v0.1v0.1

1 files changed, 100 insertions, 0 deletions

diff --git a/PositionCalculator.cs b/PositionCalculator.cs
new file mode 100644
index 0000000..8d8a94f
--- /dev/null
+++ b/PositionCalculator.cs
@@ -0,0 +1,100 @@
+using CoordinateSharp;
+using PagerParser;
+using System;
+using System.ComponentModel.DataAnnotations.Schema;
+using System.Text.RegularExpressions;
+
+namespace PagerParser;
+
+public class GpsPosition {
+    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
+    public int    GpsPositionId { get; set; }
+    public double Latitude      { get; set; }
+    public double Longitude     { get; set; }
+}
+
+public record MelwaysPage {
+    public int    PageNo    { get; set; }
+    public double Latitude  { get; set; }
+    public double Longitude { get; set; }
+}
+
+// Derive roughly-accurate GPS coordinates from a Melways page number
+// and grid number (e.g: 131 D7) by looking up the origin coordinates
+// of the Melways page from a pre-defined table, then interpolating
+// the grid number across the page to get a (very) rough position.
+public class PositionCalculator {
+    private const float PageWidth  = 0.0583403752108005f;
+    private const float PageHeight = 0.0444074086328539f;
+
+    private const int ColCount = 10;
+    private const int RowCount = 12;
+
+    private const float ColWidth  = PageWidth  / ColCount;
+    private const float RowHeight = PageHeight / RowCount;
+
+    private static readonly MelwaysPage[] MelwaysPages = {
+        new MelwaysPage { PageNo = 94,  Latitude = -38.0062431030181, Longitude = 145.153465677450 },
+        new MelwaysPage { PageNo = 95,  Latitude = -38.0061349929752, Longitude = 145.197873798763 },
+        new MelwaysPage { PageNo = 96,  Latitude = -38.0060458242193, Longitude = 145.242282747795 },
+        new MelwaysPage { PageNo = 98,  Latitude = -38.0501502394990, Longitude = 145.153538108815 },
+        new MelwaysPage { PageNo = 110, Latitude = -38.0059397267576, Longitude = 145.286740799198 },
+        new MelwaysPage { PageNo = 111, Latitude = -38.0058646642328, Longitude = 145.331164408607 },
+        new MelwaysPage { PageNo = 128, Latitude = -38.0500712736303, Longitude = 145.197997939698 },
+        new MelwaysPage { PageNo = 129, Latitude = -38.0499811306888, Longitude = 145.242424322651 },
+        new MelwaysPage { PageNo = 130, Latitude = -38.0498760059982, Longitude = 145.286857218675 },
+        new MelwaysPage { PageNo = 131, Latitude = -38.0497905233513, Longitude = 145.331303121527 },
+        new MelwaysPage { PageNo = 212, Latitude = -38.0056782419614, Longitude = 145.375580424547 },
+        new MelwaysPage { PageNo = 213, Latitude = -38.0055063939227, Longitude = 145.419999796841 },
+        new MelwaysPage { PageNo = 214, Latitude = -38.0496145139129, Longitude = 145.375744441368 },
+        new MelwaysPage { PageNo = 215, Latitude = -38.0494726543543, Longitude = 145.420190429893 },
+        new MelwaysPage { PageNo = 315, Latitude = -38.0053618479218, Longitude = 145.464319398407 },
+        new MelwaysPage { PageNo = 317, Latitude = -38.0493097638301, Longitude = 145.464666307851 }
+    };
+
+    public static GpsPosition? GetGpsPosition(ParsedPagerMessage message) {
+        if(message.MelwaysMapNo is null || message.MelwaysGrid is null)
+            return null;
+        var page = MelwaysPages.FirstOrDefault(p => p.PageNo == message.MelwaysMapNo);
+        if(page is null)
+            return null;
+
+        var match = Regex.Match(message.MelwaysGrid.ToUpper(), "^([A-HJ-K])([0-9]+)$");
+        if(!match.Success)
+            return null;
+
+        var row = int.Parse(match.Groups[2].Value) - 1;
+        var col = match.Groups[1].Value[0] - 'A';
+        if(col > 7)
+            col -= 1; // Compensate for Melways grids skipping 'I'
+
+        var position = new GpsPosition {
+            Latitude  = page.Latitude  - (row * RowHeight)  - (RowHeight  / 2),
+            Longitude = page.Longitude + (col * ColWidth)   + (ColWidth   / 2)
+        };
+
+        if(message.GridReference is null)
+            return position;
+
+        // If we have a 6-digit grid reference, try to refine the location further
+        // using the UTM coordinate system. Since zone information isn't included
+        // in a 6-digit grid reference, we use the coordinates calculated from the
+        // Melways as a starting point to find the zone.
+
+        double gridRef = (double) message.GridReference;
+
+        var origUTM = new Coordinate(position.Latitude, position.Longitude).UTM;
+        var newUTM = new UniversalTransverseMercator(
+            origUTM.LatZone,
+            origUTM.LongZone,
+            (Math.Floor(origUTM.Easting / 100000) * 100000) + (Math.Floor(gridRef / 1000) * 100),
+            (Math.Floor(origUTM.Northing / 100000) * 100000) + ((gridRef % 1000) * 100));
+
+        var coord = UniversalTransverseMercator.ConvertUTMtoLatLong(newUTM);
+
+        position.Latitude  = coord.Latitude.ToDouble();
+        position.Longitude = coord.Longitude.ToDouble();
+
+        return position;
+    }
+}
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