You can add geocodes to your existing photos with a nifty new Mac application called HoudahGeo. What's interesting about HoudahGeo, is that if you have a GPS device, you can provide a track log file from the receiver and have the application add the data to your photos. What's really helpful though, is that you can add the geocoding yourself with a user-friendly interface that requires you only to point to a location on a provided map.
I tested HoudahGeo by having it add geodata to a handful of pictures I have, then I opened the images in iPhoto and looked at their EXIF data in the Get Info box. Sure enough, the latitude and longitude information was there in Get Info.
You can download a trial version right now, and if you like it, can buy for $24.95.
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