There are a few ways we directly measure sea level. One is tide gauge observations—simply measuring the height of a tide on a fixed marker. As the tide marker is attached to land, this provides a measurement of sea level relative to land. In some areas, the land might be rising (uplift) which would mean the local sea level would be falling faster than if there was no land motion. In other areas, the land might be subsiding, which would result in higher local sea-level rise.Another method is satellite altimetry, where satellites measure the actual height of the ocean surface.
By combining these two methods, scientists can put together a picture of average global sea level changes.
There Pinky.Now you can enjoy your well deserved red without fretting.