We present the calibration of the [alpha/Fe] element in terms of ultra-violet excess for 465 dwarf stars with spectral type F0-K2. We used a single calibration, fitted to a third degree polynomial with a square of the correlation coefficient 0.74 and standard deviation 0.05 mag, for all stars due to their small colour range, 0.1 < (g - r)(0) <= 0.6 mag, and high frequency in the blueward of the spectrum which minimize the guillotine effect. Our calibration provides [alpha/Fe] elements in the range (-0.05, 0.35] dex. We applied the procedure to a high-latitude field, 85 degrees <= b <= 90 degrees with size 78 deg(2) and we could estimate the [alpha/Fe] elements of 23,414 dwarf stars which occupy a Galactic region up to a vertical distance of z = 9 kpc. We could detect a small positive gradient, d[alpha/Fe]/dz = +0.032 +/- 0.002 dex kpc(-1), for the range 0 < z < 5 kpc, while the distribution of the [alpha/Fe] element is flat for further z distances.