Geospatial Data and Queries

This section explains how to create, use and query geospatial data stored within OpenDataDSL

Introduction

In OpenDataDSL, you can store geospatial data as the following types of geometrical objects:

• Point - a single coordinate
• LineString - an array of 2 or more points
• MultiPoint - an array of points
• MultiLineString - an array of LineStrings
• Polygon - an array of linear rings where the first and last point must be the same
• MultiPolygon - an array of polygons
• GeometryCollection - an array of any other geometrical objects

You can add a property to an object which can be of any of the types described above.

Creating Geometries

There are constructor functions that you can use to construct any of the available geometries as described in this section:

Point

A point is constructed with a single coordinate in the format:

[longitude, latitude]

So, you can construct a point like so:

var = Point([long, lat])

// Example
location = Point([51.5386, -0.4956])

LineString

A LineString is constructed as empty and then add coordinates to it or using an array of coordinates as follows:

// Use the empty constructor and add coordinates
road1 = LineString()
road1.add(-73.96943, 40.78519)
road1.add(Point([52.35672, 4.91120]))

// Add the coordinates in the constructor
road2 = LineString([[-73.96943, 40.78519],[-73.96943, 41.78519]])

MultiPoint

A MultiPoint is the same as a LineString as shown below:

// Use the empty constructor and add coordinates
road1 = MultiPoint()
road1.add(-73.96943, 40.78519)
road1.add(Point([52.35672, 4.91120]))

// Add the coordinates in the constructor
road2 = MultiPoint([[-73.96943, 40.78519],[-73.96943, 41.78519]])

MultiLineString

A MultiLineString can be constructed as empty and then add LineString or MultiPoint objects to it or can be constructed with the coordinates passed in.

// Use an empty constructor and add the coordinates
road1 = MultiPoint([[-73.96943, 40.78519],[-73.96943, 41.78519]])
road2 = MultiPoint([[-73.96943, 40.78519],[-73.96943, 41.78519]])
mls = MultiLineString()
mls.add(road1)
mls.add(road2)

// Add the coordinates in the constructor
mls = MultiLineString([LineString([[0,0],[1,1]]),LineString([[2,2],[3,3]])])

Polygon

A polygon can be constructed in 3 ways:

• An empty polygon
• A single ring polygon
• A multi-ring polygon

mp1 = MultiPoint([[ 50, -1 ], [52, -1], [52, 1], [50, 1], [ 50, -1 ]])
mp2 = MultiPoint([[ 50, -1 ], [52, -1], [52, 1], [50, 1], [ 50, -1 ]])

// Create an empty polygon and add rings to it
poly1 = Polygon()
poly1.add(MultiPoint([[ 50, -1 ], [52, -1], [52, 1], [50, 1], [ 50, -1 ]]))
poly1.add(mp2)

// Create a polygon with a single ring
poly2 = Polygon([[ 50, -1 ], [52, -1], [52, 1], [50, 1], [ 50, -1 ]])

// Create a mulit-ring polygon
poly3 = Polygon([mp1, mp2])

MultiPolygon

A MultiPolygon can be constructed as empty or passing in a list of polygons, e.g.

// Create some polygons
poly1 = Polygon()
poly1.add(MultiPoint([[ 50, -1 ], [52, -1], [52, 1], [50, 1], [ 50, -1 ]]))
poly2 = Polygon([[ 50, -1 ], [52, -1], [52, 1], [50, 1], [ 50, -1 ]])

// Construct an empty multi polygon and add a polygon
mp = MultiPolygon()
mp.add(poly1)

// Create a MultiPolygon using our 2 polygons
mp2 = MultiPolygon([poly1, poly2])

GeometryCollection

A MultiPolygon can be constructed as empty or passing in a list of geometries, e.g.

// Create some geometries
road2 = MultiPoint([[-73.96943, 40.78519],[-73.96943, 41.78519]])
location = Point([51.5386, -0.4956])

// Create an empty GeometryCollection and add our geometries
gc = GeometryCollection()
gc.add(road2)
gc.add(location)

// Construct a GeometryCollection using our geometries
coll = GeometryCollection([road1, polygon, mp])

Adding Geometry to Objects

You can add any of the geometries to an object as a property which can then be used to make geospatial queries, Here is an example of creating an object with a geolocation and saving it:

BritishMuseum = object as TestGeometry
    name = "British Museum"
    location = Point([51.51958, -0.12695])
end
save ${object:BritishMuseum}

Using Geospatial Queries to find objects

You can use the find command to search for items that are within or intersect a polygon or a spherical radius of a point.

To check for items that are within a radius of a point, you use the within operator with a sphere object. The sphere object is constructed using a point and a radius in radians as follows:

// Syntax
s = Sphere([longitude, latitude], radius)

// Creating a 20 mile radius around the British Museum
// 3963.2 is the radius of the earth in miles
bm20 = Sphere([51.51958, -0.12695], 20/3963.2)

Examples

// Find a list of items that are within a 20 mile radius of a point
items = find ${object:"TestGeometry"} where location within Sphere([ 51.72961, 0.47612 ], 20 / 3963.2)

// Find items that are within a defined polygon
items = find ${object:"TestGeometry"} where location within Polygon([[ 50, -1 ], [52, -1], [52, 1], [50, 1], [ 50, -1 ]])

// Pre-define a polygon, then find items that are within that polygon
london = Polygon([[51.5386, -0.4956],[51.6445, -0.0753],[51.5205, 0.1753],[51.3479, -0.1163],[51.5386, -0.4956]])
items = find ${object:"TestGeometry"} where location within london

// Find items that intersect with a polygon
items = find ${object:"TestGeometry"} where location intersects london