orb 
Package orb defines a set of types for working with 2d geo and planar/projected geometric data in Golang. There are a set of sub-packages that use these types to do interesting things. They each provider their own README with extra info.
Interesting features
- Simple types - allow for natural operations using the
make,append,len,[s:e]builtins. - GeoJSON - support as part of the
geojsonsub-package. - Mapbox Vector Tile - encoding and decoding as part of the
encoding/mvtsub-package. - Direct to type from DB query results - by scanning WKB data directly into types.
- Rich set of sub-packages - including
clipping,simplifing,quadtreeand more.
Type definitions
type Point [2]float64
type MultiPoint []Point
type LineString []Point
type MultiLineString []LineString
type Ring LineString
type Polygon []Ring
type MultiPolygon []Polygon
type Collection []Geometry
type Bound struct { Min, Max Point }
Defining the types as slices allows them to be accessed in an idiomatic way using Go's built-in functions such at make, append, len and with slice notation like [s:e]. For example:
ls := make(orb.LineString, 0, 100)
ls = append(ls, orb.Point{1, 1})
point := ls[0]
Shared Geometry interface
All of the base types implement the orb.Geometry interface defined as:
type Geometry interface {
GeoJSONType() string
Dimensions() int // e.g. 0d, 1d, 2d
Bound() Bound
}
This interface is accepted by functions in the sub-packages which then act on the base types correctly. For example:
l := clip.Geometry(bound, geom)
will use the appropriate clipping algorithm depending on if the input is 1d or 2d, e.g. a orb.LineString or a orb.Polygon.
Only a few methods are defined directly on these type, for example Clone, Equal, GeoJSONType. Other operation that depend on geo vs. planar contexts are defined in the respective sub-package. For example:
-
Computing the geo distance between two point:
p1 := orb.Point{-72.796408, -45.407131} p2 := orb.Point{-72.688541, -45.384987} geo.Distance(p1, p2) -
Compute the planar area and centroid of a polygon:
poly := orb.Polygon{...} centroid, area := planar.CentroidArea(poly)
GeoJSON
The geojson sub-package implements Marshalling and Unmarshalling of GeoJSON data. Features are defined as:
type Feature struct {
ID interface{} `json:"id,omitempty"`
Type string `json:"type"`
Geometry orb.Geometry `json:"geometry"`
Properties Properties `json:"properties"`
}
Defining the geometry as an orb.Geometry interface along with sub-package functions accepting geometries allows them to work together to create easy to follow code. For example, clipping all the geometries in a collection:
fc, err := geojson.UnmarshalFeatureCollection(data)
for _, f := range fc {
f.Geometry = clip.Geometry(bound, f.Geometry)
}
Mapbox Vector Tiles
The encoding/mvt sub-package implements Marshalling and Unmarshalling MVT data. This package uses sets of geojson.FeatureCollection to define the layers, keyed by the layer name. For example:
collections := map[string]*geojson.FeatureCollection{}
// Convert to a layers object and project to tile coordinates.
layers := mvt.NewLayers(collections)
layers.ProjectToTile(maptile.New(x, y, z))
// In order to be used as source for MapboxGL geometries need to be clipped
// to max allowed extent. (uncomment next line)
// layers.Clip(mvt.MapboxGLDefaultExtentBound)
// Simplify the geometry now that it's in tile coordinate space.
layers.Simplify(simplify.DouglasPeucker(1.0))
// Depending on use-case remove empty geometry, those too small to be
// represented in this tile space.
// In this case lines shorter than 1, and areas smaller than 2.
layers.RemoveEmpty(1.0, 2.0)
// encoding using the Mapbox Vector Tile protobuf encoding.
data, err := layers.Marshal() // this data is NOT gzipped.
// Sometimes MVT data is stored and transfered gzip compressed. In that case:
data, err := layers.MarshalGzipped()
Decoding WKB from a database query
Geometries are usually returned from databases in WKB format. The encoding/wkb sub-package offers helpers to "scan" the data into the base types directly. For example:
row := db.QueryRow("SELECT ST_AsBinary(point_column) FROM postgis_table")
var p orb.Point
err := row.Scan(wkb.Scanner(&p))
db.Exec("INSERT INTO table (point_column) VALUES (ST_GeomFromWKB(?))", wkb.Value(p))
Scanning directly from MySQL columns is supported. By default MySQL returns geometry data as WKB but prefixed with a 4 byte SRID. To support this, if the data is not valid WKB, the code will strip the first 4 bytes, the SRID, and try again. This works for most use cases.
List of sub-package utilities
clip- clipping geometry to a bounding boxencoding/mvt- encoded and decoding from Mapbox Vector Tilesencoding/wkb- well-known binary as well as helpers to decode from the database queriesencoding/wkt- well-known text encodinggeojson- working with geojson and the types in this packagemaptile- working with mercator map tilesproject- project geometries between geo and planar contextsquadtree- quadtree implementation using the types in this packageresample- resample points in a line string geometrysimplify- linear geometry simplifications like Douglas-Peucker
