Google Maps co ordinates

[Giffo]

I have got these co ordinates off Goggle maps and they are different from how they are setup on my Garmin.

-25° 1' 37.62", +152° 31' 18.03"




I have had a look at the setup on my GPS and which one is more accurate

hddd mm.mmm' or

hddd mm'ss.s"

[BarryJ]

I wouldn't expect one or the other to be more accurate; they are just different ways of expressing the same thing. If you are getting differences between Google Earth & your GPS, it is probably because they are using different datums.

[Giffo]

A bit like saying Doug is wrong and Doug don't know sh!t,the same thing, expressed different ways !laugh1laugh1

[Pirate Pete]

A bit like saying Doug is wrong and Doug don't know sh!t,the same thing, expressed different ways !

laugh1 laugh1 laugh1

[BarryJ]

A bit like saying Doug is wrong and Doug don't know sh!t,the same thing, expressed different ways !laugh1laugh1
Got it in one, Giffo. 2thumbsup laugh1

[Giffo]

Awaiting reply from the Noosa Bypass (Giffo piles up the sand bags)laugh1

[Dave]

even with the trip to the plumbers.. he is still slow to reload... poor Mrs Doug

[Doug]

What? Did someone speak?

No, it was just The Three Stooges - two Tasmanians and a Souff Orstralian who has been exported.

No-one important. :D:D:D

[Giffo]

What about the bloke from the sugar town Doug ?

[Doug]

You mean the Candy-Floss-Firie?

I was ignoring him, naturally.

[Keithy P]

Even when operating with a pair of retread eyes a zipper up his trunk, Dougs still sharper then a pimp on payday. pointlaugh

[Doug]

"Even when operating with a pair of retread eyes a zipper up his trunk, Doug's still sharper then a pimp on payday. pointlaugh "

Does that surprise you? I'm an officer, after all!

[BakerBill]

The latest version of Google Earth has four lat/long types, the older version only had three. I use Degrees, Decimal Minutes. Occasionally the zeros can be a hassle, but generally this is the same as my GPS to enter points. If I get any new marks I usually put them in to find out exactly where they are. Some have ended up being on dry land, so a handy tool when you get used to it. It is also good to work out distances for trips on the water.

[Giffo]

Thanks for the info BakerBill,I will look at Google earths settings.See Doug,a most useful reply,you could do this from time to time laugh1laugh1

[BarryJ]

I have been searching Google Earth Help trying to determine what map datum they use and finally found it:
What Is a Datum? While a projection is used in mapping to define the earth on a flat surface, a datum is used to describe the actual shape of the earth in mathematical terms. This is because the earth's surface is not perfectly round, but ellipsoid in shape. A datum also defines the association of latitude and longitude coordinates to points on the surface of the earth, and defines the basis for elevation measurements.
As with projections, there is more than one mathematical interpretation of the earth's shape. Google Earth uses WGS84 datum.

If your GPS is using a different map datum, there is likely to be some sort of variation between the two (can be of the order of 200-300m).

[Doug]

"See Doug,a most useful reply,you could do this from time to time "

Ok, I'll give it a try......

WGS84 is the default datum setting on all three of my GPS (one nautical and two hand-held land-based ones).

Did it work?

[BakerBill]

Perhaps to clarify a little. Using Degrees, Decimal Minutes as previoulsy stated. Google Earth shows a local wreck as 33(o for degree)3.270'S and 137(o for degree)59.800'E In my GPS it would show as S 33.03.270 and E 137.59.800 If you put a zero in front of the middle 3 in the south co-ord, it dissapears once you hit enter or ok in Google Earth. It just takes a little playing and checking to get used to it. And what's more this co-ord is readily available on more than one site on the net, so bad luck guys, no secret revealed here.............lol

[Keithy P]

Heya Giffo and all,

I meant to say the other day,

The other thing to remember when dealing with digital co ordinate systems is that at about 30 degrees south, one degree= =/-100km so 0.001 degree gives you a precision of 100m? and 0.0001 degree = +/- 10m or so (I'm not that flash with math) where as one minute = 1.61km and one second= 26.77m. Thats precision, accuracy is a different thing all together, I dont know how accurate Google earth is but your GPS is gennerally =/- 10/15m if you are getting a differential position.

