As far as I understand, altitude is a tricky business. (https://support.wikitude.com/support/discussions/topics/5000078080).
Since my app will cover a completely flat area (I live in the Netherlands), I would like to fix the user's altitude as well as that of the POIs, so a near POI will always appear at the top of the screen/at eye height.
The closest I get to an answer is in the above post. I would really appreciate some sample code and directions to the class/file that I should change to establish such.
Thank you so much for your consideration.
I'm sorry this answer took as long as it did. It seems your issue somehow fell through the cracks.
What you are doing should be easily achieved by not setting an altitude at all for your AR.GeoLocation instances. The altitude will then default to the AR.CONST.UNKNOWN_ALTITUDE value, causing your POIs to appear at the device height by default.
Here is the relevant reference page.