PDA

View Full Version : silly question (maybe?)



cmg
4th November 2005, 06:03 AM
Im pretty sure I know the answer to this question, but just need someone to back me up who knows what they are doing.

When Im given some latitude/longitude figures, and I put them into Oziexplorer, Im guessing that the map datum doesnt matter, ie: lat/long figures are going to be the same spot (eg: a carpark) on a map no matter which map datum is used.

mmmmmm, not sure if that makes sense :confused: , how about this,

Joe Bloggs gives me a lat/long figures from a waypoint, but he is using map datum X, I takes those figures and plug them in to Ozi using map datum Y, is the waypoint still going to be correct?

I figure it is, but not 100% sure

arthurking83
4th November 2005, 06:13 AM
Map Datum is very important, as the co-ordinates will be off by hundreds of meters!!

Pluto
4th November 2005, 07:44 AM
...Joe Bloggs gives me a lat/long figures from a waypoint, but he is using map datum X, I takes those figures and plug them in to Ozi using map datum Y, is the waypoint still going to be correct?...Wrong!

Waypoints that are entered using the wrong datum can be hundereds of metres out. If the coordinates are supplied without the datum, you need to chase down the supplier and ask them.

When manually entering a waypoint into OziExplorer, it's wise to check the datum.

MarkkyMarkk
4th November 2005, 08:48 AM
Lat/Longs are not absolute, they are dependant on the datum being used. A single location on earth will have different lat/longs for each & every datum.

The datum is a mathematical model (spheroid - a fattened sphere) of the world that is positioned & sized in space to accurately represent the earth's surface. Some datums, such as AGD66 & AGD84, are created to be used only for a particular section of the world (in this case Australia) and if used for other parts of the world, can be extremely inaccurate. Other datums (WGS84 & GDA94) are designed to be worlwide, but there is a trade off in accuracy.

In fact, the difference between AGD66/84 & WGS84/GDA94 will put your position out by approx 200m in a NE direction, on the east coast of Aust.

So always check your datum when entering Lat/Longs! They are not all the same.

As for map projections.... (see cartography101.)

Mark.T

cmg
4th November 2005, 09:15 AM
in that case I will go back to the source and find out the original datum

thanks alot guys

mitchofsutho
4th November 2005, 02:12 PM
What the guys have said about datums is 100% correct. If you have problems tracking down your source for the datum the chances are (in Australia) that the datum is probably one of two AGD66 (AGD84 - similar) or GDA94 (WGS84 - near enough to the same in Aust.). If the locations are important, and you have some location coordinates to compare, you can attempt to identify the datum by entering some of the coordinates, giving both datums a go. That way you may find out which datum has been used. This is certainly not the best approach, but if you have no option, it's worth ago.