GPS mark clarification

[sharkbait]

Hi all,

question for the gurus:

Why do some marks only contain 2 digits in the final column (minutes???) ?

For example

S 27.56.31 E 153.26.15

Was this an outdated method or have they been shortened???

Do I simply add a " 0 " at the end when punching these marks in ?

to what degree does the final digit effect the Unit's capability of finding a mark accurately?

Cheers, Kris.

[Gorilla_in_Manila]

I'm no guru but I think thats seconds Kris.
60 seconds in a minute.

Hang on ........

KERRY!!!!!!!!

[Morlers]

I think Gorilla is right and the two digits could be seconds and would be less than 60. Three digits would normally be decimal minutes.

Kerry is the expert on this subject so best get his help on this.

:) :)

Morlers

[banshee]

At a guess I would say they have just droped the last digit,if it's in the same format that most people I know use,I think the difference in distance between a 0 and 9 in this column is 16 metres.I would just add a 0.But like the others I would be interested in Kerrys' input.

[ba229]

lol Aren't any of you guys prepared to go out on a limb before you are corrected :-)

I'll stay silent now.

[bay_firey]

Kris

I am thinking that the marks are in d'm's and for best accuracy you would have to check what format your GPS is set to. If it is set to d'm's then no wories just punch away, but if it si set to d'm.m then you will get an error in the actual position. I don't think the distance would be too greta though ??

As said before Kerrrryyyyyy

[oscar]

27:56.31 S 153:26.15 E decimal of a minute.
27:56'31 S 153:26'15 E seconds.
You can have:
DEG/MIN.MM
DEG/MIN.MMM
DEG/MIN/SEC
--------------
oscar

[Cheech]

We want Kerry,,, we want Kerry,,,, we want Kerry,,,

oops did I type that or just think it??

[ba229]

lol. if you post a reply Kerry you just might get a standing ovation ;D

[Kerry]

:-X well I wouldn't know either ??? Those coordinates could be anything.

Those could be S 27° 56.31' E 153° 26.15' or S 27° 56' 31" E 153° 26' 15", they could have been DM.m written as DMS or DMS written as DM.m or .....

Really publishers could eliminate all/any confusion with the inclusion of some simply identifiers like degrees ° minutes ' or seconds " symbols but then they probably don't really know what they were meant to be in the first place. Oh and not to forget the datum, presume the publishers quality asure that as well ;D

If they are minutes and meant to be minutes then dropping the last digit could be around 25 metres. If they are seconds and meant to be minutes (or minutes and meant to be seconds) then in this particular example a bit over 400 metres.

Unsigned :D

[sharkbait]

::) Sorry fellas - what I maybe should have asked is:

if I am given a mark that looks like this:

S 27.56.31 E 153.26.15


and my gps requires an extra digit on the end, like this:

S 27.56.310 E 153.26.150


Am I right to just add the "0" on the end, or is it a different datum/ ??? ??? ???

[Lucky_Phill]

Just add the 0, Kris.

The problem being, of course, IS Datum.

In a round about way, if the mark you were given was in AGD66 and your GPS is in WGS84, then that mark could be up to 500mtrs from where you will be looking.

Lots of ways around this.

1. #When recieving GPS marks, ask for Datum.( always )

2. # Do a calculation if you know the mark is in AGD, to convert to WGS.

3. # Plot mark in GPS in WGS ( as should be default )
Then set GPS default to AGD and plot it in again, go over mark in AGD, turn unit off and reset in WGS and go over the mark again.( like this will work ;) )

4. That mark is for the 1 mile off the Gold Coast and is probably a ' Bait #Ground " Mark. ( big guess ! )

5. #have a beer.

6. #for your info. #At Brisbane Longitude, a mark in AGD66 that has to be converted to WGS84 will need to have the following conversion.

AGD to WGS # # minus 104 Lat # # and # add 067 Long ( notice the 3 numbers ) ?

All of the above info is my opinion ( guess/ educated )and should not be used as the only source for navigation. #Any further info can be gleened by sending 1ctn of XXXX Gold to me ! #:o :D ;) ;) :P :P :P :P :P :P

Cheers # Phill

[Cheech]

Sharkbait,

Unfortunately the answer is the same. The point I think Kerry was making discussing degree and minutes etc was that some coordinates are actually correct to be

S27.56.31 E153.26.15

We are just use to seeing them as

S 27.56.310 E 153.26.150

That is because the position format we mostly use is

hddd mm.mmm

But there are other formats that only go to 2 decimal places instead of 3 decimal places. Thus making

S 27.56.31 E 153.26.15 correct.

