Coordinate systems and vertical datums

Most accuracy confusion comes down to datums. Your data can be geometrically perfect and still read as 'off by a constant' if the vertical reference isn't what you expected.

Horizontal vs. vertical

The horizontal coordinate system places you on the map (latitude/longitude or a projected grid). The vertical datum defines what 'elevation zero' means — and that's where the surprises live.

Ellipsoidal vs. orthometric

Raw GPS/RTK heights are ellipsoidal — measured from a mathematical model of the earth. Site and civil work almost always want orthometric height, which relates to mean sea level. The two differ by a regional offset, so ellipsoidal elevations look shifted by a roughly constant amount — about 90 feet in Florida, and a different figure everywhere else.

Choosing your vertical datum

You choose the vertical datum when you upload. Orthometric heights are not a North-America-only feature:

• EGM2008 — the global geoid. Works anywhere in the world, and the usual choice outside North America.
• NAVD88 (GEOID18) — the US vertical datum, and what survey work in the United States is specified against.
• EGM96 — an older global geoid. Choose it only when a client or an existing dataset requires it.

Which options you're offered depends on where your site is. DroneFlow measures which datums actually resolve at your coordinates instead of assuming a region — NAVD88 reaches Alaska and northern Mexico, for instance, but not Calgary, Hawaii or Puerto Rico. You're shown the datums that genuinely work for your project, each with the offset it applies at that location.

How DroneFlow handles it

Elevations are delivered on the datum you chose, so contours, cut/fill, and checkpoints line up with your survey. DroneFlow also probes what the data actually arrived on rather than trusting the setting, and labels every deliverable with the result. If elevations look uniformly high or low, a datum mismatch is the first thing to check — not a processing error.