Earth is not a sphere and therefore the calculation is only a approximation valid at sea level.
1 degree° = 60 arc minutes ' 1 arc minute ' = 60 arc seconds '' 1 arc second '' at equatorial sea level = 1852m/60 = 30.86666667m
Formlua: cos(degree latitude) * (1852/60) Example   0° latitude and 1 arc second:   cos(0) * 30.866666667 = 1 * 30.866666667 = 30.866666667 Example 45° latitude and 1 arc second: cos(45) * 30.866666667 = 0.7071067811865476 * 30.866666667 = 21.82602931286047 Example 60° latitude and 1 arc second: cos(60) * 30.866666667 = 0.5000000000000001 * 30.866666667 = 15.433333333500004
Fig. 1: Schematic presentation of decrease toward the earth's poles.