Or, like you have assumed, they simply did leave off the last "0". But would never know which without more info.

The datum discussion is that most of our marks are in Map Datum WGS 84, but there are a heap of other formats that are used in mapping. eg, I have a map program that is in AGD 66. so if I pick up a gps point from my map and plug it into my gps that is set to standard WGS 84, then I will not be in the right place as there is a small difference between WGS 84 and AGD 66 (20m - 50m?). Close enough if I was just after an approx location. So what I have to do to get the exact spot is to change the setting on my GPS to AGD 66, plug in the coordinate, and then change the setting back to WGS 84.

Hope this helps (and I hope I have it correct)

Cheech

[Kerry]

::)
if I am given a mark that looks like this:

S 27.56.31 # # E 153.26.15

and my gps requires an extra digit on the end, like this:

S 27.56.310 # E 153.26.150


Am I right to just add the "0" on the end, or is it a different datum/ ??? #??? ???


IF you KNOW the format is ACTUALLY Degrees Minutes and Decimal minutes (D°M.mmm') and IF your GPS is set to accept D°M.mmm' then yes add a zero

If you don't know what the format actually is then adding a zero (0) is making assumptions.

The datum is a different issue to this, this is a
FORMAT ISSUE (http://cqnet.com.au/~user/aitken/gps_x/format.htm) and mind you a very common problem.



.... small difference between WGS 84 and AGD 66 (20m - 50m?). Close enough if I was just after an approx location.

Cheech, that should be approx 200 metres, ranges from around 170 to 220 metres across the country.

Cheers, Kerry.
#

[Lucky_Phill]

Yes Kerry, we agree. I believe the dif between AGD and WGS at Brisbanes Lat, is 170mtrs


Phill

[Jilly]

hey just sedning a post so i can send a a pm...lol. sorry guys
love ya's
me ooxx

[Kerry]

Yes Kerry, we agree. #I believe the dif between AGD and WGS at Brisbanes Lat, is #170mtrs


Phil, Relative to 27°S 153°E (Brisbane there abouts) the difference between AGD and WGS84/GDA94 is 213.3 metres (+/- 0.050m).


.... AGD to WGS # # minus 104 Lat # # and # add 067 Long ( notice the 3 numbers ) ?

With respect those numbers the difference AGD to WGS/GDA in decimal minutes is 0.095' less in latitude and 0.054' increase in longitude.

That makes 27° 00.000'S 153° 00.000'E AGD becomes 26° 59.905'S 153° 00.064'E in WGS84/GDA94

Cheers, Kerry.
#

[Cheech]

That much?? that surprises me. Sounds like it must be from what you and phil said. As an example, recently I picked up the wreck on the beach that is south of tangers, and after puting it into my gps (in correct AGD) and seting the gps back to WGS, I plugged the WGS spot into my AGD map to see what the difference was, and it did not look to be that much.

Anyway, even though I always convert to the right format to suit, at least now I know what the exact variations are.

Apart from that, the rest of the advise I gave sharkbait was right,,, wasn't it? At least in simple terms.

[Needmorerum]

So what do I do when I am given a mark like 23.41.769 - 151.38.525 and my GPS only takes xx.xx.xx - xxx.xx.xx
The last set of numbers have one more than I can fit in, and of the last set of numbers the first digit only goes up to 5 on the GPS. Is it something to do with the GPS I have. It is one of the old Magellan Pioneers'. I have looked throught the menu on it and changed things around and tried it, but honestly, I have no idea what I am doing.

Any help would be appreciated,
Corry

[SeaSaw]

Corry, #It sounds like your sounder is expecting Degrees, Minutes & Seconds rather than Degrees, Minutes & Decimal Minutes. #That would explain why it only goes up to 5 (i.e. 60 seconds in a minute, so it would go to 59 and then back to 00)

I am not sure how to convert between the two but at a guess it would be simply a case on multipling the decimal minutes by 60 to convert to seconds e.g.

