Add polygon area calculation #21
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@@ -4,7 +4,10 @@ public typealias LocationRadians = Double | |
| public typealias RadianDistance = Double | ||
| public typealias RadianDirection = Double | ||
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| struct Constants { | ||
| static let metersPerRadian = 6_373_000.0 | ||
| static let equatorialRadius:Double = 6378137 | ||
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There was a problem hiding this comment. Specify |
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| } | ||
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| /** | ||
| A `RadianCoordinate2D` is a coordinate represented in radians as opposed to | ||
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@@ -285,3 +288,67 @@ public struct Polyline { | |
| return closestCoordinate | ||
| } | ||
| } | ||
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| public struct Polygon { | ||
| var polygonCoordinates: [[CLLocationCoordinate2D]] | ||
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There was a problem hiding this comment. GeoJSON represents a polygon as a collection of rings (one outer ring and multiple inner rings). Similarly, the Mapbox Maps SDKs for iOS and macOS represent a polygon as an outer polygon containing multiple inner polygons. Either way, it’s nice to be able to work with a formal structure instead of a nested array. Consider replacing this property with: var rings: [Ring]or: var outerRing: Ring
var innerRings: [Ring]where |
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| var polygonArea: Double { | ||
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There was a problem hiding this comment. This property already lives inside a struct named There was a problem hiding this comment. Add a comment with a permalink to the specific commit of the Turf package you’re porting this code from, in case we need to track down a discrepancy in the future. |
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| var area:Double = 0 | ||
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| if (!polygonCoordinates.isEmpty && polygonCoordinates.count > 0) { | ||
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There was a problem hiding this comment. The more idiomatic way to express this in Swift is There was a problem hiding this comment. If we use a non-optional |
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| area += abs(ringArea(polygonCoordinates[0])) | ||
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| for coordinate in polygonCoordinates.suffix(from: 1) { | ||
| area -= abs(ringArea(coordinate)) | ||
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There was a problem hiding this comment. Note that each item in A more functional approach would be: let area = abs(ringArea(polygonCoordinates.first ?? []))
- polygonCoordinates.suffix(from: 1)
.map { abs(ringArea($0)) } // convert the inner rings to their areas
.reduce(0, +) // sum the inner areasThere was a problem hiding this comment. If we use separate let area = abs(outerRing.area)
- innerRings
.map { abs($0.area) } // convert the inner rings to their areas
.reduce(0, +) // sum the inner areas |
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| } | ||
| } | ||
| return area | ||
| } | ||
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| /** | ||
| * Calculate the approximate area of the polygon were it projected onto the earth. | ||
| * Note that this area will be positive if ring is oriented clockwise, otherwise it will be negative. | ||
| * | ||
| * Reference: | ||
| * Robert. G. Chamberlain and William H. Duquette, "Some Algorithms for Polygons on a Sphere", JPL Publication 07-03, Jet Propulsion | ||
| * Laboratory, Pasadena, CA, June 2007 http://trs-new.jpl.nasa.gov/dspace/handle/2014/40409 | ||
| * | ||
| */ | ||
| private func ringArea(_ coordinates: [CLLocationCoordinate2D]) -> Double { | ||
| var p1: CLLocationCoordinate2D | ||
| var p2: CLLocationCoordinate2D | ||
| var p3: CLLocationCoordinate2D | ||
| var lowerIndex: Int | ||
| var middleIndex: Int | ||
| var upperIndex: Int | ||
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There was a problem hiding this comment. These six variables are always written inside each iteration of the There was a problem hiding this comment. Use a tuple consisting of three coordinates to ensure that all three are set simultaneously: let controlPoints: (CLLocationCoordinate2D, CLLocationCoordinate2D, CLLocationCoordinate2D)
if index == coordinatesCount - 2 {
controlPoints = (coordinates[coordinatesCount - 2], coordinates[coordinatesCount - 1], coordinates[0])
}
// … |
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| var area: Double = 0 | ||
| let coordinatesCount: Int = coordinates.count | ||
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| if (coordinatesCount > 2) { | ||
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There was a problem hiding this comment. Nit: drop the parentheses (×3). |
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| for index in 0...coordinatesCount - 1 { | ||
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There was a problem hiding this comment.
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| if (index == coordinatesCount - 2) { | ||
| lowerIndex = coordinatesCount - 2 | ||
| middleIndex = coordinatesCount - 1 | ||
| upperIndex = 0 | ||
| } else if(index == coordinatesCount - 1) { | ||
| lowerIndex = coordinatesCount - 1 | ||
| middleIndex = 0 | ||
| upperIndex = 1 | ||
| } else { | ||
| lowerIndex = index | ||
| middleIndex = index + 1 | ||
| upperIndex = index + 2 | ||
| } | ||
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| p1 = coordinates[lowerIndex] | ||
| p2 = coordinates[middleIndex] | ||
| p3 = coordinates[upperIndex] | ||
| area += (p3.longitude.toRadians() - p1.longitude.toRadians()) * sin(p2.latitude.toRadians()) | ||
| } | ||
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| area = area * Constants.equatorialRadius * Constants.equatorialRadius / 2 | ||
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| } | ||
| return area | ||
| } | ||
| } | ||
| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,19 @@ | ||
| { | ||
| "type": "Feature", | ||
| "properties": {}, | ||
| "geometry": { | ||
| "type": "Polygon", | ||
| "coordinates": [ | ||
| [ | ||
| [125, -15], | ||
| [113, -22], | ||
| [117, -37], | ||
| [130, -33], | ||
| [148, -39], | ||
| [154, -27], | ||
| [144, -15], | ||
| [125, -15] | ||
| ] | ||
| ] | ||
| } | ||
| } |
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I’m not sure a generic Constants struct adds much value to the file, since these constants are internal to the library.