Assistance please

[Mallard]

The attached is a description of 'Datum' from the help menu of Mapsource.

Datum—select from over 100 available map datums, allowing MapSource to easily match up with virtually any paper chart or map, or manually enter information for a user defined map datum.

Forgive my ignorance, but could anyone expand on this please and advise me:

a) what a datum is
b) what use is it to me.

 

[st13phil]

The attached is a description of 'Datum' from the help menu of Mapsource.

Datum—select from over 100 available map datums, allowing MapSource to easily match up with virtually any paper chart or map, or manually enter information for a user defined map datum.

Forgive my ignorance, but could anyone expand on this please and advise me:

a) what a datum is
b) what use is it to me.

All maps are a representation of physical reality and need a datum, which is the origin or zero point on the globe, from which all the map coordinates are defined. Different mapping standards use different map datums, or zero points.

For example, I could create a set of maps of the world with my house as the origin (the ST13Phil datum). You could also create a set of maps of the world with your house as the origin (the Mallard datum). If I were to give someone the coordinates of a place in terms of degrees north and degrees west using the ST13Phil datum, say N 2.345 and W 1.234, then this would represent a different physical location on the globe to N 2.345 / W 1.234 using the Mallard map datum (unless you lived in my house, which you don't).

So, when someone gives you a set of coordinates that identify a particular physical place (say, the Chip Shop down the road from me) then they also need to tell you the datum that they've used so you can use the combination of the datum and coordinates to correctly identify where my local chippy is.

HTH, Phil

 

[Mallard]

Thanks for that Phil. It's a pity Mapsource couldn't have explained that as well as you. Mine is set to WGS72 (whatever that is) - so, if I understand you correctly, there's no need to change it unless I wish to, or to agree with someone else.

 

[Taff]

Thanks for that Phil. It's a pity Mapsource couldn't have explained that as well as you. Mine is set to WGS72 .. if I understand you correctly, there's no need to change it unless I wish to..

Strange, because mine is set to WGS84. I assume this setting gets used far more than others.

Cracking answer though.

 

[DodgyGeeSer]

Strange, because mine is set to WGS84

That's because you don't live in Mallard's house.

Mine too is WGS84 (except when it's set to Ireland 1965 (Irish National Grid)..

 

[YosemitePaul]

The most useful Datum in this country is 'BNG' = British National Grid; ie. what we all use as a map reference.

Not sure about the whole Garmin range, but BNG is available at a software update on the 2720 and as standard on the 2820.

Paul