GPS error

[Mike_02]

Having just used my GPS a fair bit over the last 2 weeks i have a few problems..!

It is just a simple Garmin GPS 12.

The first problem is the it seems to be about 140 meters out.

The second (which causes the frst i think) is to do with inputting the lat and long into it.

My display gives me the option to enter dd,mm,sss

But the almanac only gives the seconds as 2 digits eg dd,mm,ss

So my question is what do i input for the last digit in ym GPS.....oh and it's not anything to do with the setup of the system...??

 

[bedouin]

The Garmin GPS12 is a nice little unit...

If you are consistently 140 metres out the most likely cause is that you have the wrong "Datum" set. Most UK charts are now printed to WGS84 Datum, but up until a couple of years ago OSGB36 was more common. Without going into the details of a datum is, the same coordinates in two different datums (or should that be data) may differ by a hundred metres or so.

Check the chart - it WILL say somewhere what datum it uses. Somewhere on the Garmin setup you will find the option to change the datum - I would recommend sticking to WGS84.

As to the third digit - to be honest who cares? Two digits gives you accuracy to within about 20 metres - which is about as accurate as GPS can be. If you are bothered then set it to 0, but if you are relying on that degree of accuracy think again!

 

[brianrunyard]

Mike

"The first problem is the it seems to be about 140 meters out."
Have you set your GPS to the same Datum as the chart?
My chart of Poole Harbour states:-
"SATELLITE DERIVED POSITIONS
Positions obtained from satellite navigation systems are normally referred to WGS (1984) Datum. Such positions should be moved 0.03 mins SOUTHWARD and 0.08 mins EASTWARD to agree with this chart."
That equates to approx 100 metres.

Hope that helps,


Brian
http://homepage.ntlworld.com/brunyard

 

[Mike_02]

I have checked the datum and i am using the correct one. But i'm not usign a chart.

My test was to programme the lat and long for the end of alderney breakwater from the almanac.....then i went for a long walk...at the end of the breakwater i was 0.14km away....!!!

 

[bedouin]

Let that be a lesson to you - don't trust Waypoints in Almanacs!

Seriously - it is better to work out your own waypoints. Almanacs have been known to make mistakes, and with something like the breakwater in Braye there could be a number of interpretations as to where to put the waypoint.

 

[philip_stevens]

You say you "programed" it. Do you mean you put the chart position of the breakwater in, or did you accept the GPS position of the breakwater??

If you input the position, and didn't accept the GPS position, you may well have copied the position from an almanac that was using the ED datum. That would also have been different from the WGS84 position, by about the same as has been said earlier.

If you had accepted the given GPS shown position, you would have seen a course and distance travelled by the time you had walked to or from the end.

If you have the GPS with you at home, try changing the datums to see the difference in your position. Some places, it is worse than others.

If you do a search of earlier threads for a position of a French beacon, you will find one from me, where I put in a position taken off the chart, that was enough out that it would have put me on the rock instead of 150yds off of it.

There was an article in PBO earlier this year (I think this year) concerning the difference in datum positions.

regards,
Philip

 

[charles_reed]

Braye breakwater

As the breakwater was never completed, there are foundations (which you could see at LW springs) a further 200 yds from the end of the breakwater.

Perhaps you assumed the end of the breakwater was really the end!!

Whereas the writers of the Almanac may have known about it.

In fact the Alderney waypoint, as opposed to the breakwater end, is safely to sea.

 

[vyv_cox]

Point 1. Are you sure the almanac reading is dd,mm,ss? I never take anybody else's waypoint figures but I'm fairly sure that mine gives them as dd,mm.mm, i.e decimal minutes, not seconds. This might give a little inaccuracy but probably not as much as you are looking for. Every Garmin I have owned had the facility to change the format of waypoints.

Point 2. Are the waypoints you are taking the precise points, or a safe distance off? If you compare with an on-land lighthouse or similar fixed single point you may be more certain, but when it comes to a collapsing breakwater I'm not sure.

Point 3. Again, Garmins I own/have owned have a facility to adjust position for inaccuracies. Before doing this you need to be very sure that you are using the correct chart datum, that you are in an absolutely known position and that no other errors may be present. However, my experience, using a GPSMAP, is that we invariably are shown as sailing over the land when doing canals and narrow harbour entrances. GPS is accurate but there are technical reasons why it does not always read spot-on. It could be that this facility has been used on your set unknown to you, accounting for the error.

