Hi,
With this idea, you can only find approximate distance and you have to take samples at very high speed…
What is the issue, let me explain…
Near the center of erth if u will find difference of langitude say x it will correspond to distance X..
but at north or south pole longitude difference of x corresponds to distance Y …
but X will not be equal to Y even though longitude difference is same as x…
If you want to find difference in ur city or state, than its fine …