NOAA is currently working on three major updates to vertical datums. They’re used for everything from nautical charts and flood maps to infrastructure planning. A datum is a reference system that starts at an agreed data point. You’re probably familiar with at least one horizontal datum — latitude and longitude. If you’ve been to a mountaintop and seen a sign with the distance above sea level, you’re also familiar with vertical datums, which measure heights. Here’s a rundown on what we’re updating and why.