From my observations it also seems to vary a lot depending on how the person recording behaves. Some people just walk around logging reading along the way. Others go out of the way to find interesting above background level radioactive objects and place the measurement equipment right next to those. Like finding 2 pieces of granite having 6x the background level in a giant pile of other rocks.

Another interesting pattern I saw while browsing around was increased level next to every fancy cast iron light fixture along a specific bridge. The bridge was made mid 20th century so I don't think they are old enough to have used thorium gas light mantles.

> From my observations it also seems to vary a lot depending on how the person recording behaves. Some people just walk around logging reading along the way.

I think if this is only what they allowed it would be a useful since it would measure real world exposure.