Latitude and longitude calculate distances on the Earth in degrees. Latitude marks distances from north to south and vice versa, with longitude from east to west. Converting latitude distance to miles is simple, but longitude is different because the distance between longitude points gets wider as you travel closer to the equator. Calculating and converting these distances requires knowledge in numerous types of math, including geometry and trigonometry, to get it precise.
Take the latitude point, in the number of degrees, of your starting point -- Point A.
Locate the latitude of your finishing point -- Point B -- along the same longitude. If you're on the northern hemisphere, this number is higher if you're travelling north and less if going south. It is the opposite on the southern hemisphere.
Subtract the lower number from the higher number to get the distance in latitude degrees.
Multiply this number by 69 to get the distance in miles.
Measure the distance from point A to B in longitude using the same method of subtracting the smallest longitude from the largest.
Locate and record the latitude that these two longitude points travel along. The distance from one longitude degree to the next is 69 miles if the latitude is between 1 and 7 degrees.
Subtract one mile from 69 if the latitude is between 8 and 12. Subtract an extra mile for every three latitude degrees up to 18. Subtract another mile for every two degrees from 19 to 33 and one mile for every degree after that
Multiply the first number you got (from measuring the distance in longitude) by the second number (calculating the miles by latitude position) to get the mileage.
Use the Pythagorean Theorem with the above steps to calculate mileage across a diagonal line -- a distance that crosses multiple latitude and longitude. Multiply each distance in miles you get by itself (square them) and then add the two squares together. The square root of this number is your actual distance in miles.