.750 decminal minutes = 45 seconds #(.750 x 60 = 45)

In your example of a mark of 23.41.769 - 151.38.525, I think the correct mark would be 23.41.46 - 151.38.31. #Someone else might be able to confirm if this is the right way to convert - over to Kerry. #;D

Cheers,

Mark

[Kerry]

The Pioneer can be configured to accept dDD° MM.mm' (DEGMIN) dDD° MM' SS" (DEGMINSEC) and UTM.

Limited to whole seconds in DEGMINSEC or 2 decimal minutes in DEGMIN.

23.41.769 - 151.38.525 would appear to be 23° 41.769' - 151° 38.525' based on the latitide decimal minutes being greater than 59.


Your receiver appears to be currently configured to DEGMINSEC (limited by only accepting 59 seconds). Not a very good format to use at all, so forget any thoughts of conversion.

In the Pioneer Press [MENU} until SETUP is displayed and press [ENTER]. Press [ENTER] again and you should see the coordinate system flashing. Use the RIGHT/LEFT arrow's to scroll the coordinate systems and press [ENTER] when DEGMIN is displayed.

Enter those particular coordinates by rounding as 23° 41.77' - 151° 38.53'


....and changed things around and tried it, but honestly, I have no idea what I am doing.

Can highly recommend reading the manual [smiley=2thumbsup.gif]

Cheers, Kerry.

[ba229]

It's been a good thread this one. It got me thing about some marks i have got of the internet and out of books.

Marks such as 32.58.36S 151.45.09E

I have been entering them as ddd.mm.mm and just add zeros to the end of them

When i go to these spots there is nothing there.

Just looking at me etrex

Could it be that i should enter them as ddd.mm'ss.s? and then i might find them ?

Hmmmm might have a play with this thing.

Kerry out of interest, how far out would my marks be if this was the case and i was in sydney (roughly)?

[Kerry]

Based on mistaking 32° 58.36'S 151° 45.09'E for 32° 58' 36"S 151° 45' 09"E would be around 450 metres.

As for datum difference around Sydney then based on 33S 152E about 216 metres.

Really all published or internet marks should be treated as suspect as there's generally no guarantee as to the certainity of the format or the datum.

The "unknown" is what some publishers call a "fudge factor", when really it's "trash for cash"

Cheers, Kerry.

[MulletMan]

Coordinates are normally expressed in hours – minutes – seconds
27044’13 is obviously 27 hours 44 minutes 13 seconds. As one hour equals sixty minutes then this “one hour” can also be called “one degree” - makes it easier!
Your GPS SETUP MENU x NAVIGATION will omit the main “degree” figure and simply show MM.MMM’. It does this because the primary degree figure (27) cannot be altered to anything else whereas the “seconds” can. The MMM is simply 1/1000ths of the MINUTE. So, in the GPS setup, you scratch about till you find the MM.MMM’ setting and select it (it is usually the default one anyway!) …. And remember that this principle applies to BOTH Latitude and Longitude. Now, the above coordinate will look like this:
27044’135
If you had a hot spot to locate with a coordinate of say:
27037’272 you would not be able to insert it all (the 2 bit) into the GPS if you had the SETUP as in the first example above (not enough digits!!)
However the latter example with its 7 available digits would accept the coordinates quite happily and be more marginally more accurate.
The difference is not quite as huge as you would imagine but it is necessary if you are looking for a very isolated peak and need to be as close as possible! It can also be seen by the red lines on the sample chart above that it is now simple to plot in the “.272” bit and vice versa from the chart (or your mate) to your GPS.
1 degree of Latitude = 60 nautical miles so S270 to S280 = 60 nautical miles
1 Nautical Mile = 1832 metres
1/10th. Nautical Mile = 183 metres
1/100th. (MM) = 18.3 metres
1/1000th. (MMM) = 1.83 metres
So in summary, the difference in putting the 2 (.002) into your GPS makes a difference of about 2 metres!
Also important to check your GPS is set up to WGS84 being the last decent update from hydrographic surveys in Qld, i.e. your latest chart is already 20 years old!
As even the best of GPS’s still contain errors of up to 50 metres +/-, the offending “third digit” is not all that critical. If you can only see “two digits” in your last grouping of coordinates, go to the SETUP menu and look for the MM.MMM one.
By the way, Longitude is a different ball game altogether and although we can guesstimate on it sometimes, it follows totally different rules than the 1 nm = 60 minutes! In our latitude however, it can be (almost) said that 1 nm of Longitude also just about equals 1 nml – but not exactly! About 70% of the cost of a GPS unit is supposadly in the antenna so without a good one and as high possible with no obstructions about it, the better it is!

