GeoJSON

GeoJSON is a text format for geographic features, built on JavaScript Object Notation (JSON). Its current definition is RFC 7946, published by the Internet Engineering Task Force (IETF) in August 2016 as a Standards Track document. The abstract: "GeoJSON is a geospatial data interchange format based on JavaScript Object Notation (JSON)." It "uses a geographic coordinate reference system, World Geodetic System 1984, and units of decimal degrees."

The pieces

• Geometry types: seven of them, "Point", "MultiPoint", "LineString", "MultiLineString", "Polygon", "MultiPolygon", and "GeometryCollection".
• Feature: one geometry plus a "properties" member for attributes. RFC 7946 says a Feature object "represents a spatially bounded thing." A commonly used identifier "SHOULD be included" as a member named "id".
• FeatureCollection: an array of features. This is what most GeoJSON files hold.
• Media type: "application/geo+json".

Rules that trip people up

• Axis order: "The first two elements are longitude and latitude, or easting and northing, precisely in that order." Longitude comes first.
• One coordinate system: all coordinates use "the World Geodetic System 1984 (WGS 84) [WGS84] datum, with longitude and latitude units of decimal degrees." The 2008 specification allowed other systems through a "crs" member. RFC 7946 removed it because that approach "has proven to have interoperability issues." Other systems can be used only "where all involved parties have a prior arrangement."
• Polygons: each ring is "a closed LineString with four or more positions," with identical first and last positions. Exterior rings run counterclockwise and holes clockwise, though parsers "SHOULD NOT reject Polygons that do not follow the right-hand rule."
• Elevation: an optional third value is "the height in meters above or below the WGS 84 reference ellipsoid."

Where it shows up

BLM's public ArcGIS REST services for the PLSS and for MLRS land status list "geoJSON" among their supported query formats, alongside JSON and PBF.

Why it matters in mining claim research

BLM's MLRS mining claim layer is stored in geographic coordinates on NAD 83. GeoJSON is defined on WGS 84. NOAA's National Geodetic Survey lists them as separate systems: GRS 80 is the "Basis for NAD 83 and related datums," while WGS 84 is "Defined by the U.S. Department of Defense." Mining claim polygons moved into GeoJSON should be transformed, not simply relabeled, and the step noted. A swapped axis order is the other common error. It produces positions that are invalid or nowhere near the western states, which is easy to spot once someone looks. See Datum.