#1 Choose a reference point and find its lat/lon coordinates. Any point that is close to your operational area will work nicely, but it's particularly convenient in the Northern Hemisphere to choose a point that is southeast of your operational area. That sets things up so that all of your x,y coordinates will be positive. Reference Point: 39.978477 lat, -69.793358 log (3958'42.5"N 6947'36.1"W) #2 Find the relative coordinates of any point in your operational area by subtracting the coordinates of your reference point. Coordinates chosen: 40.417001 lat, -69.996868 log Relative coordinates: 40.417001-39.978477=0.438524 lat -69.996868-(-69.793358)=-0.20351 long #3 Calculate the conversion factor for changes in latitude and longitude to changes in distance at the reference point. We'll use the arc length formula for this, so s = r , where s is distance, r is radius, and is the angle. To get a conversion factor it's easy to just use = 1 . For latitude, r is just the radius of Earth since the distance between lines of latitude does not change. But for longitude, r=r earth coslat where > lat is the latitude of your reference point. So once you do this calculation you'll end up with two different values of s, one for latitude and one for longitude. These are your conversion factors, so let's call those s lat and s lon