How do tide gauges measure sea-level change?

An acoustic tide gauge on the coast of Tasmania, Australia. Credit: © Commonwealth of Australia (Geoscience Australia) 2016. This product is released under the Creative Commons Attribution 4.0 International License

Tide gauges installed along coastlines, or on ocean platforms, rely on sensors to make a continuous record of water height, by the minute, hour, or day. These records are made in comparison to a reference point known as the geoid, or mean (average) sea level and its imagined extension over land areas.