Map Datum wow what a differance

[GIN51E]

Well i just got my new GPS, Explorist 600 with the lot mapping software ect,

although i'm not going to take it out of the box until Tuesday i however did install the discover Aus mapping software, and thought i would save a few waypoints out of my old GPS i punched in the cords and wow it was saying my house was in the middle of the national park, now this got me very depressed as whats the point of mapping software if its not acurate then i thought i would check the map datum and changed the GPS to WGS84 and bang it now gives the correct location for my house where as before i had it on AUS84 now i thought AUS84 would be best as i am in australia but gee why such a difference why not just have one standard map datum and cord system for everyone to use wouldn't that make life much easier? anyone i guess i will have to stick with WGS84 as that is what the mapping software appears to be in but what is the most common map datum used and why?


also might be a dumb question but i always wanted to know what NMEA Rate is? can't find it in the manual so whats it there for?

[The Garner Family]

WGS84 is used as the standard datum for geocaching. You may find other applications may use different datums.

[EcoTeam]

The Sydney street directories all still use AGD66 - really annoying.
I believe the Melbourne Sydway has switched to WGS84(GDA84)

EcoDave

[riblit]

also might be a dumb question but i always wanted to know what NMEA Rate is? can't find it in the manual so whats it there for?

You can read about it <a href="http://vancouver-webpages.com/peter/nme ... t">here</a>

[riblit]

... but gee why such a difference why not just have one standard map datum and cord system for everyone to use wouldn't that make life much easier? anyone i guess i will have to stick with WGS84 as that is what the mapping software appears to be in but what is the most common map datum used and why?...?

<br>

Some answers are <a href="http://www.colorado.edu/geography/gcraf ... l">here</a>

[Chwiliwr]

There is another explanation here that is more specific to Australia and explains the shift to GDA94 (Not GDA84). The reference to AUS84, if it was not a typo, is probably to the datum AGD84 which superseeded AGD66 in some but not all Australian states before all the states adopted GDA94 some years ago.

[alex]

Very basically, AUS66 was a standard adopted by federal and state authorities in 1966 to define a shape of the world which was the 'best fit' for Australia. AUS84 was a refinement of this in 1984. The Global Positioning System uses a world 'best fit' which, coincidentally, was also defined in 1984, hence WGS84.

Clear as mud, but it covers the ground.

[Chwiliwr]

Having never owned a portable GPS I didn't understand the references to AUS66 and AUS84 as they didn't appear to be datums. They obviously, to me now, refer to the AGD66 and AGD84 datums on the GPS and some of the sofware I have looked at over the last few days. The GIS software (MapInfo) that I use only refers to the true datum names. (The NAVMAN bluetooth GPS connected to the IPAQ that I use is only ever in WGS84 datum and I have to use the GIS software to convert coordinates from one datum to another if necessary)

Also it is interesting to note that the AGD66 datum, and a number of the other countries datums, were defined from a particular point on the earth's surface. In the case of AGD66 it was from the Johnson Geodetic Station.

The WGS84 datum is basically the orbital centre of the 24 GPS satellites.

Looking back at the posts I don't think we have quite answered the original post properly.

Part of the questions was 'what is the the most common map datum used and why?'

If we talk about GPS and caching then their answer is WGS84 as that is the one the satellites and the geocaching web sites use.

However if they start using a physical map for any reason then they will have to adopt the the datum that the map uses if it is anything other than GDA94 as that is the only way they can relate their GPS position to the map.

However if the physical map uses the GDA94 datum they don't have to change their GPS datum as for all practical purposes GDA94 and WGS84 are the same for anything measured with a portable GPS.

[Kerry]

It's really only since the advent of GPS that geocentric type datums have been able to be better defined and refined. Even WGS84 has been 're-accessed" and effectively re-aligned several times since first defined.

Really there is no "common map" datum (as such yet) but in time most countries are heading towards a compatible geocentric datum as it makes life so much easier and a lot less confusing. These compatible datums might be called different things (WGS84, NAD83, GDA94 etc) in different countries but for all intents and purposes they achieve the same purpose.

In the meantime one simply has to match the datum with the GPS/Chart etc and this will probably need to be considered for quite a few years to come.

Cheers, Kerry.

[LazyLeopard]

You should see the puzzled looks on tourists' faces as they stand on the Meridian at the Royal Observatory at Greenwich and peer at their GPS receivers. WGS84 is about 200 feet off...