The Z values we report are measured from the top of the AeroPoints. This is because when they are used as ground control points and when manual processing is required we align images using pixels corresponding to the top of the AeroPoint. 

We recommend using this data as it is a correct reading of the surface level of that point in the corresponding images. 

If you need to know the height of the ground underneath the AeroPoint, you can subtract the thickness of the AeroPoint (32mm) from the survey report's height data by downloading the survey report in CSV or excel format  and reducing the "NAD83 (2011) Height (m)" (usually column G) and "NAVD88 Height (m)" (usually column J) values for each AeroPoint by 0.032.