From 8693ab8098ab68cd1be3a64fb9e9b4717aff0902 Mon Sep 17 00:00:00 2001
From: ferdiflash <yeti-oilman-0u@icloud.com>
Date: Thu, 30 Apr 2026 12:46:28 +0200
Subject: [PATCH] Add functions that specify UTM zone for conversion

---
 README.md                                     | 13 ++++++++++
 .../CLLocation+UTMCoordinate.swift            | 14 ++++++++++-
 ...CLLocationCoordinate2D+UTMCoordinate.swift | 14 +++++++++--
 .../UTMConversionTests.swift                  | 24 +++++++++++++++++++
 4 files changed, 62 insertions(+), 3 deletions(-)

diff --git a/README.md b/README.md
index e90c082..55fe8c4 100644
--- a/README.md
+++ b/README.md
@@ -66,3 +66,16 @@ let utmCoordinate = UTMCoordinate(northing: 7034313, easting: 569612, zone: 32,
 let datum = UTMDatum(equitorialRadius: 6378137, polarRadius: 6356752.3142)
 let coordinate = utmCoordinate.coordinate(datum: datum)
 ```
+
+### UTM Zone
+
+It is possible to specify the UTM zone for conversion, e.g. if you want consistent UTM coordinates at a border of two zones.
+
+```swift
+import CoreLocation
+import UTMConversion
+
+let location = CLLocation(latitude: 51.357402, longitude: 12.002263)
+let utmCoordinateInZone33 = utmCoordinate.coordinate() // Coordinate is in 33, but at the border to 32.
+let utmCoordinateInZone32 = utmCoordinate.coordinate(zone: 32)
+```
diff --git a/Sources/UTMConversion/CLLocation+UTMCoordinate.swift b/Sources/UTMConversion/CLLocation+UTMCoordinate.swift
index d0862b5..0220d86 100644
--- a/Sources/UTMConversion/CLLocation+UTMCoordinate.swift
+++ b/Sources/UTMConversion/CLLocation+UTMCoordinate.swift
@@ -14,5 +14,17 @@ public extension CLLocation {
         let zone = coordinate.zone
         return TMCoordinate(coordinate: coordinate, centralMeridian: zone.centralMeridian, datum: datum).utmCoordinate(zone: zone, hemisphere: coordinate.hemisphere)
     }
-    
+
+    /**
+        Calculates the UTM coordinate of the receiver
+
+        - Parameter zone: The UTM zone to use
+        - Parameter datum: The datum to use, defaults to WGS84 which should be fine for most applications
+
+     */
+    func utmCoordinate(zone: UTMGridZone, datum: UTMDatum = UTMDatum.wgs84) -> UTMCoordinate {
+        let coordinate = self.coordinate
+        return TMCoordinate(coordinate: coordinate, centralMeridian: zone.centralMeridian, datum: datum).utmCoordinate(zone: zone, hemisphere: coordinate.hemisphere)
+    }
+
 }
diff --git a/Sources/UTMConversion/CLLocationCoordinate2D+UTMCoordinate.swift b/Sources/UTMConversion/CLLocationCoordinate2D+UTMCoordinate.swift
index d79dffd..fdc3458 100644
--- a/Sources/UTMConversion/CLLocationCoordinate2D+UTMCoordinate.swift
+++ b/Sources/UTMConversion/CLLocationCoordinate2D+UTMCoordinate.swift
@@ -14,8 +14,18 @@ public extension CLLocationCoordinate2D {
         let zone = self.zone
         return TMCoordinate(coordinate: self, centralMeridian: zone.centralMeridian, datum: datum).utmCoordinate(zone: zone, hemisphere: hemisphere)
     }
-    
-    
+
+    /**
+     Calculates the UTM coordinate of the receiver
+
+     - Parameter zone: The UTM zone to use
+     - Parameter datum: The datum to use, defaults to WGS84 which should be fine for most applications
+
+     */
+    func utmCoordinate(zone: UTMGridZone, datum: UTMDatum = UTMDatum.wgs84) -> UTMCoordinate {
+        TMCoordinate(coordinate: self, centralMeridian: zone.centralMeridian, datum: datum).utmCoordinate(zone: zone, hemisphere: hemisphere)
+    }
+
     /**
      The UTM grid zone
      */
diff --git a/Tests/UTMConversionTests/UTMConversionTests.swift b/Tests/UTMConversionTests/UTMConversionTests.swift
index dc18846..6a7e412 100644
--- a/Tests/UTMConversionTests/UTMConversionTests.swift
+++ b/Tests/UTMConversionTests/UTMConversionTests.swift
@@ -97,6 +97,30 @@ class UTMConversionTests: XCTestCase {
         let oslo = osloUTM.location(datum: UTMDatum(equitorialRadius: 6378137, polarRadius: 6356752.3142))
         XCTAssertEqual(oslo.coordinate.latitude, 59.912814611065265)
         XCTAssertEqual(oslo.coordinate.longitude, 10.760192985178369)
+        
+    }
+
+    func testCustomZone() {
+        // Coordinate located in UTM zone 33
+        let lat = 51.357402
+        let lon = 12.002263
+
+        // echo "12.002263 51.357402" | proj -f "%.5f" +proj=utm +zone=32 +datum=WGS84 +units=m +no_defs
+        let northing32 = 5693849.66049
+        let easting32 = 709023.99157
+        let zone: UTMGridZone = 32
+
+        let neumarkt2D = CLLocationCoordinate2D(latitude: lat, longitude: lon).utmCoordinate(zone: zone)
+        XCTAssertEqual(neumarkt2D.northing, northing32, accuracy: 0.0001)
+        XCTAssertEqual(neumarkt2D.easting, easting32, accuracy: 0.0001)
+        XCTAssertEqual(neumarkt2D.zone, zone)
+        XCTAssertEqual(neumarkt2D.hemisphere, .northern)
+
+        let neumarkt = CLLocation(latitude: lat, longitude: lon).utmCoordinate(zone: zone)
+        XCTAssertEqual(neumarkt.northing, northing32, accuracy: 0.0001)
+        XCTAssertEqual(neumarkt.easting, easting32, accuracy: 0.0001)
+        XCTAssertEqual(neumarkt.zone, zone)
+        XCTAssertEqual(neumarkt.hemisphere, .northern)
     }
 }