I'm working on a usability study of an interface that shows a globe. You're currently able to click, hold, and drag the globe to move it around (i.e. view the other side of Earth). In testing, I've found users can do this intuitively just fine.
You're also able to zoom in, but to do so, you need to "Ctrl + left click and drag, or right click and drag." Users utterly failed at figuring out how to do this in testing, even when presented with written instructions on the screen. (It's also terribly trackpad unfriendly.)
So, my question is, has there been any research into the best practices or the best keyboard/mouse combos for how to zoom? My initial guess would be to model it off of Google Maps (and allow for double clicking, using the scrolly wheel, and having visible +/- buttons), since a good number of people are familiar with that app, but I'd prefer to point to something more academically based.
Also, note that this is not a web app; it's standalone downloadable software.