The Environment Agency uses a network of boreholes accross the country. Each one has its own datum - a height in metres fixed relative to mean sea level.
This height is expressed as metres above ordnance datum (mAOD).
The groundwater levels we provide for each borehole are all relative to its site datum.
How often are groundwater levels measured?
The data we collect is fed into this service once a day under normal conditions.
We increase this frequency whenever there's a greater risk of flooding.