A bit of an outlier, but worth mentioning, AR apps can also use just the device's GPS sensor to identify its location in the environment and use that information to annotate what is in view. I use the word annotate because GPS tracking is not as accurate as any of the techniques we have mentioned, so it wouldn't work for close-up views of objects. But it can work just fine, say, standing atop a mountain and holding your phone up to see the names of other peaks within the view or walking down a street to look up Yelp! reviews of restaurants within range. You can even use it for locating and capturing Pokémon.

As an introduction to developing for augmented reality, this book focuses on all kinds of target tracking. This way, each of our projects can be built using either handheld or eyewear AR devices. Where a project's user experience can be enhanced on a more advanced device or technique, we'll try to include suggestions and instructions for supporting that too.