Friday, March 17, 2017

Geographic Distances: A Quick Trip Around the Great Circle

Recently, I wanted to calculate the distance between locations on the Earth. Finding a handy solution, I thought some reader might be interested. In my situation, location data included ZIP codes (American postal codes). Also available to me is a look-up table of the latitude and longitude of the geometric centroid of each ZIP code. Since the areas identified by ZIP codes are usually geographical small, and making the "close enough" assumption that this planet is perfectly spherical, trigonometry will allow distance calculations which are, for most purposes, precise enough.

Given the latitude and longitude of cities 'A' and 'B', the following line of MATLAB code will calculate the distance between the two coordinates "as the crow flies" (technically, the "great circle distance"), in kilometers:

DistanceKilometers = round(111.12 * acosd(cosd(LongA - LongB) * cosd(LatA) * cosd(LatB) + sind(LatA) * sind(LatB)));

Note that latitude and longitude are expected as decimal degrees. If your data is in degrees/minutes/seconds, a quick conversion will be needed.

I've checked this formula against a second source and quickly verified it using a few pairs of cities:


% 'A' = New York
% 'B' = Atlanta
% Random on-line reference: 1202km
LatA = 40.664274;
LongA =  -73.9385;
LatB = 33.762909;
LongB = -84.422675;
DistanceKilometers = round(111.12 * acosd(cosd(LongA - LongB) * cosd(LatA) * cosd(LatB) + sind(LatA) * sind(LatB)))

DistanceKilometers =

        1202


% 'A' = New York
% 'B' = Los Angeles
% Random on-line reference: 3940km (less than 0.5% difference)<0 .5="" br="" difference="">
LatA = 40.664274;
LongA =  -73.9385;
LatB = 34.019394;
LongB = -118.410825;
DistanceKilometers = round(111.12 * acosd(cosd(LongA - LongB) * cosd(LatA) * cosd(LatB) + sind(LatA) * sind(LatB)))

DistanceKilometers =

        3955



 References:

"How Far is Berlin?", by Alan Zeichick, published in the Sep-1991 issue of "Computer Language" magazine. Note that Zeichick credits as his source an HP-27 scientific calculator, from which  he reverse-engineered the formula above.

"Trigonometry DeMYSTiFieD, 2nd edition", by Stan Gibilisco (ISBN: 978-0-07-178024-7)

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