Topographic mapping.
Many Alaskan islands were never on NAD27 but rather on independent datums. NADCON conversion program provides transformation from St. Lawrence Island Datum to NAD83 (original 1986 realization) - making the transformation appear to user as if from NAD27.
Definition: JSON
{ "$schema": "https://proj.org/schemas/v0.7/projjson.schema.json", "type": "GeodeticReferenceFrame", "name": "St. Lawrence Island", "ellipsoid": { "name": "Clarke 1866", "semi_major_axis": 6378206.4, "semi_minor_axis": 6356583.8 }, "scope": "Topographic mapping.", "area": "United States (USA) - Alaska - St Lawrence Island.", "bbox": { "south_latitude": 62.89, "west_longitude": -171.97, "north_latitude": 63.84, "east_longitude": -168.59 }, "id": { "authority": "EPSG", "code": 6136 } }