These are measured from the top of the casing down to the top of the water level. The height of the casing is subtracted from the number.

Most are measured using an e-line.

This map is interactive. Click on a spot/well and you'll get a popup with data. Sometimes there will be more than one page for that for that well so notice in the upper right hand covern of the pop-up if it is 1 of 2, for example.

The link to a datasheet listing historic levels is the very last item on the popup.