Just noted a bug where the difference in altitude between the viewer and a location would not be taken into account. The bug is inside of ARHelper.DistanceBetween where an altitude difference of 0 is always used. That's fine in Texas, but not in other places. :)
 
Quesion is, how to deal with this? If we don't have a GPS lock we probably won't have altitude. Many data sources may have lat / long but no altitude. We could have a configuration property called AltitudeCalculation with options:
 
Always - Always calculate the difference
Never - Pretend the world is flat
Auto - If ARItem and user location both have altitudes, calculate difference. Otherwise assume "flat".