A question of accuracy

[banshee]

A couple of anglers that find their way onto my boat from time to time claim that the best way of relocating a school of fish after you have traveled over them is to take land marks.They say by all means hit the waypoint button,but this is not accurate enough to find them again.These blokes are far more experienced than me,but for the life of me I can't see how some tree waveing six to ten miles away lined up with some mountain/hill even further away can be more accurate than a GPS.Any offshore blokes out there want to put their two bobs worth in?

[mackmauler]

I agree with you banshee, never heard that one before!

[jaybee]

yeh depends if ya sounder is shooting straight, to the side, the front or back of the boat as well. ???

[Volvo]

Backtrack over the trail on your GPS, might haveta do it coupla times but it works. As for land marks ya wanna be pretty close to the coast.
Cheers

[Kerry]

So what might be their reasoning ??? As far as I'm concerned offshore one can't see any trees anyway, sometimes it hard to tell which are the closest, Noumea or Australia

But really on most days the accuracy of "most" GPS receivers is about the same or less than the distance between the transducer and the GPS (on many 5-6 m boats) and yet some still don't thing their GPS is good enough but then forget where their transducer is situated anyway.

So when these bods take their land (tree) marks are they standing over the transducer or standing over the windscreen

Cheers, Kerry.

[banshee]

G'day Kerry,I have to admit the waveing tree thing was a bit of bullshit chucked in by me to make my side of the story sound better,but seriously,these blokes swear by land marks for accuracy.Thanks for everyones input,think I'll stick to the GPS.

[Rev]

Horses for courses, they're most likely used to doing it that way and haven't crossed over that technological barrier!

Remember, their method has been used for much longer than GPS.

But there is no point for you to go backwards many years, keep using the GPS and take them to that spot or school of fish on a foggy morning!

[banshee]

Caught up with one of the blokes I refered to,he explained it like this.When you are still,a GPS plots a squigley line,this in itself shows the system is flawed/inaccurate.When you pass over a school of fish, depending on what cone angle you use,there is nothing saying the fish are directly under your boat,they could be anywhere up to sixty feet to one side(32-52fathoms).They claim these two things combined render the GPS useless.Their method- locate the fish on the sounder,do a couple of drifts to pin point the school(bites),then take land marks for a speedy return to the spot.They will only use the GPS for general positioning or if land marks cannot be had.Any thoughts on this?

[mackmauler]

Total rubbish, but thats ok, each to their own

[Kerry]

No logic in that at all, really what's the sounder got to do with the accuracy of the GPS.

Be interested in hearing what these blokes think the accuracy of a GPS is?

Cheers, Kerry.

[Rev]

Got a suggestion here!

Next time ask the question over a cuppa tea not a few beers!

Doesn't make sense to me either. ???

If they knew how to use a G.P.S. to some its better functionality they would be much better off.

[banshee]

A magazine was produced(TBF'95)that claimed at best 10m at worst 50m.A statement was also made that the mark would be in a different position each time a satelite not in the original configureation was used by the unit?

[team_mongo]

Banshee,

Modern GPS is extremely accurate - enuf to drop a bomb through a window! The inaccuracy you are referring to is whats called selective availability (SA) which was introduced by the US Department of defense. This was so their bombs were more accurate then others. SA was from the DOD adding whats called a dither to the the satellite time clock. Only they know the error introduced(which was constantsly changing), and thus they could correct for it. SA was disabled by the clinton administration several years back now. And your GPS should have an accuracy of < 15 meters, 95% of the time.

George

[Kerry]

A magazine was produced(TBF'95)that claimed at best 10m at worst 50m.A statement was also made that the mark would be in a different position each time a satelite not in the original configureation was used by the unit?

Lots of "claims" about accuracy back in those days , the days of myths and misconceptions.

Cheers, Kerry.

[CHRIS_aka_GWH]

GPS is extremely accurate - I've got most of the unlit beacons at the pin saved & my unit is spot on every time.

Triangulation is the method your mates are refering to. It is most accurate when the seperation of the landmarks along each line is much greater than the seperation of your boat to the closest object.

Along our coastline this is rare when fishing offshore. This together with the fact our landmarks are usually poorly defined like saddles in a mountain range & you are moving around in a 1 to 3m swell mean each time you take a triangulation the angles normally differ by a least a couple of degrees at best. A couple of degrees over several kilometres (the distance to the closest object) means a GPS wins hands down. My GPS triangulates in 3 dimenions using 5 to 7 satelites most of the time that alone means a higher degree of accuracy as opposed to 2 lines of sight in 2D used in your mates method.

Visual triangulation is very handy though & worth learning.

chris