How is gauge pressure defined?

Gauge pressure is defined as the pressure relative to the ambient atmospheric pressure. To understand this, it is important to start with the concept of absolute pressure, which is the total pressure exerted, including atmospheric pressure.

By subtracting 1 ATA (atmospheric pressure at sea level) from the absolute pressure, you determine the pressure above atmospheric pressure, which is what gauge pressure represents. This means that gauge pressure measures the pressure of a system, excluding any influence from the surrounding atmospheric pressure, allowing divers and engineers to understand the actual pressure being exerted that isn't affected by the weight of the atmosphere.

The other choices do not capture the definition of gauge pressure accurately. For instance, defining gauge pressure at the surface level does not account for how pressure varies with water depth, and expressing it in terms of absolute pressure minus sea level pressure is not standard terminology, as sea level pressure can vary due to weather conditions. Adding atmospheric pressure instead of subtracting it would also misrepresent the nature of gauge pressure.