The GeoJSON Format

February 15, 2018 4 minutes read

GIS • Notes

General notes and remarks

Abreviations of interest,

WKB - Well Known Binary.
WKT - Well Known Text.

Introduction

GeoJSON is a geospatial data interchange format based on JavaScript Object Notation (JSON). It defines several types of JSON objects and the manner in which they are combined to represent data about geographic features, their properties, and their spatial extents. GeoJSON uses a geographic coordinate reference system, World Geodetic System 1984, and units of decimal degrees.

It is specified by the technical note RFC7946[1][3]

Summary of representations

Encodes a variety of geographic data such as,

Geometry: A region of space. The geometry types are, * Point
- LineString
- Polygon
- MultiPoint
- MultiLineString
- MultiPolygon
- GeometryCollection
Feature: A spatially bounded entity. Features contain Geometry objects as defined above.
FeatureCollection: A list of Features. FeatureCollection contains Feature objects defined above.
ForeingMembers: GeoJSON allows the inclusion of extra object members not specified in [1]

The `Geometry`

Core concept: the positon

Fundamental geometry construct consisting of an array of numbers that we will call coordinates. There must be at lest two elements in the array: longitude and latitude or easting and northing. The third optional element is elevation or altitude.
All GeoJSON coordinates must be in the geographic coordinate reference system, using the World Geodetic System 1984 (WGS 84)[1].
The positions are core concepts in building,

One position for a Point geometry.
Array of positions for a LinearString or MultiPoint geometry.
Array of LinearString in case of a Polygon, or MultiLineString geometry
Array of Polygon coordinates in case of MultiPolygon.

A line between two positions is a straight Cartesian line.

Examples of Geometries

Point

The template is as follows,

{
	"type": "Point",
	"coordinates": [lon, lat]
}

Remember that lon and lat are in the WGS84 coordinate system. The following is a valid point,

     {
         "type": "Point",
         "coordinates": [100.0, 0.0]
     }

LineString

     {
         "type": "LineString",
         "coordinates": [
             [100.0, 0.0],
             [101.0, 1.0]
         ]
     }

Polygon

Consider the concept of linear ring. A linear ring is a closed LineString with four or more positions,

The first and last positions are equivalent, and they MUST contain identical values; their representation SHOULD also be identical.
A linear ring is the boundary of a surface or the boundary of a hole in a surface
A linear ring MUST follow the right-hand rule with respect to the area it bounds, i.e., exterior rings are counterclockwise, and holes are clockwise.

For type Polygon, the coordinates member MUST be an array of linear ring coordinate arrays.

No holes:

     {
         "type": "Polygon",
         "coordinates": [
             [
                 [100.0, 0.0],
                 [101.0, 0.0],
                 [101.0, 1.0],
                 [100.0, 1.0],
                 [100.0, 0.0]
             ]
         ]
     }

With holes:

     {
         "type": "Polygon",
         "coordinates": [
             [
                 [100.0, 0.0],
                 [101.0, 0.0],
                 [101.0, 1.0],
                 [100.0, 1.0],
                 [100.0, 0.0]
             ],
             [
                 [100.8, 0.8],
                 [100.8, 0.2],
                 [100.2, 0.2],
                 [100.2, 0.8],
                 [100.8, 0.8]
             ]
         ]
     }

MultiPolygon

For type MultiPolygon, the coordinates member is an array of Polygon coordinate arrays.

Crossing the antimeridian

In representing Features that cross the antimeridian[2] , it is adviced to split the geometries so that neither of them cross the antimeridian. For example like the following MultiLineString

   {
       "type": "MultiLineString",
       "coordinates": [
           [
               [170.0, 45.0], [180.0, 45.0]
           ], [
               [-180.0, 45.0], [-170.0, 45.0]
           ]
       ]
   }

`Feature`

A Feature object represents a spatially bounded thing. Every Feature object is a GeoJSON object no matter where it occurs in a GeoJSON text.

A Feature object has a "type" member with the value "Feature".
A Feature object has a member with the name "geometry" whose value SHALL be either a Geometry object as defined above or, in the case that the Feature is unlocated, a JSON null value.
A Feature object has a member with the name "properties". The value of the properties member is an object (any JSON object or a JSON null value).

The following example illustrates the way a Feature object is specified [3],

{
  "type": "Feature",
  "geometry": {
    "type": "Point",
    "coordinates": [125.6, 10.1]
  },
  "properties": {
    "name": "Dinagat Islands"
  }
}

Foreing Memebers

For the Feature below, we can specify a foreing member called title and the GeoJSON will still be valid[1].

   {
       "type": "Feature",
       "id": "f1",
       "geometry": {...},
       "properties": {...},
       "title": "Example Feature"
   }

Libraries that deal with GeoJSON

Another interesting resource for GeoJSON is geojson.io