Added GISTool.convertToDegrees(fromMeters:atLatitude:) (#59)

Also added GISTool.convertToCoordinate(fromPixelX:pixelY:atZoom:tileSideLength:projection:) and GISTool.metersPerPixel(atZoom:latitude:tileSideLength:) (they were in MapTile before)

Sources/GISTools/Algorithms/Conversions.swift

@@ -5,13 +5,16 @@ import Foundation
 
 // Ported from https://github.com/Turfjs/turf/tree/master/packages/turf-helpers
 
+// MARK: Measurements
+
 extension GISTool {
 
     /// Unit of measurement.
     public enum Unit {
         case acres
         case centimeters
         case centimetres
+        /// Latitude
         case degrees
         case feet
         case inches
@@ -31,7 +34,7 @@ extension GISTool {
     public static func factor(for unit: Unit) -> Double? {
         switch unit {
         case .centimeters, .centimetres: return earthRadius * 100.0
-        case .degrees: return earthRadius / 111_325.0
+        case .degrees: return earthRadius / (GISTool.earthCircumference / 360.0)
         case .feet: return earthRadius * 3.28084
         case .inches: return earthRadius * 39.370
         case .kilometers, .kilometres: return earthRadius / 1000.0
@@ -98,3 +101,62 @@ extension GISTool {
     }
 
 }
+
+// MARK: - Pixels
+
+extension GISTool {
+
+    /// Converts pixel coordinates in a given zoom level to a coordinate.
+    public static func convertToCoordinate(
+        fromPixelX pixelX: Double,
+        pixelY: Double,
+        atZoom zoom: Int,
+        tileSideLength: Double = GISTool.tileSideLength,
+        projection: Projection = .epsg4326)
+        -> Coordinate3D
+    {
+        let resolution = metersPerPixel(atZoom: zoom, tileSideLength: tileSideLength)
+
+        let coordinateXY = Coordinate3D(
+            x: pixelX * resolution - GISTool.originShift,
+            y: pixelY * resolution - GISTool.originShift)
+
+        if projection == .epsg4326 {
+            return coordinateXY.projected(to: projection)
+        }
+
+        return coordinateXY
+    }
+
+    /// Resolution (meters/pixel) for a given zoom level (measured at `latitude`, defaults to the equator).
+    public static func metersPerPixel(
+        atZoom zoom: Int,
+        latitude: CLLocationDegrees = 0.0, // equator
+        tileSideLength: Double = GISTool.tileSideLength)
+        -> Double
+    {
+        (cos(latitude * Double.pi / 180.0) * 2.0 * Double.pi * GISTool.equatorialRadius / tileSideLength) / pow(2.0, Double(zoom))
+    }
+
+}
+
+// MARK: - Meters/latitude
+
+extension GISTool {
+
+    public static func convertToDegrees(
+        fromMeters meters: Double,
+        atLatitude latitude: CLLocationDegrees)
+        -> (latitudeDegrees: CLLocationDegrees, longitudeDegrees: CLLocationDegrees)
+    {
+        // Length of one minute at this latitude
+        let oneDegreeLatitudeDistance: CLLocationDistance = GISTool.earthCircumference / 360.0 // ~111 km
+        let oneDegreeLongitudeDistance: CLLocationDistance = cos(latitude * Double.pi / 180.0) * oneDegreeLatitudeDistance
+
+        let longitudeDistance: Double = (meters / oneDegreeLongitudeDistance)
+        let latitudeDistance: Double = (meters / oneDegreeLatitudeDistance)
+
+        return (latitudeDistance, longitudeDistance)
+    }
+
+}

Sources/GISTools/GeoJson/BoundingBox.swift

@@ -77,17 +77,12 @@ public struct BoundingBox:
                 northEast.longitude += padding
 
             case .epsg4326:
-                // Length of one minute at this latitude
-                let oneDegreeLatitudeDistance: CLLocationDistance = GISTool.earthCircumference / 360.0 // ~111 km
-                let oneDegreeLongitudeDistance: CLLocationDistance = cos(southWest.latitude * Double.pi / 180.0) * oneDegreeLatitudeDistance
+                let latLongDegrees = GISTool.convertToDegrees(fromMeters: padding, atLatitude: southWest.latitude)
 
