Why does iPhone Maps app sometimes ask to enable Wi-Fi for "accuracy", even though Location Services (GPS) is enabled? Background:

I have an iPhone 3GS with iOS 5.1, and a 3G data plan that I keep enabled. I also keep "Location Services" (GPS) always on, since I use applications that take advantage of it.

However, I make a point of keeping Wi-Fi turned off because it's a battery sucker and I seldom use the phone within range of a base-station for which I'd be authorized &ndash; and I dislike the practice of hopping onto unsecured "free" networks, even briefly.

Scenario:

Sometimes when I need to look up an address or directions I use the Maps app.  When I launch the app for the first time in what seems a while, I see a dialog telling me I should "enable Wi-Fi to improve accuracy".

I forget the exact text for the dialog, but that's the general message and the buttons are "OK" and "Settings". ("OK" apparently meaning "I get it, now go away", as opposed to "OK, enable it.")  I find the dialog both irritating, and puzzling.  When I dismiss it, I still get an accurate location, far as I can tell!

So I'd like to know:


Why is it that Maps is telling me the location accuracy would be improved with Wi-Fi enabled?  Is there some problem with GPS that Wi-Fi can somehow compensate for?  I fail to see how this would be helpful.  Isn't GPS accurate enough?
In terms of dealing with the annoyance: Is there a way for me to silence that dialog once and for all?  I dislike continuing to acknowledge something I've been informed of dozens of times already.


Thanks!
The iPhone uses assisted GPS to improve the accuracy of the location system using GPS and cell towers. Wi-Fi networks are used to help determine the device's location; this is called a hybrid positioning system (XPS).

Prior to iOS 4 iOS tapped into the Skyhook Wireless database of Wi-Fi hotspots and their locations. Apple disclosed in April 2010 that they had switched over to their own proprietary database.


  If Location Services is on, your device will periodically send the geo-tagged locations of nearby Wi-Fi hotspots and cell towers in an anonymous and encrypted form to Apple, to augment the crowd-sourced database of Wi-Fi hotspot and cell tower locations. In addition, if you are traveling (for example, in a car) and Location Services is on, a GPS-enabled iOS device will also periodically send GPS locations and travel speed information in an anonymous and encrypted form to Apple, to be used for building up a crowd-sourced road traffic database. The crowd-sourced location data gathered by Apple does not personally identify you.


For your security concerns, read this Apple press release.