Digital elevation model - why does the scale have negative numbers?

[ChrisC]

I have been working on a project to map out a coral reef plot. In the plot we have ground control points with known distances which were recorded into PhotoScan. So the scale in the DEM that PhotoScan created goes from -1.03 meters to 1.22 meters (see attached pdf of the DEM).

I am wondering why the negative baseline? 

My wild guess is that since I could not tell PhotoScan what the lowest point is, then PhotoScan just arbitrarily picks a starting point and measures everything up or down relative to that point? Is that correct or is something else going on?

If anyone could shed some light on this I would greatly appreciate it!

--Chris