Last summer I finally realized the joys of iPhone ownership. The iPhone 3GS I purchased has built-in GPS and a proper magnetic compass that appears to work well enough. Yes, the GPS is not the best. It would have been nice to see a WAAS-enabled SiRF StarIII (or StarIV) chip in there. Regardless, it works pretty darn well and has proven to be sensitive enough for offroad trail use with a significant tree canopy.
The positional accuracy of the iPhone GPS appears to be fairly good. Using the MotionX-GPS app (more about that in a moment) on my iPhone I captured a track and viewed it in Google Earth. I compared the captured track with a .kml overlay of GDOT roads, the NAVTEQ streetbase found in GE, and the GE (coarse) aerial imagery. In my unscientific eyeball and onscreen measurement test, the typical error I found for the captured track was in the 20-40ft range, based on the position relative to the road visible in the imagery. I'm not certain about the positional accuracy of the imagery. The NAVTEQ data appears to be quite close to the road centerline visible in the imagery, while the GDOT roads were generally worse, positionally speaking. The captured track had errors as great as 300ft in a couple areas, 50-100ft in others. For the most part, however, the MotionX-GPS captured track was well under 50ft compared to the street centerline viewed on the imagery.
After testing a number of GPS apps, the one I finally settled on is the MotionX-GPS app by Fullpower. As of 1/17/10, it is only $2.99 on the App Store. I originally intended to purchase a newer, mid-grade GPS device to take on the trails. I was due for a modern, updated device with plenty of data storage after owning a couple older consumer GPS units. While the MotionX-GPS does not replace a new rugged handheld GPS for hiking or serious technical use, it does a fine job in a vehicle. The tracks and waypoints found on GeorgiaOverland are captured using MotionX-GPS and a 3GS iPhone. Read up on the app, see what you think. A couple highlights that I have found useful is the pre-caching of imagery/road tiles for use in the field and the ability to import .gpx files and export .kmz and .gpx files. In addition to an editable description (plus time, date, and more), the Tracks and Waypoints can include a photo, captured by the iPhone. The only drawback I see is that MotionX does not allow a full resolution photo to be attached to the location as it automatically saves the photo in a lower resolution.
Overall, MotionX-GPS is a capable app, and at only $2.99 it is a great value.