datums aint datums ????

[nofrills]

howdy all

as most would know from a previous thread my gps carked it last week.So today i got a new one,a lowrance global map 4000 m.now heres the thing.

my old unit was/is a lowrance globalnav IIa and im told the datum it uses is WGS 84.i cant find it in any infomation menus in the unit or the manual . now when i take these marks and put them into a globalmap 4000 they are slightly out.i know this already as i use a global map in a mates boat .they are close enough to make them look like you are in the correct place but far enough out that i cant find some small patches i sometimes fish on.the new unit also uses WGS 84.

im told buy someone who is supposed to be a guru with lowrance gear the reason for this is the old unit uses a 5 channel reciever and the new one a 12.this makes the new unit alot more accurate and so it is putting the marks that the old unit took and placing them in the correct place.
i know the yanks had that built in error factor thinngy a while back,i think the stopped that to a degree didnt they.
anyway basic problem is,old unit,i could stop ontop of what i was looking for almost every time.new unit,cant find jack unless its the size of a cricket field(only when using marks from old unit of course)
if i get the old unit to run again ill be right cause i can use them side buy side and sort it out but id like to know what yous reccon.
cheers scott

[Kerry]

....my old unit was/is a lowrance globalnav IIa and im told the datum it uses is WGS 84.i cant find it in any infomation menus in the unit or the manual . ....

WGS84 it was and any difference in that WGS84 the current one is about 3/5ths of 5/8th of ..... and without going into an explanation why lets just say it don't matter, actually it's so little it's irrelevant.

....now when i take these marks and put them into a globalmap 4000 they are slightly out....

"Slightly out" is how much ????

....but far enough out that i cant find some small patches i sometimes fish on....

So how far is far enough ?????

....the new unit also uses WGS 84....

Well they all do they simply have to.

....im told buy someone who is supposed to be a guru with lowrance gear the reason for this is the old unit uses a 5 channel reciever and the new one a 12.this makes the new unit alot more accurate....

What a load of crap, actually absolute crap !!!!

....i know the yanks had that built in error factor thinngy a while back,i think the stopped that to a degree didnt they....

Totally, well it's set to zero anyway and that's the way it will stay.

....anyway basic problem is,old unit,i could stop ontop of what i was looking for almost every time.new unit,cant find jack unless its the size of a cricket field(only when using marks from old unit of course)....

Now sounds like there is a datum or format problem.

....ill be right cause i can use them side buy side and sort it out but id like to know what yous reccon....

What I reckon really well it's what some call a "user" issue

Cheers, Kerry.

[nofrills]

kerry if you keep quoting me all the time ill start to think your stalking me.
so if you think its me stuffing up the system give me some clues.i dont drive around playing with the buttons on the *&%#$ing dash all day.
both units run and have always run in the factory preset modes you wack a number in drive to it bang on the plotter zoom the thing right up drive to it and bingo.
that user issue statement is crap.i shouldnt need to be a astronaught to use 2 units of the same brand so something is not right.
maybee you have no decient marks so have never had the problem
cheers scott

[SeaSaw]

I am very interested in this discussion, as I also have an old GlobalNav, in fact the earlier model than yours Scott as it is does not have a II at the end, so I assume it is the earlier model.

Mine also does not have any information in the menus or the manual as to what Datum it uses, so a while ago I send an email to Lowrance to see if they new. The response I got was they are 99% sure it is WGS84. Why they cannot be 100% sure, i dont know ???

As I also have some very valuable (to me) GPS points captured using this reciever, and I want to make sure they will convert when I get a new set, I let the GlobalNav plot a course around my yard in the exact location I store the boat i.e a know physical location. Then I captured a point that is in about the middle of the plot that was created ..... because the boat moved around in about a 10m area while it was sitting still - as they do

Hopefully I will be able to repeat this process when I get a new GPS and should be then able to calculate any offset by comparing the two GPS marks or at least be sure of what Datum being used in my current set. Is my thinking on this going to work ... your thougths appreciated .... Kerry [smiley=helpa.gif]

Scott, maybe we can solve both our problems at once as my reciever should be compatible with your unit and your new unit would have configuable Datums - now we just have to get our boats together in a fixed location

Mark

[banshee]

I am more concerned on how I'm going to get my marks into the new system,I have read a couple of articles that state the system we are now using will slowly deteriorate to the point where in about 2008-2010 it will be totaly useless.No doubt units useing the new system will cost an arm and a leg till the companies recoup some of their costs.Does anyone know if the new system will use the same format/datum as we use now or will it be something totaly different?

[nofrills]

hey mark

im going in to ross's aka fin addicts next week he also has a global nav IIa so ill take the new one with me and see what that tells me.ive fished with dazza a fair bit and i know some of my marks are accuratley on his hand held so ill check that out as well.ill let you know what happens for sure.if you want me to bring the new one up give me a pm and dazza and i will come for a drive.
cheers scott

[Kerry]

Scott, A datum is a very explicity defined entitity with regards what's being discussed here anyway, WGS84 is WGS 84 is WGS84.

SO to answer some of your questions then how about some actual answers, like what (how far in metres etc) is "slightly out", which can be related to "how far is far enough" like how far on the ground ????? Some specifics would help.

