ATGs use probes (sensors) to monitor both fuel and water height.  Most underground tanks will have some water at the bottom and regulations require that less than 1 inch of water is maintained in the tank at all times. Sensors are accurate to 1/1000 of an inch for level measurements. Sensors also measure temperature. It’s important to know the fuel temperature because fuel expands and contracts with temperature changes. The warmer the fuel, the larger the volume change. For example, a rise of one degree Fahrenheit in a 10,000-gallon gasoline tank will increase the volume by 7 gallons.
A sensor placed in the dry interstitial space or annual space of a double-walled underground storage tank checks for fluid that is either leaving the inner wall or entering from the outer wall.  A drop in the fluid level indicates that either the interstitial fluid has left the outer wall of the tank or mixed with the product in the inner wall.  Either situation indicates a leak, which the sensor is designed to detect.