-                let longitudeDistance: Double = (padding / oneDegreeLongitudeDistance)
-                let latitudeDistance: Double = (padding / oneDegreeLatitudeDistance)
-
-                southWest.latitude -= latitudeDistance
-                northEast.latitude += latitudeDistance
-                southWest.longitude -= longitudeDistance
-                northEast.longitude += longitudeDistance
+                southWest.latitude -= latLongDegrees.latitudeDegrees
+                northEast.latitude += latLongDegrees.latitudeDegrees
+                southWest.longitude -= latLongDegrees.longitudeDegrees
+                northEast.longitude += latLongDegrees.longitudeDegrees
 
             case .noSRID:
                 break // Don't know what to do -> ignore

Sources/GISTools/Other/MapTile.swift

@@ -70,8 +70,8 @@ public struct MapTile: CustomStringConvertible, Sendable {
         let pixelX: Double = (Double(x) + 0.5) * GISTool.tileSideLength
         let pixelY: Double = (Double(y) + 0.5) * GISTool.tileSideLength
 
-        return MapTile.pixelCoordinate(
-            pixelX: pixelX,
+        return GISTool.convertToCoordinate(
+            fromPixelX: pixelX,
             pixelY: pixelY,
             atZoom: z,
             tileSideLength: GISTool.tileSideLength,
@@ -89,14 +89,14 @@ public struct MapTile: CustomStringConvertible, Sendable {
         // Flip y
         let y = (1 << z) - 1 - y
 
-        let southWest = MapTile.pixelCoordinate(
-            pixelX: Double(x) * GISTool.tileSideLength,
+        let southWest = GISTool.convertToCoordinate(
+            fromPixelX: Double(x) * GISTool.tileSideLength,
             pixelY: Double(y) * GISTool.tileSideLength,
             atZoom: z,
             tileSideLength: GISTool.tileSideLength,
             projection: projection)
-        let northEast = MapTile.pixelCoordinate(
-            pixelX: Double(x + 1) * GISTool.tileSideLength,
+        let northEast = GISTool.convertToCoordinate(
+            fromPixelX: Double(x + 1) * GISTool.tileSideLength,
             pixelY: Double(y + 1) * GISTool.tileSideLength,
             atZoom: z,
             tileSideLength: GISTool.tileSideLength,
@@ -161,7 +161,8 @@ public struct MapTile: CustomStringConvertible, Sendable {
 
     // MARK: - Conversion pixel to meters
 
-    /// Converts pixel coordinates in a given zoom level to EPSG:3857.
+    /// Converts pixel coordinates in a given zoom level to a coordinate.
+    @available(*, deprecated, renamed: "GISTool.convertToCoordinate", message: "This method has been moved to the GISTool namespace")
     public static func pixelCoordinate(
         pixelX: Double,
         pixelY: Double,
@@ -170,34 +171,30 @@ public struct MapTile: CustomStringConvertible, Sendable {
         projection: Projection = .epsg4326)
         -> Coordinate3D
     {
-        let resolution = metersPerPixel(at: zoom, tileSideLength: tileSideLength)
-
-        let coordinateXY = Coordinate3D(
-            x: pixelX * resolution - GISTool.originShift,
-            y: pixelY * resolution - GISTool.originShift)
-
-        if projection == .epsg4326 {
-            return coordinateXY.projected(to: projection)
-        }
-
-        return coordinateXY
+        GISTool.convertToCoordinate(
+            fromPixelX: pixelX,
+            pixelY: pixelY,
+            atZoom: zoom,
+            tileSideLength: tileSideLength,
+            projection: projection)
     }
 
     // MARK: - Meters per pixel
 
     /// Resolution (meters/pixel) for a given zoom level (measured at `latitude`, defaults to the equator).
+    @available(*, deprecated, renamed: "GISTool.metersPerPixel", message: "This method has been moved to the GISTool namespace")
     public static func metersPerPixel(
         at zoom: Int,
         latitude: Double = 0.0, // equator
         tileSideLength: Double = GISTool.tileSideLength)
         -> Double
     {
-        (cos(latitude * Double.pi / 180.0) * 2.0 * Double.pi * GISTool.equatorialRadius / tileSideLength) / pow(2.0, Double(zoom))
+        GISTool.metersPerPixel(atZoom: zoom, latitude: latitude, tileSideLength: tileSideLength)
     }
 