[sharkbait]

Thanks everyone for answering my queries ;D

what a wealth of knowledge to be able to tap into :o

Thanks, Again

Kris

[Kerry]

.... Coordinates are normally expressed in hours – minutes – seconds 27°44’13 is obviously 27 hours 44 minutes 13 seconds.

Sorry not obvious (to me). If it was obviously Degree Minutes and Seconds then why not actually make it Degrees Minutes and Seconds?

27°44’13 is ambiguous 27°44’13" is not and clearly intended to be DMS.

Sorry but this is rather confusing. First mentions that 27°44’13 is obviously Degrees Minutes and Seconds"? Debatable, and then goes on to setting MM.MMM' (1/1000ths of a MINUTE) and now the above coordinate looks like 27°44’135 ? well if 27°44’13 is Degrees Minutes and Seconds? then configuring to MM.MMM' the above "same" coordinate should look like this 27°44.217' otherwise following on that 27°44’13 is Degrees Minutes and Seconds then 27°44’135 should rightly be taken to be 27°44’ 13.5" (in the same context).



.... If you had a hot spot to locate with a coordinate of say: 27°37’272 you would not be able to insert it all (the 2 bit) into the GPS if you had the SETUP as in the first example above (not enough digits!!)....

But really it is quite simple to mistake as these days there are very few GPS receivers that don't have Degrees Minutes and Seconds format as dDD° MM' SS.s" (to 1/10 of a second) so 27°37’272 could be input/accepted as 27°37.272' (DM.m)or 27°37’ 27.2" (DMS) without a problem. For sure with some older receivers and specifically some particular makes there were some limited to whole seconds but in today's environment whole seconds is an antique limitation.

Many GPS receivers also have a decimal degrees format such as dDD.ddddd° (5 decimal degrees) so what's stopping someone from entering 27°37’272 as 27.37272° ?? has been known to happen. So the available digits doesn't really reflect the format.


.... As even the best of GPS’s still contain errors of up to 50 metres +/-, the offending “third digit” is not all that critical.

Although it's probably better for users to think it could be 50 metres +/- than believe the claims of sub-metre or even worse accuracy within a "few feet" 50 metres is probably not the norm these days.

The system specifications are 13 metres (SIS) 95% of the time world average with worst case scenario (down to 22 satellites) at 36 metres 95% of the time. The system specifications are based on a 24 satellites constellation and currently there are 28 on-line.

Most recreational receivers probably fit into the less than 7 metres some of the time, less than 10 metres the majority of the time and less than 15-20 metres most of the time but not all of the time as there's the other 5% of the time where accuracy "can be" bad, sometimes real bad, at times even tens of kilometres. It's this "other" 5% that one doesn't want to want to occur when say running the beacons in the dark solely on GPS. #

So in affect users who simply drop the last digit for decimal minute inputs "could" introduce a position error several times greater than generally available accuracy. Even rounding 3 decimal minutes to 2 decimal minutes has the possibility of around 13 metres, which is equal to the system's accuracy specifications. #


Cheers, Kerry.

PS that 1nm=1832 metres should be 1852 metres.

[MulletMan]

If they weren't confused with my explanation Kerry, they sure as shit are now after viewing yours! Good pick up on the typo in 1832 though! My sloppiness must cease immediately!

[Kerry]

Well sometimes one thinks long and hard as to if trying to spell things out as they are will confuse further or not confuse.

But generally what the hell, can only tell it how it is and if anybody wants to dispute that then lets do that.

Cheers, Kerry.

[sharkbait]

OK - my GPS has an 'accuracy' meter (for want of a better way to describe it) which when I am on moreton bay usually reads about 4 - 15m.

Is this accurate? Say, given that my GPS is programmed for Brisbane's time zone - and accuracy is at 10m, does that mean I am within 10m of the co-ordinates currently being displayed?

