An old navigators trick: if the points are on the same longitude, one can use
the approximation that one minute of arc is one nautical mile. The conversion
between statute miles and nautical miles is about 6000/5300 and you may use
your favorite MKS to Engineering unit conversion. If you are working on a
chart, merely measure the distance between point b and c with a divider, find
the number of minutes on the chart scale and do the above arithematic.