Gravity varies in relation to latitude due to a combination of factors. The earth spins faster the nearer to the equator you get, the force from this slightly counteracts gravity. Also the earth is not a perfect sphere, at the equator it bulges this means that any point their is further from the centre of the earth and feels a slightly smaller force due to gravity.
Gravity also varies in accordance to altitude, the further you get from the surface to smaller the force of gravity but you have to go a long long way before you escape the Earths gravity completely.
So with values for both latitude and altitude it is possible to find what the value of g should be at and location on earth. The formula used by this application is from the the UK's National Physics Laboratory website and it is as follows :
g = 9.780 327 (1 + A sin2 L - B sin2 2L) - 3.086 × 10-6 H m/s2
A = 0.005 302 4
B = 0.000 005 8
L = latitude
H = height in metres above sea level
The uncertainty in the value of g so obtained is generally less than ±5 parts in 105.
The app is available here.