 

[LadyInBed]

I think most GPS units can be set to display DD MM SS or DD MM.mmm
All my chart lat/long scales are in DD MM.mmm format.
Are charts still being produces in DD MM SS format?

 

[vyv_cox]

I guess you could read a chart in whatever format you wanted, although a little inconvenient. The question referred to waypoints read from an almanac.

 

[kingfisher]

Charts in DD MM SS

Yep, in the good old US of A, they still use seconds. I was in Maine in July, and it takes quite some adjusting to get the waypoints right.

Obi-Wan
http://sirocco31.tripod.com

 

[duncan]

the information in the almanac is based on a datum; this will be quoted somewhere in the publication. That needs to be the same as the one the unit is set to.
secondly try a different breakwater - but I am sure this is a red herring and the figure shown will be for the current end position! If it isn't and you are using a 1964 almanac then both the datum and the geographical position will be wrong!

 

[Twister_Ken]

GPS for a breakwater walk

presumably you were worried about the vis closing in?

 

[Guest]

Post deleted by danfoley

 

[hlb]

Firstly. Missing off the last two digits is going to through you out a few yards. But more important the waypoint position will be in safe water, not on the quay, so walking to it is not an option. Also as pointed out, the quay at Braye extends much further out, if under water mostly.

No one can force me to come here-----------
----- I'm a Volunteer!!!

Haydn

 

[oldharry]

I agree whole heartedly! Who needs pinpoint accuracy when afloat? Having learned my skills in the day when doubling the angle on the bow was the height of sophistication, it amuses me to hear complaints that the GPS may be 50metres adrift in its position fixing.

Use your eyes if its that close. Beats any GPS....

Trouble is, as with any innovation, if it is capable of a high degree of accuracy, then this creates an expectation that it WILL be accurate - and a corresponding disappointment when it proves not to be.

Keeping the grandchildren amused on a long car journey, my old Garmin 12 came well in to its own, providing an ongoing source of information, and amusement - ' Grandad did you know we are only averaging 53mph - Dad does over 65 on this bit...' and the mapping screen (with previously entered waypoints to add interest) was an enduring source of interest as we 'navigated' the M4.

What I found significant later was the variation on the trace of the return journey - same road, but quite significant differences of position at times - sometimes as much as 200 metres.

I was also quite struck with the difference betwen the car speedo, and the GPS speed readings - with the speedo at 70 mph the GPS consistently read 64mph! 'Well officer, I wasnt speeding - according to the GPS I was only going....' I wonder if that would stand up before the magistrates as a defence? Doubt it somehow.

 

[charles_reed]

Spurious accuracy

Go to 3 places of decimals and people assume inordinate accuracy.

GPS is just one of many navigational aids - and just as likely to be wrong as any of the others.

It is plain commonsense to have at least 3 navigational inputs, you'll usually find one way out of agreement with the rest.

Officially the current GPS system is beyond its use-by date - it will be becoming increasingly inaccurate as hardware deteriorates, software becomes scrambled and orbits decay.

So I don't know whether the original posting was a send-up or incredibly naive.
Whichever it was methodologically suspect (one reading, no population) and remarking on the obvious.
You'll never get an accurate X-reference on the edge of a chart - they're all merctor projections and only to scale in the middle.

 

[tcm]

Re: Decaying reliabilty?

I'm not sure that accuracy of gps will "decay", in the style of a gradually more and more inebriated navigator, say? At least one advantage of modern electronics is that they either "work" or they're "knackered".

 

[duncan]

Re: Decaying reliabilty?

I find the talk of inaccuracy facinating.
Although I will admit to having DGPS I spend a fair few hours comparing charted objects (not buoys 'cos they move) with my position and I have never experienced the sort of 'inaccuracy' discussed. Even with the DGPS off you get the general impression that it's all 'within a boat length 'sort of stuff - say 30ft all the time.

 

[philip_stevens]

Re: Decaying reliabilty?

30ft inaccuracy /new/forums/images/icons/shocked.gif That could be disastrous for someone with a 35footer /new/forums/images/icons/frown.gif They will have to put the GPS aerial up on the pulpit /new/forums/images/icons/wink.gif

regards,
Philip