Read in ascii topology data and extract a square chunk centred around a given latitude and longitude (code provides ottawa and montreal targets). Expects 30 arc-second map of canada.

Program.cs

using System;
using System.IO;
using System.Collections.Generic;
using System.Linq;
using System.Windows.Forms;

namespace AscMapSubset
{
    static class Program
    {
        /// <summary>
        /// The main entry point for the application.
        /// </summary>
        [STAThread]
        static void Main()
        {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);

            var canLat = 41.004166667f;
            var canLon = -147.995833333f;
            var cellSize = 0.008333333f;

            // montreal
            var monLat = 45.4987368f;
            var monLon = -73.5907014f;                

            // ottawa
            var ottLat = 45.4240305f;
            var ottLon = -75.6954888f;

            var area = new List<List<string>>();
            var areaRadius = 5;

            var y = (int)Math.Round(Math.Abs(monLat - canLat) / cellSize);
            var x = (int)Math.Round(Math.Abs(monLon - canLon) / cellSize);

            var height = 5040;

            var startX = x - areaRadius;
            var startY = height - (y - areaRadius);
            var endX = x + areaRadius;
            var endY = height - (y + areaRadius);

            var lines = File.ReadAllLines(@"can3d30.asc")
                            .Where((line, lineNum) => lineNum > 5)
                            .ToList();

            for (int i = startY; i >= endY; i--)
            {
                var line = lines[i];
                var cols = line.Split(new[] { ' ' }, StringSplitOptions.RemoveEmptyEntries);

                area.Add(cols.Where((col, idx) => idx >= startX && idx <= endX).ToList());
            }

            var output = new List<string>();

            var currLine = area.Count - 1;
            for (int lonStep = startX; lonStep <= endX; lonStep++)
            {
                var col = 0;
                for (int latStep = startY; latStep >= endY; latStep--)
                {
                    var currLat = canLat + ((height - latStep) * cellSize);
                    var currLon = canLon + (lonStep * cellSize);

                    output.Add(currLat + " " + currLon + " " + area[currLine][col]);

                    col++;
                }
                currLine--;
            }
            
            File.WriteAllLines(@"output.asc", output);
        }
    }
}

Expected 30 arc-send map of Canada provided here: can3d30