Works with decimal or DMS, like 40°17′24″N 80°14′24″W.
What is a datum?
A datum is the starting point for coordinates: a model of Earth's shape plus where it is pinned. Change the datum and the same fence post gets a different latitude and longitude. Mixing two datums is one of the most common reasons RTK points "miss" by a few feet.
The datums in the table
- NAD 27 is the old North American datum, pinned at a ranch in Kansas. Old maps and deeds may use it. It can be off from today's numbers by tens of yards (NGS: NAD 27).
- NAD 83 is today's official US datum. It has been updated several times: 1986, HARN (state by state in the 1990s), NSRS2007, and 2011. Most RTK networks use NAD 83(2011) (NGS: NAD 83).
- WGS 84 is GPS's own datum. It follows the whole Earth, while NAD 83 moves with the North American plate. So the gap between them grows a little every year.
Where the numbers come from
- NAD 27, NAD 83(1986), HARN and NSRS2007: NGS Coordinate Conversion and Transformation Tool (NCAT), asked live for your point (it uses NGS's NADCON 5 grids).
- WGS 84: EPSG:8970, "ITRF2014 to NAD83(2011) (1)", NGS's published 14-parameter transformation, run in your browser for today's date. We treat WGS 84 as matching ITRF2014, which is within a few centimeters for current WGS 84.
- All shifts are from NAD 83(2011) at your point to the other datum. NCAT also reports its own uncertainty for each shift.
What's next
NGS plans to replace NAD 83 with a new frame (NATRF2022) that will be much closer to WGS 84. Until then, NAD 83(2011) is the official one. See NGS: New Datums.
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