As you go about your trip, the app is capturing time and location information. At the end of your trip, you press an "Export" button to generate a 2D QR bar code, and then take a photo of the bar code with the same camera you've been using to take the photos. If you've captured a lot of images during your trip, the app may generate multiple QR codes, all of which you need to take photos of.
Next, you move your thousands of photos from the camera to a folder on your Mac or PC. Developer Michael Diguet has written a desktop app for Mac and Windows that is downloadable for free. What does the desktop app do? It goes through the images from your camera, grabs the QR bar codes, and then matches the time stamp in the EXIF photo information data to time and location information stored in the QR codes. Each image is then updated with the GPS information. At the end of the process, you can import the images with the GPS data into your favorite photography app.
In order to test-drive the app, I used while running some errands earlier in the week. My camera? A Canon PowerShot SX30IS "Super Zoom", with no built-in GPS capability. At each stop on my little trip, I took a picture, then shook the iPhone to capture the GPS information in GPS4CAM. The app vibrates the phone to let you know that it has done a GPS capture, so I was concerned when it vibrated twice (indicating a double capture) at a few stops. At the end of my errand run I "exported" the QR code and dutifully snapped a photo.
The GPS4CAM desktop app is quite minimalistic, just asking for the location of the input and output folders for your photos. After creating a couple of folders, moving the images off of my camera and into the input folder, and then letting the app do its work, it took less than a minute to geotag all of the images.
By the way, you can also look at maps of each trip -- that's shown in the main post image at the top of this review. A red dot indicates a location where that can be zoomed into to reveal other more precise locations, while a green dot is a single tag point.
After importing those photos into iPhoto, I was happy to see that they did properly reflect the locations of where the photos were taken. That's going to save my wife -- the person in our household who does most of the organization and publishing of our vacation photos -- a lot of time, since she will no longer have to manually enter locations.
GPS4CAM puts the photos of the QR codes into a special folder so you don't import them into iPhoto or Aperture, and also generates a .GPX (GPS eXchange Format) file if you want to throw that into Google Earth.
GPS4CAM works flawlessly and quickly, and is an excellent addition to the app library of any photographer with an iPhone.