Adds radiusInMeters(latitudeDelta:) to GeoCoordinate

Signed-off-by: Markus Mueller <mmlr@gmx.net>

Sources/GPXKit/DistanceCalculation.swift

@@ -144,4 +144,11 @@ extension GeoCoordinate {
         let d = R * c
         return d
     }
+
+    //  one degree of latitude is always approximately 111 kilometers (69 miles)
+    func radiusInMeters(latitudeDelta: Double) -> Double {
+        let topCentralLat: Double = latitude - latitudeDelta / 2
+        let topCentralLocation = Coordinate(latitude: topCentralLat, longitude: longitude, elevation: 0)
+        return distance(to: topCentralLocation)
+    }
 }

Tests/GPXKitTests/GeoCoordinateExtensionsTests.swift

@@ -0,0 +1,20 @@
+import Foundation
+@testable import GPXKit
+import XCTest
+
+final class GeoCoordinateExtensionsTests: XCTestCase {
+    func testRadiusForDelta() {
+        let location = Coordinate(latitude: 51.323331, longitude: 12.368279, elevation: 110)
+
+        for degree in stride(from: 1.0, to: 180.0, by: 1) {
+            let expected = degree * 111.045 / 2.0 * 1000
+            //  one degree of latitude is always approximately 111 kilometers (69 miles)
+            XCTAssertEqual(
+                expected,
+                location.radiusInMeters(latitudeDelta: degree),
+                accuracy: 139 * degree,
+                "Radius \(location.radiusInMeters(latitudeDelta: degree)) for \(degree) not in expected range \(expected)"
+            )
+        }
+    }
+}