Relative Sea Level Rise is the additive product of Vertical Land Movement (VLM, land moving up or down) and any change in the actual height of the sea surface (up or down). For a tide gauge, the measurements must be taken at the same place on the same structure (pier, dock, wharf): Sea Surface Height by a modern tide gauge and VLM by a Continuously Operating GPS station attached to the same structure. NOAA attributes the differences in RSLR rate to differences in the rate of VLM (in most cases, “negative VLM” or downward).