Old coordinates need their original datum
NAD27 (EPSG:4267) uses the Clarke 1866 ellipsoid. A latitude and longitude copied from an old US map is not necessarily a WGS84 GPS position. Enter latitude first and longitude second, with west longitudes negative. Decimal degrees and degrees, minutes and seconds are both accepted.
This page handles geographic NAD27. For easting and northing in a State Plane zone, use the NAD27 State Plane page and retain the original units and zone.
NADCON grids and accuracy
For covered points in the contiguous US and Alaska, the converter downloads the NOAA NADCON grid and recalculates the result. The catalogued grid accuracy is 0.15 m in CONUS and 0.5 m in Alaska. Without a grid, the fallback is a three-translation approximation rated at 10 m or 12 m respectively.
Published NGS coordinate pairs tested in six states differ by up to 0.28 m. Some Anchorage controls differ by up to 0.89 m, exceeding the Alaska figure; that limitation remains recorded in the tests. These are NADCON 2.1 grids, not NADCON5, and their accuracy does not include every WGS84 realization or epoch difference.
Check coverage before using the result
The implemented grids concern US coverage; a NAD27 label on Canadian or Mexican data does not establish that the same transformation applies. Source: NOAA NGS NADCON.
Frequently asked questions
Is NAD27 the same as NAD83?
No. NAD83 uses a different geocentric datum. Relabelling NAD27 coordinates does not transform them; a grid or suitable regional transformation is required.