So basically, if google is only accurate to lets say 200m (I have no idea, it could be 2 km fo all I know but when you zoom right in it gets really boxy which means its not that accurate, but thats a whole other lesson) but if they are giving you a co ordinate in DD MM SS and one second has a prcision of 26.77m then thats a false precision... because its only accurate to 200m

So in summery, presuming you are using the same datum, which you probably are, the co ordinates you are getting off google are actually a position somewhere within the accuracy of google earth + presision of co ordinate + accuracy of your GPS. (200m+26.77m+ 10/15 m)

I think that makes sense... it may not... it may not even be exactly right, please correct me if I'm wrong.

But Doug, please dont pick on my speeling or 'Grammar: the public school system is to blame

[Giffo]

Thanks Keith,if I can get close to it, I can sound around and find what I am looking for.

[4.50_Drifter]

I use Ozi Explorer guys, it costs about $100 but then you can upload nautical charts and save as many spots as you like directly to your GPS. It's gold for marking rocks, reefs, wrecks etc.

If you have a larger boat you can even run Ozi as a chart plotter system direct from a laptop, however be sure to put the laptop in a pelican tough case with foam inserts and some breathing room.

Enjoy!

[BakerBill]

Certainly not disagreeing with the technical suff Keith, but it seems a lot closer than 200 metres. I've taken local creek entrances from Google Earth and put them into my GPS in boat. Most seem accurate within about 20metres. So i suggest it is up to the individual what they want to get out of it.

[Keithy P]

Yeah Bill as I say, I dont really know how accurate it is as I've never used it like that... I just stare at reefs longingly on it when the weathers bad... or at night time when there is nothing on the tele...

[Andy]

One thing I found with google is that the maps a flat, but the earth is round and bumpy! So in places you get larger errors than others. If you follow a straight strech of highway on google earth you will see places where they have attempted to correct this.

[DvrDve]

keithy 1 minute= 1863 m it will be
u r dealing with nautical miles not statutory(land miles)

1nm =1863m
1land mile= 1.6 ..m dont know after 1st decimal - i dont use em\
edit
ps doug any one can be an officer- they even made me one long long time ago when the dead sea was only sick

[Keithy P]

Yeah Andy, I hadn't thought about that. When I was in the Kimberlies, a mate was right into his geography and he overlaid the google veiw of the park we worked in with the corresponding topographical map. Both where flat projections of a mountainous surface so you would think you could just layer one over the other, but he had to stretch the topo map and distort it and it looked stupid. I didnt understand it at the time but that was probebly due to the datums they used as Barry mentioned. A dutum is a mathmatical representation of the shape of the earth, which they call a geoid. WGS84 (world geodetic system) is measured from the centre of the earth to best fit the shape of the globe where as AGD66/84 (australian geodetic datum) is what a lot of our paper maps in australia use and is best fit for the geiod in australia. The thing with all these digital mapping systems is that someone had to enter them into the system and convert them to the datum they are using, which is done mathmatically and also causes errors, and there are the errors that occur when the map is digitised which is why sometimes you see roads or rivers go all skewiff.

Any way.... something like that.....

DvrDve I think what I meant was that 1 minute longatude = 1.61km at 30 degrees south, just using that as an egsample as they get shorter and shorter as they go towards the poles.

to be honest I dont really know about miles or knots, they did tell us something about it at boat school but numbers go in one ear out the other.....

[DvrDve]

keithy
i believe that s why we use the the numbers running up and down the map North and south
not across E &W--- to read distances of/measure distances


they stay the same all across the map/world
1deg =60 nm
1min 1nm =1863 m
sadly i still work a lot with paper maps

[BarryJ]

...................... I didnt understand it at the time but that was probebly due to the datums they used as Barry mentioned. A dutum is a mathmatical representation of the shape of the earth, which they call a geoid. WGS84 (world geodetic system) is measured from the centre of the earth to best fit the shape of the globe where as AGD66/84 (australian geodetic datum) is what a lot of our paper maps in australia use and is best fit for the geiod in australia. ............................
AGD66 and AGD84 are two different datums from 1966 and 1984 respectively. As time goes on, the datums are refined as better technology allows refinement of the formula used in the mathematical representation. There are also different versions of the WGS datum depending on the year they were published.

You will even find variations between paper maps depending on the datum on which they are based so you need to check the map legend to determine which datum it uses; particularly if you are trying to relate what you see on paper to what you see on your GPS or Google Earth.