And in the other scenario - If I punch in a mark that I want to go to (and given datum etc are the same) does that mean when I come up on the mark, I am within 10m of the co-ordinates I punched in, or is it 10m plus the 50+/- that kerry mentions?

Cheers, Kris

[blaze]

use your gps to get you somewhere near the mark and then use your sounder, you may find a better spot
cheers
blaze

[sharkbait]

lol ok blaze ;D

Maybe I should clarify that these questions are more out of interest than anything to do with fishing or not being able to find a particular spot.

cheers, Kris.

[Kerry]

Kris, that 'accuracy' meter as you term it doesn't indicate accuracy (as such) but rather an Estimate of Position Error and the key word is "estimate" as acucracy can only be determined relative to an actual known position and really a GPS receiver doesn't actually "know" where it is, just where it thinks it could be as there's influences that this value simply can not factor into the Estimate.

Estimated Position Error (EPE) is not a standard and manufacturers implement this in their own propriety way and then can differ between different models of the same make.

Basically this estimate should only be used a a relative guide and not as an absolute indication of accuracy. So in an absolute sense just because it shows 10 metres, doesn't "necessarily mean" you are within 10 metres of the the true coordinates. If it was showing 10 metres then went to 8 then 7 over a short period of time one might assume that the integrity/confidence of the position fix improved during this time, not that your position is accurate to 7 metres.

Estimated Position Error values are based on several factors, some are standard but others are propriety and manufacturers don't publish their methods.

When Selective Availability (SA) was discontinued in May 2000 just about all manufacturers were caught out as EPE implementations defined pre SA didn't fit a system without SA. In fact the moment SA was set to zero the real accuracy from one particular make/model improved by a factor of 7 and yet EPE only chamged by a factor of 2. Sort of indicated that the EPE was a bit of a rort and actually meant very little.

Cheers, Kerry.

[Kerry]

This is an Accuracy v EPE comparison from a day in about March 2003. I suppose one thing that stands out is the fairly flat consistency of EPE relative to actual accuracy. Metres wise it's not a great deal but % wise it is quite a bit and as one might guess indicates manufacturers tend to make things sound better than they actually are.

Being a common garden variety recrational GPS, I suppose the other thing is the actual accuracy ~ 6 metres @ 95%, which isn't bad considering the system specifications but then this is one day and all days can be different. However don't expect all days to be like this especially in a mobile situation.

http://cqnet.com.au/~user/mattk/gps_x/epe1153.jpg

Cheers, Kerry.

[Lucky_Phill]

OK.

I have done a ' Manual " test.

GPS ( Garmin 120 )in( AGD66 ) in boat in driveway, gave me a position.

GPS ( Garmin 126 ) in ( WGS84 ) in boat in driveway, gave me a position.

The difference was 104 Lat and 067 Long.

So I'll have to stand by me original statement. Now that driveway is located somewhat above sea level, and West of Morteon Bay.

I'll stick to this equation when doing my conversions.

Bloody good info Kerry, and jeez, doesn't it get technical.

And at the end of the day, if you are 6 meters away or 600, does it really matter ? cause GPS marks do Never gaurantee fish.

Cheers Phill

[Kerry]

Phil, Maybe your driveway moved ;D but then that's not as funny as it sounds as it is in fact "moving" but not the difference between the actual difference and what some of the generic AGD66/WGS84 receiver conversions reckon. Basically all receivers only use 1 (one) formula to cover the entire country but it's not the same across the country so it's a bit of give and take in the way receivers compute this difference.

Adopting your own conversion, the difference is approx twice the systems accuracy capability.

Australia is "close" enough to get away with only one conversion option but take a look at the U.S and they require something like 11 "different" options all for the ONE (NAD27) datum shift.

It's your choice you can stick to an average that many receivers give or you can use the proper values. This is one of the positives when using WGS84, there isn't any issues with conversions.

No it takes a lot more than GPS marks to catch fish but the following, which occured this year is a little bit more than 6 or 600 metres for that matter, how does over 20 odd kilometres sound :-X and this occured over 2 days 8).

THAT OTHER 5 PERCENT (http://cqnet.com.au/~user/aitken/gps_x/GPS%20The%20other%205%20percent.htm)


Cheers, Kerry.