If I use my iPhone navigation app overseas will it use data and if so what are the cost likely to be ?

If I use my iPhone navigation app overseas will it use data and if so what are the cost likely to be ?

It depends. Which app is it?

Expensive ($30+) GPS navigation apps where you have pre-purchased a country map should not need to stream map data off the internet, but the safest way to use them would be to put your phone into airplane mode to disable the possibility of it using data.

The GPS positioning signal is received from satellites, independently of the mobile network, so as long as the GPS app has a map built in, it should not need to use the internet.

Some of the cheaper or free apps do stream map data from the internet though -- especially the in-built Maps app that comes with the iPhone, so don't use them while you're in global roaming mode.

Global roaming is outrageously expensive: Vodafone charges $10 per megabyte, Telstra $15 and Optus $20. All the carriers offer some discount plans if you pre-purchase megabytes in bulk before you go.

A much better option is to use a prepaid SIM card at your destination, or a special global roaming SIM card with cheap data. We've got many articles on AusBT covering good SIM cards for travelling -- but picking the best one depends on where you're going. If you can reply with further info on the countries you're visiting, I can make some more specific recommendations.

Following up on Dan's advice...

Airplane Mode also turns off GPS, so if you want to just turn off the data, go to Settings -> General -> Network and turn off the "Cellular Data" option.

(This is also the place to find the "Enable 3G" option, which is itself a nifty hack for reducing the amount of data you use if you need to turn it on. Turning off 3G while using data means that (a) your battery life will improve, (b) you will get maps, but sloooooowly, and (c) you'll have the chance to spot enormous emails before they've downloaded 10MB attachments over your data pipe.)

Having just had the irritating experience of having to wait two days for my French mobile network operator to register my phone and switch on mobile Internet (seriously), I also used my fallback maps tactic: find somewhere that has free wifi, set your route, zoom into it at a scale that makes sense to your trip, and then leave. The iPhone's Maps app stores a limited amount of map data in memory, which means you'll be able to follow along. Be careful you don't reset the route by mistake, though!

if you know where you're going but just need a local map for the immediate area you can actually browse the map while connected elsewhere via Wi-Fi, scroll around the area you need to cache the map data, once you're at the location you can view the maps and track that same immediate location without a data connection, have done this in countries where I didn't have a local SIM at the time and it did the job