XAML-Map-Control/MapControl/Shared/MapGraticule.cs

// XAML Map Control - https://github.com/ClemensFischer/XAML-Map-Control
// © 2020 Clemens Fischer
// Licensed under the Microsoft Public License (Ms-PL)

using System;
#if WINDOWS_UWP
using Windows.UI.Xaml;
#else
using System.Windows;
#endif

namespace MapControl
{
    /// <summary>
    /// Draws a graticule overlay.
    /// </summary>
    public partial class MapGraticule : MapOverlay
    {
        public static readonly DependencyProperty MinLineDistanceProperty = DependencyProperty.Register(
            nameof(MinLineDistance), typeof(double), typeof(MapGraticule), new PropertyMetadata(150d));

        /// <summary>
        /// Minimum graticule line distance in pixels. The default value is 150.
        /// </summary>
        public double MinLineDistance
        {
            get { return (double)GetValue(MinLineDistanceProperty); }
            set { SetValue(MinLineDistanceProperty, value); }
        }

        private double GetLineDistance()
        {
            var pixelPerDegree = ParentMap.ViewTransform.Scale * ParentMap.MapProjection.UnitsPerDegree;
            var minDistance = MinLineDistance / pixelPerDegree;
            var scale = 1d;

            if (minDistance < 1d)
            {
                scale = minDistance < 1d / 60d ? 3600d : 60d;
                minDistance *= scale;
            }

            var lineDistances = new double[] { 1d, 2d, 5d, 10d, 15d, 30d, 60d };
            var i = 0;

            while (i < lineDistances.Length - 1 && lineDistances[i] < minDistance)
            {
                i++;
            }

            return lineDistances[i] / scale;
        }

        private static string GetLabelFormat(double lineDistance)
        {
            if (lineDistance < 1d / 60d)
            {
                return "{0} {1}°{2:00}'{3:00}\"";
            }
            
            if (lineDistance < 1d)
            {
                return "{0} {1}°{2:00}'";
            }
            
            return "{0} {1}°";
        }

        private static string GetLabelText(double value, string format, string hemispheres)
        {
            var hemisphere = hemispheres[0];

            if (value < -1e-8) // ~1mm
            {
                value = -value;
                hemisphere = hemispheres[1];
            }

            var seconds = (int)Math.Round(value * 3600d);

            return string.Format(format, hemisphere, seconds / 3600, (seconds / 60) % 60, seconds % 60);
        }
    }
}