经度是分和秒是按60进位，如果要做运算第一步就是转换成浮点数，之后就是计算和还原。

using System.Text.RegularExpressions;

public static double DegreeFromString(string str)
{
    Match match = Regex.Match(str,
        @"(?<hour>[0-9]+)°(?<minute>[0-9]+)(′|')(?<second>[0-9]+(\.[0-9]+)?)(″|""|'')");
    if (match.Success)
    {
        double hour = 0, minute = 0, second = 0;
        double.TryParse(match.Result("${hour}"), out hour);
        double.TryParse(match.Result("${minute}"), out minute);
        double.TryParse(match.Result("${second}"), out second);
        return hour + minute / 60 + second / 3600;
    }
    else return 0;
}

public static string DegreeToString(double degree)
{
    double minute = (degree - (int)degree) * 60;
    double second = (minute - (int)minute) * 60;
    return string.Format("{0:#}°{1:#}′{2:#.00}″", degree, Math.Abs(minute), Math.Abs(second));
}

private void button1_Click(object sender, EventArgs e)
{
    Console.WriteLine(DegreeToString(DegreeFromString("20°53'41''")));
    Console.WriteLine(DegreeToString(DegreeFromString("10°11'20.5''")));
    Console.WriteLine(DegreeToString(DegreeFromString("20°53'41''") - DegreeFromString("10°11'20.5''")));
    Console.WriteLine(DegreeToString(DegreeFromString("10°11'20.5''") - DegreeFromString("20°53'41''")));
}
输出：

21°54′41.00″
10°11′20.50″
11°42′20.50″
-11°42′20.50″