the datum from a list of choices. You have to have them both set to the same datum - WGS84 normally.

However, do check out how different the positions are - i.e. calculate it in feet - to see if it's so small as to be in the noise.
1.0000 minutes latitude = 1 mile
0.1000 minutes latitude = 600 ft (1 cable)
0.0100 minutes latitude = 60 ft
0.0010 minutes latitude = 6 ft (1 fathom)
0.0001 minutes latitude = 7 inches

This means that if you are agreeing to the second decimal place you are within 60 ft (north south). I'd expect you to agree to that figure and then be close but not exact on the third figure with normal GPS accuracy.

yes all gps I have (handheld, USB chip to the PC, and the fixed one) are to the same WGS84.
I just swapped once the fixed GPS WGS84<-> ED50 as the SHOM Paper chart of Glenan Archipelago in South Brittany is (at least was 3-4 years ago) still under ED50, and I was curious to see the difference between the two; but ever since everything is in WGS84.

The difference between the handheld and the others is variable (which excludes a datum difference anyway), I confess I have not spent days comparing the two gps, but distinctly remember having computed differences up to 10-20-30m, say roughly 0.01min, which may not be a big deal on most occasions but if one sails in a river 50m wide is a lot more significant.

Anyway, if noone else noticed such differences it must be my old set.. time for a new one :)
 

Amulet

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yes all gps I have (handheld, USB chip to the PC, and the fixed one) are to the same WGS84.
Sorry, I WAS teaching you to suck eggs. I am surprised by 30 metres difference, I think 10 metres is about the extreme you'd expect to see.

My further egg sucking advice would be not to assume that charts are accurate to that tolerance.
 

maxi77

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Back when SA was active many of the lower price sets used shortcuts in the maths which were covered by SA so the user did not see them. As it is it is possible that different sets uses slightly different algorithms anyway and this may well show up as slight variations.
 
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