     /// Resolution (meters/pixel) for a given zoom level measured at the tile center.
     public var metersPerPixel: Double {
-        MapTile.metersPerPixel(at: z, latitude: centerCoordinate().latitude)
+        GISTool.metersPerPixel(atZoom: z, latitude: centerCoordinate().latitude)
     }
 
     // MARK: - Private

Tests/GISToolsTests/Algorithms/ConversionTests.swift

@@ -0,0 +1,40 @@
+@testable import GISTools
+import XCTest
+
+final class ConversionTests: XCTestCase {
+
+    func testMetersPerPixelAtEquator() {
+        var mppAtZoomLevels: [Double] = Array(repeating: 0.0, count: 21)
+        mppAtZoomLevels[0] = 156_543.03392804096
+
+        for zoom in 1...20 {
+            mppAtZoomLevels[zoom] = mppAtZoomLevels[zoom - 1] / 2.0
+
+            XCTAssertEqual(GISTool.metersPerPixel(atZoom: zoom),
+                           mppAtZoomLevels[zoom],
+                           accuracy: 0.00001)
+        }
+    }
+
+    func testMetersPerPixelAt45() {
+        var mppAtZoomLevels: [Double] = Array(repeating: 0.0, count: 21)
+        mppAtZoomLevels[0] = 110_692.6408380335
+
+        for zoom in 1...20 {
+            mppAtZoomLevels[zoom] = mppAtZoomLevels[zoom - 1] / 2.0
+
+            XCTAssertEqual(GISTool.metersPerPixel(atZoom: zoom, latitude: 45.0),
+                           mppAtZoomLevels[zoom],
+                           accuracy: 0.00001)
+        }
+    }
+
+    func testMetersAtLatitude() throws {
+        let meters = 10000.0
+        let degreesLatitude1 = try XCTUnwrap(GISTool.convert(length: meters, from: .meters, to: .degrees))
+        let degreesLatitude2 = GISTool.convertToDegrees(fromMeters: meters, atLatitude: 0.0).latitudeDegrees
+
+        XCTAssertEqual(degreesLatitude1, degreesLatitude2, accuracy: 0.00000001)
+    }
+
+}

Tests/GISToolsTests/Other/MapTileTests.swift

@@ -161,33 +161,10 @@ final class MapTileTests: XCTestCase {
     }
 
     func testMetersPerPixelAtEquator() {
-        var mppAtZoomLevels: [Double] = Array(repeating: 0.0, count: 21)
-        mppAtZoomLevels[0] = 156_543.03392804096
-
-        for zoom in 1...20 {
-            mppAtZoomLevels[zoom] = mppAtZoomLevels[zoom - 1] / 2.0
-
-            XCTAssertEqual(MapTile.metersPerPixel(at: zoom),
-                           mppAtZoomLevels[zoom],
-                           accuracy: 0.00001)
-        }
-
-        // Alternative
         let worldTile = MapTile(x: 0, y: 0, z: 0)
-        XCTAssertEqual(worldTile.metersPerPixel, mppAtZoomLevels[0], accuracy: 0.00001)
-    }
-
-    func testMetersPerPixelAt45() {
-        var mppAtZoomLevels: [Double] = Array(repeating: 0.0, count: 21)
-        mppAtZoomLevels[0] = 110_692.6408380335
+        let mppAtZoom0 = 156_543.03392804096
 
-        for zoom in 1...20 {
-            mppAtZoomLevels[zoom] = mppAtZoomLevels[zoom - 1] / 2.0
-
-            XCTAssertEqual(MapTile.metersPerPixel(at: zoom, latitude: 45.0),
-                           mppAtZoomLevels[zoom],
-                           accuracy: 0.00001)
-        }
+        XCTAssertEqual(worldTile.metersPerPixel, mppAtZoom0, accuracy: 0.00001)
     }
 
     func testQquadkey() {