Drag'n'drop one of your photos in the right location along the track. (Try to place it as accurately in the time dimension as possible. For example, if the clock of your camera happened to be off by 10 seconds, find a location such that the GPS time stamp and photo time stamp differ by exactly 10 seconds in the right direction. Make sure you drop the photo so that it "snaps" onto the track.)

Then Aperture asks if you want to tag all other photos (see the bottom of the map screen). Press the button and it does exactly what you would expect it to do: it places all other photos with the same offset (difference between GPS time and camera time).

Notes:

• The automatic geotagging tool affects all matching photos in the entire project. It is therefore best to use it only in a newly created project that holds just photos from one session.
• It is a bad idea to mix photos taken with two different cameras unless you have synchronised the clocks (or fixed the time stamps after importing the photos).

A typical workflow:

1. Check the clock of the camera and compare it with the real time (e.g., GPS time) – preferably soon after the photo shoot. The clocks in cameras are lousy, and there may be a lot of clock drift; it can be difficult to guess what was the right time offset if you do geotagging several months after shooting the photos.
2. Import all photos to a new project. Import photos from only one camera.
3. Import all GPS tracks. Delete the tracks that you don't need, and then display all relevant tracks.
4. Use the automatic geotagging tool (= what I described above) and try to get everything as correct as possible. You can fine-tune the offset and try the automatic placement again, if necessary.
5. Do all manual geotagging (= fix GPS errors, add coordinates to photos that were taken when there was no GPS signal, etc.) only after that. Do not touch the automatic tagging tool ever again; you can otherwise easily lose your manual edits.
6. Do whatever you want with your photos. Merge the new project with an old project if you don't want to have a new project. Repeat for another camera, if needed.