Climate Central’s sea level rise and coastal flood maps are based on peer-reviewed science in leading journals. As these maps incorporate big datasets, which always include some error, these maps should be regarded as screening tools to identify places that may require deeper investigation of risk.
Outside of the United States, maps are based on global-scale datasets for elevation, tides, and coastal flood likelihoods. Inside the U.S., more accurate but still imperfect data are used.
Areas lower than the selected water level and with an unobstructed path to the ocean are shaded red. By default, areas below the water level but that appear to be protected by ridges (and in the U.S., levees) are not shaded.
https://sealevel.climatecentral.org/maps/
The Short Answer: NASA measures sea level around the globe using satellites. The Jason-3 satellite uses radio waves and other instruments to measure the height of the ocean's surface – also known as sea level. It does this for the entire Earth every 10 days, studying how global sea level is changing over time.
Rising sea level is one of the clearest signs of global warming. It's also one of the biggest problems that global warming is causing.