wreck in the bay

[Dino]

If you are such an expert with your equipment, you shouldn't have a need to to follow him!

[xxxxhornet]

guys, guys, guys,

Somebody has to say this......

Rather than puffing our chests out about what bigtime gps users we all are, how about helping us lesser folk who are learning the ropes....isnt that what this whole site is intended for.

I believe a simple question was asked about 6 or 7 posts ago.......

cheers - just my 2 cents worth.

[Lucky_Phill]

Ok, since I started with the reply, I'll help out.

In your Settings page ( sometimes they are called navigation ) in your GPS you will find all sorts of " Datum ". Please ensure that this is set to WGS84 as you will find this matches the " Datum " on most Navigation Charts from Sunmap etc.

You will find the " Datum " in the Legend area of the chart.

The difference between AGD and WGS Datum is 170 meters South West. There is a formula for working this out, but I won;t go into it here.

Let's just say, make sure your GPS is in WGS84 Datum and that anyone who gives you a GPS mark tells you what Datum is being used.

Datum is probably a fancy word for Data or Information that describes the lines and squiggley things on the chart.

This Datum thing has been a problem for quite some time now. My advice to anyone buying a GPS is to READ the instruction manual, from start to finish. If still in doubt, post the question here.

Another thing to remember, is that some GPS's , particuarly the hand held jobs only go to 2 numbers in the seconds as opposed to Hard wired units. Therefore < and it is only my opinion, that Hand Helds are not as accurate as hard wired units. How can they be ?

For more info on calculating Datum, GPS points, long and lats from charts etc, please send me next weeks Lotto numbers and I will fill you in.

???


Phill

[peterbo3]

Phill,
I will get those Sat night lotto numbers over to you first up Sun morning.

[Lucky_Phill]

I need them by Friday arvo Peter

[Kerry]

.... The difference between AGD and WGS Datum is 170 meters South West. There is a formula for working this out, but I won;t go into it here.

Phill, probably needs some clarification as the difference between AGD and WGS is approx 200 metres North East. AGD to WGS effectively reduces the Latitude and increases the Longitude.

Example (for anybody who wants to check their GPS)

35 00.000'S 150 00.000'E AGD66
is
34 59.906'S 150 00.071'E WGS84 (GDA94)

However (and not wanting to confuse things that are already confused) if one was to use "AGD coordinates" and input them into a GPS "set to WGS84" then the coordinates have not changed but the point on the ground has "physically" shifted South West, but this type of scenario is a little different when one is referring to the difference between AGD and WGS, which is technically North-east.

Another thing to remember, is that some GPS's , particuarly the hand held jobs only go to 2 numbers in the seconds as opposed to Hard wired units. Therefore < and it is only my opinion, that Hand Helds are not as accurate as hard wired units. How can they be ?

Accuracy and Precision are 2 different things and there's certainly nothing in the thinking that hand held accuracy is any way different to any other style of GPS.

Not too many handhelds around these days that don't have the same precision capabilities as any other fixed model. Actually when handhelds were restricted in the output so were many other models. Had a lot to do with manufacturers thinking, GPS accuracy specs at the time in the days of selective availability.

When the system was only capable of less than a hundred metres it was a bit deceptive showing accuracy to a foot, meant bugga all but give users more decimal places and they'll swear it's as accurate as the display shows, which is a misconception.

Cheers, Kerry.

[Kerry]

On the subject of "Handheld GPS accuracy" a little comparison between 2 GPS receivers in May 2003.



- Data recorded concurrently side by side over 24 hours
- both using the Standard Positioning Service mode
- 95% accuracy is 4.3m & 4.8m
- one is a superceded handheld
- one cost $500, the other is $25,000+

The contrast is what it's all about and that contrast is which one is 0.5 metres "more accurate" than the other and which one is worth $24,500+ more than the other.

Cheers, Kerry.

[Lucky_Phill]

That's all very well, but have you ever tried to back track on the Track Log on a hand held.

The pixel count is poor compared to the Hard wired units. This is what I'm trying to get across.

Pixel count and screen size comparing HH and HW is not contest.

Phill

[CHRIS_aka_GWH]

if only we could rate topics like we can photos now - THIS ONES AN 8.5 - entertaining, informative, bit of biffo - good stuff.

I'm actually intrigued that some guys seem not to have read their owners manuals, most of this is in there - at least my Lowrance has it. I often sit down with a beer, get out my motor, GPS & sounder manuals & reread them. I'd do it twice a month & I still come across stuff I didn't know.

chris

[spike2]

whats a G.P.S

[Lucky_Phill]

G.P.S stands for:-

Geothermalnytricladone Pyroflamolightoboomo Systiometricoboringo

I think ?

[jaybee]

can you pronounce them as well as u spell em phil

[Lucky_Phill]

OK Chris, here's more.


Latitude : 1 degree = 60 minutes
1 minute = 1852 meters or 1 nautical mile
0.1 of a minute = 185 meters

and the Longitude minute = 1640 meters ( mostly ), because of the Earths curviture, it would be 1654 meters at Brisbanes Longitude.

Going back to 0.1 of a minute = 185 meters

153.24.100 is the same as 153.24.10 as per Hand held, ie:- 185 meters past 153.24.000. Now, 153.24.155 on a Hard wired is 286.75 meters past 153.24.00, but the hand held will tell you ( as it will read 153.24.15 ) it is 277.5 meters past 153.24.000

This may be only 10 meters difference, and in the scheme of things not important, but sounding in water less than 20 meters with a 10 degree transducer means you miss the object, ledge, reef, wreck etc alltogether. 10 meters either side of a channel, is.........bottom.

And onto the next one............nuffin like a good debate, if that's what this !

Converting AGD to WGS.

Decrease Latitude by 095 and increase Longitude by 058, which equates to pretty close to 170 meters. Now, having said that, The Department of Transport Navigation Charyts say:- decrease Latitude by 5.8" and increase Longitude by 3.9", whatever that means ? Just thought I'd put that one in for no apparent reason, but it is true.....................


The thing is, as I have said, Initialize your unit to WGS and at sealevel, get all your marks in WGS, read your manual, have a beer, and if you need confusing, please post your request here... ??? 8)

Phill

[Big_Muddie]

Most interesting. I checked my handheld - Lowrance GlobalNav 12 - purchased April 2000. Precision is to 3 decimal places.

[Kerry]

Phil, well what is ??? so called accuracy issues between 2 digit (out of date) handhelds v so called hardwired (that also were no different in their day) or pixel count and screen size, they ain't even close to the same, make up your mind.

No 10 metres is not important and if anybody thinks it really is then they are using their equipment without fully understanding the operation and limitations of the equipment.

Actually it's a lot more than 10 metres with some of those now old (and basically obsolete) receivers BOTH Handheld, fixed mount or any mount.

But if 10 metres isn't important then 216 odd metres (which is what the difference actually is around Brisbane) isn't even close to 170, where does this BS come from .

The one thing that really makes sense is "read the manual" [smiley=2thumbsup.gif], it's amazing how many have this "appliance" approach and never bother reading the manual. Like Chris this also intrigues me.

Maybe the question to some should be "What's so scary about the manual", it's really quite interesting


Cheers, Kerry.