Coordinate System

A coordinate system turns a position into numbers. GIS uses two broad kinds, always on some datum. See Datum.

Geographic coordinates

Latitude and longitude, in degrees, measured on an ellipsoid. BLM's MLRS mining claim layer is stored this way: its metadata gives "GCS_North_American_1983," EPSG code 4269, with its angular unit in degrees. A degree is not a fixed ground distance, which is why areas and lengths are normally computed after projecting. See Projection.

Projected (grid) coordinates

NOAA's National Geodetic Survey (NGS) describes a map projection as "a mathematical transformation of latitudes and longitudes on the surface of a sphere or ellipsoid representing the Earth to grid coordinates (northing, easting or y, x values) on a plane." Two grids dominate U.S. work:

• UTM: USGS: "The UTM system consists of 60 zones, each 6-degrees of longitude in width. The zones are numbered 1-60, beginning at 180-degrees longitude and increasing to the east." Coordinates are in meters. USGS Professional Paper 1395 explains that the central meridian at the Equator is the origin, "with an x coordinate of 500,000 m."
• State Plane: NGS says the State Plane Coordinate System was "originally created in the 1930s to support surveying, engineering, and mapping activities throughout the U.S. and its territories," and its maps show "the 1927 and 1983 versions of SPCS." BLM's Manual of Surveying Instructions (2009) notes that "Townsite surveys, mineral surveys, highway surveys, and surveys for other engineering projects are typically made in feet."

USGS on UTM versus latitude and longitude: "One system is no more or less accurate than the other."

Order and labels

GeoJSON requires longitude before latitude (RFC 7946); other formats differ. A coordinate pair without its system, datum, units, and zone is not safe to use.

The PLSS has its own basis

The PLSS is laid out according to "the mean bearing referenced to the true meridian at the point of record," which BLM's Manual calls "a basis of bearing that is not orthogonal with reference to a plane surface" (chapter II). The Manual also warns that some local surveys only look like State Plane: "measurements and everything except the initial coordinate pair in the survey fail to actually be in the State plane system."

Why it matters in mining claim research

Mining claim data arrives in many systems: NAD 83 geographic coordinates in BLM layers, GPS readings, grid coordinates, and bearings tied to PLSS corners. Each has to be identified before the pieces are combined. The Manual's rule for survey records is a good rule for research too: coordinate data "will include the correct datum and adjustment date or epoch of the NSRS control used" (section 2-32).