Are you absolutely sure there was never a PCF used in the Global Nav, the formats are identical and no, one doesn't need to be an astronaut but one does need to appreciate it's a little bit more invloved than driving a toaster, commonly referred to as the "appliance" approach.

Cheers, Kerry.

[Kerry]

I am more concerned on how I'm going to get my marks into the new system,I have read a couple of articles that state the system we are now using will slowly deteriorate to the point where in about 2008-2010 it will be totaly useless.No doubt units useing the new system will cost an arm and a leg till the companies recoup some of their costs.Does anyone know if the new system will use the same format/datum as we use now or will it be something totaly different?

Like what "New system" are we talking about here and lets hope the answer is not Galileo!

I have read a couple of articles that state the system we are now using will slowly deteriorate to the point where in about 2008-2010 it will be totaly useless

Where's all this myth stuff coming from ???


Cheers, Kerry.

[bay_firey]

Just a thought (dangrous i know)

What a bout the format

deg, min, sec
deg, min.(decimal)min

? ? ?

[SeaSaw]

..... PCF ??? huh .... whats that in non techo talk

Scott, thanks for the offer, I am in no real hurry as I am not replacing my unit in the short term. I was really more trying to help sort out your probs as my reciever is probably compatible with your unit, but if you take your old and new unit down to Fins and compare the GPS point at the same location, that should tell you what is going on.

Mark

[banshee]

Article by Paul Dornbusch of G.P.S. Surveys in Darwin,Titled Today's G.P.S. accuracy,page 20 in the North Austrailian Fishfinder,the last parragragh titled "The future of G.P.S." It rambles on with a lot of stuff concerning new satellites, different frequencies and states we are going to need new units(over simplified,but I type at about five words a minute).

[Kerry]

....It rambles on with a lot of stuff concerning new satellites, different frequencies and states we are going to need new units....

Well we'll only need new units if we wish to take advantage of civil dual frequency and the improved accuracy it will/can provide.

But really all this talk about the system being totally useless by 2008-2010 is rather way off the mark.

All existing receivers will still work just like they do now but also with some improvement in accuracy due to new satellite technology as with every launch of a Block IIR things get better and right now (this minute) GPS IIR-13 is waiting some last minute glitches for lift-off.

This one still doesn't carry the first of the second civil frequencies (or other new and additional military freq) but it does have other improvements (as does all of the block IIR's). The first of the modified IIR's (known as IIR-M) with the additional freq's won't be launched until about April 2005. But one satellite won't make the additional civil freq useable and this will take some time until enough IIR-M's and IIF's are in orbit.

Cheers, Kerry.

[Kerry]

....Hopefully I will be able to repeat this process when I get a new GPS and should be then able to calculate any offset by comparing the two GPS marks or at least be sure of what Datum being used in my current set. #Is my thinking on this going to work ... your thougths appreciated .... Kerry ....

Mark, There should be no need to calc offsets or anything like that as if you are sure of your settings.

The Global Nav/II/IIa's did not have any datum options and hence was WGS84 as this is what the system uses but over the years there has been some fine tuning of the parameters, which define WGS84 but this was in about the uptenth decimal place and quite irrelvant to recreational use.

Similar with GDA94 and WGS84 (both earth centred datums), GDA94 is a more modern definition than WGS84 (at the same point in time) but as data became more refined parameters have been fine tuned over the years but nothing your receiver will ever be able to detect.

Many of the first Lowrances (with no datum options) had a PCF (Position Correction Factor) option, which basically allowed a user to display coordinates effectively in any datum. The receiver still used WGS84 (as such) internally, derived its WGS84 coords then applied the PCF to display the reuqired coord set. Normally the PCF is set to 0,0 (default) so with a Global Nav the PCF is something to check as it affects all displayed coords.

The other issue with some of the earlier Lowrance's (with the first of the datum options) was that changing the datum DID NOT change the coordinate, which really is quite misleading and probably one of the many things that have caused confusion with coordinates etc.

I suppose the thing to check is that the coordinates you have are actually correct/known (in the first place) as making adjustments/offsets for these (if required) doesn't necessarily make the coords correct in any new unit.

Cheers, Kerry.

[nofrills]

hey kerry

toaster lol .
the pcf is off,ie,all zero's and has always been.both are set on magnetic north(prolly not relevant ???)
the problem with telling how far out it is is that with small structures,i cant find them to tell ???.and on larger reefs.which i can still find ive always used plotter icons for my trolling so theres no chance there.
when i go to ross's ill take a mark on both unit's in the same place and see what i get.

PS if i turn up the knob on my toaster from 2 to 4 will this make the bread cook faster or will it just take longer? ??? ???

cheers scott

[Kerry]

Scott, yes your right magnetic north not relevant at all with regards coordinates.

So how "far out" do you suspect and how old are the waypoints (when were they originally saved) you are using.

With regards the 5 v 12 channel myth, the number of channels really has not a great impact on accuracy (in recreational terms) and in fact a 5 channel receiver can/will use all satellites it can see, even if there's more than 12, which a 12 channel receiver can't do.

If you have some idea of how far things are (supposedly out) then that would give something to work on.

Cheers, Kerry.

....PS if i turn up the knob on my toaster from 2 to 4 will this make the bread cook faster or will it just take longer?

What model/make of toaster and also require time of day