Simple Features

Simple Features is a 2D geometry library that provides Go types that model geometries, as well as algorithms that operate on them.

It's a pure Go Implementation of the OpenGIS Consortium's Simple Feature Access Specification (which can be found here). This is the same specification that GEOS, JTS, and PostGIS implement, so the Simple Features API will be familiar to developers who have used those libraries before.

Table of Contents

• Geometry Types
• Marshalling and Unmarshalling
• Geometry Algorithms
• GEOS Wrapper
• Examples

Geometry Types

Type	Example	Description
Point		Point is a single location in space.
MultiPoint		MultiPoint is collection of points in space.
LineString		LineString is curve defined by linear interpolation between a set of control points.
MultiLineString		MultiLineString is a collection of LineStrings.
Polygon		Polygon is a planar surface geometry that bounds some area. It may have holes.
MultiPolygon		MultiPolygon is collection of Polygons (with some constraints on how the Polygons interact with each other).
GeometryCollection		GeometryCollection is an unconstrained collection of geometries.
Geometry		Geometry holds any type of geometry (Point, MultiPoint, LineString, MultiLineString, Polygon, MultiPolygon, or GeometryCollection). It's the type that the Simple Features library uses when it needs to represent geometries in a generic way.
Envelope		Envelope is an axis aligned bounding box typically used to describe the spatial extent of other geometric entities.

Marshalling and Unmarshalling

Simple features supports the following external geometry representation formats:

Format	Example	Description
WKT	POLYGON((0 0,0 1,1 1,1 0,0 0))	Well Known Text is a human readable format for storing geometries. It's often the lowest common denominator geometry format, and is useful for integration with other GIS applications.
WKB	<binary>	Well Known Binary is a machine readable format that is efficient for computers to use (both from a processing and storage space perspective). WKB is a good choice for transferring geometries to and from PostGIS and other databases that support geometric types.
GeoJSON	{"type":"Polygon","coordinates":[[[0,0],[0,1],[1,1],[1,0],[0,0]]]}	GeoJSON represents geometries in a similar way to WKB, but is based on the JSON format. This makes it ideal to use with web APIs or other situations where JSON would normally be used.
TWKB	<binary>	Tiny Well Known Binary is a multi-purpose compressed binary format for serialising vector geometries into a stream of bytes. It emphasises minimising the size of the serialised representation. It's a good choice when space is at a premium (e.g. for storage within a web token).

Geometry Algorithms

The following algorithms are supported:

Miscellaneous Algorithms	Description
Area	Finds the area of the geometry (for Polygons and MultiPolygons).
Centroid	Finds the centroid of the geometry.
ConvexHull	Finds the convex hull of the geometry.
Distance	Finds the shortest distance between two geometries.
Envelope	Finds the smallest axis-aligned bounding-box that surrounds the geometry.
ExactEquals	Determines if two geometries are structurally equal.
Length	Finds the length of the geometry (for LineStrings and MultiLineStrings).
PointOnSurface	Finds a point that lies inside the geometry.
Relate	Calculates the DE-9IM intersection describing the relationship between two geometries.
Simplify	Simplifies a geometry using the Ramer–Douglas–Peucker algorithm.

Set Operations	Description
Union	Joins the parts from two geometries together.
Intersection	Finds the parts of two geometries that are in common.
Difference	Finds the parts of a geometry that are not also part of another geometry.
SymmetricDifference	Finds the parts of two geometries that are not in common.

Named Spatial Predicates	Description
Equals	Determines if two geometries are topologically equal.
Intersects	Determines if two geometries intersect with each other.
Disjoint	Determines if two geometries have no common points.
Contains	Determines if one geometry contains another.
CoveredBy	Determines if one geometry is covered by another.
Covers	Determines if one geometry covers another.
Overlaps	Determines if one geometry overlaps another.
Touches	Determines if one geometry touches another.
Within	Determines if one geometry is within another.
Crosses	Determines if one geometry crosses another.

GEOS Wrapper

A GEOS CGO wrapper is also provided, giving access to functionality not yet implemented natively in Go. The wrapper is implemented in a separate package, meaning that library users who don't need this additional functionality don't need to expose themselves to CGO.

Examples

The following examples show some common operations (errors are omitted for brevity).

WKT

Encoding and decoding WKT:

// Unmarshal from WKT
input := "POLYGON((0 0,0 1,1 1,1 0,0 0))"
g, _ := geom.UnmarshalWKT(input)

// Marshal to WKT
output := g.AsText()
fmt.Println(output) // Prints: POLYGON((0 0,0 1,1 1,1 0,0 0))

WKB

Encoding and decoding WKB directly:

// Marshal as WKB
coords := geom.Coordinates{XY: geom.XY{1.5, 2.5}}
pt := geom.NewPoint(coords)
wkb := pt.AsBinary()
fmt.Println(wkb) // Prints: [1 1 0 0 0 0 0 0 0 0 0 248 63 0 0 0 0 0 0 4 64]

// Unmarshal from WKB
fromWKB, _ := geom.UnmarshalWKB(wkb)
fmt.Println(fromWKB.AsText()) // POINT(1.5 2.5)

Encoding and decoding WKB for integration with PostGIS:

db, _ := sql.Open("postgres", "postgres://...")

db.Exec(`
    CREATE TABLE my_table (
        my_geom geometry(geometry, 4326),
        population double precision
    )`,
)

// Insert our geometry and population data into PostGIS via WKB.
coords := geom.Coordinates{XY: geom.XY{-74.0, 40.7}}
nyc := geom.NewPoint(coords)
db.Exec(`
    INSERT INTO my_table
    (my_geom, population)
    VALUES (ST_GeomFromWKB($1, 4326), $2)`,
    nyc, 8.4e6,
)

// Get the geometry and population data back out of PostGIS via WKB.
var location geom.Geometry
var population float64
db.QueryRow(`
    SELECT ST_AsBinary(my_geom), population
    FROM my_table LIMIT 1`,
).Scan(&location, &population)
fmt.Println(location.AsText(), population) // Prints: POINT(-74 40.7) 8.4e+06

GeoJSON

Encoding and decoding GeoJSON directly:

// Unmarshal geometry from GeoJSON.
raw := `{"type":"Point","coordinates":[-74.0,40.7]}`
var g geom.Geometry
json.NewDecoder(strings.NewReader(raw)).Decode(&g)
fmt.Println(g.AsText()) // Prints: POINT(-74 40.7)

// Marshal back to GeoJSON.
enc := json.NewEncoder(os.Stdout)
enc.Encode(g) // Prints: {"type":"Point","coordinates":[-74,40.7]}

Geometries can also be part of larger structs:

type CityPopulation struct {
    Location   geom.Geometry `json:"loc"`
    Population int           `json:"pop"`
}

// Unmarshal geometry from GeoJSON.
raw := `{"loc":{"type":"Point","coordinates":[-74.0,40.7]},"pop":8400000}`
var v CityPopulation
json.NewDecoder(strings.NewReader(raw)).Decode(&v)
fmt.Println(v.Location.AsText()) // Prints: POINT(-74 40.7)
fmt.Println(v.Population)        // Prints: 8400000

// Marshal back to GeoJSON.
enc := json.NewEncoder(os.Stdout)
enc.Encode(v) // Prints: {"loc":{"type":"Point","coordinates":[-74,40.7]},"pop":8400000}