connyay / orb

Types and utilities for working with 2d geometry in Golang

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 geojson sub-package.
• Mapbox Vector Tile - encoding and decoding as part of the encoding/mvt sub-package.
• Direct to type from DB query results - by scanning WKB data directly into types.
• Rich set of sub-packages - including clipping, simplifing, quadtree and 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 box
• encoding/mvt - encoded and decoding from Mapbox Vector Tiles
• encoding/wkb - well-known binary as well as helpers to decode from the database queries
• encoding/wkt - well-known text encoding
• geojson - working with geojson and the types in this package
• maptile - working with mercator map tiles
• project - project geometries between geo and planar contexts
• quadtree - quadtree implementation using the types in this package
• resample - resample points in a line string geometry
• simplify - linear geometry simplifications like Douglas-Peucker