Setting up your map datum
Your GPS settings should match the Grid & Datum of the map you are using – details of which should be on the map itself, it is very possible that local maps might be using different systems. In the UK we set it up for British National Grid.
You should take into consideration that many countries have/use several Grids & Map Datum’s.
Details of Map Datum being used by the mapmaker should be on the map itself, it doesn’t follow that these are being used by your mapmaker or are the settings for use in your GPS. The settings we suggest are the commonly used settings for that country.
Many countries use the UTM system (Universal Transverse Mercator) which is a worldwide standard for geo-referencing. The grid uses one kilometre steps with the earth being divided into 60 zones the first one around the Bearing Straits; even countries used to statute system (miles/feet) use it. The UPS system is the UTM equivalent near the poles.
I hope this helps