Method for the quantification of the fugitive gas flow from a dispersed source (A) by monitoring with a remote detection optical instrument mounted on an aircraft (UAV) which moves at a determined height along a plane (S) perpendicular to the direction of the wind field (u), such wind field being known through suitable positioning of meteorological stations within and in areas neighboring the site to be monitored according to known techniques and use of commercially available diagnostic meteorological models. By this instrument discrete vertical measurements are carried out of the fugitive gas concentration averaged over said height along the whole width (W) of the plane (S) to yield corresponding mean vertical concentration values and, according to mean wind speed values detected at said discrete vertical measurements, a value of the fugitive gas flow (Q) is obtained by integrating the product of the mean vertical concentration values and of the corresponding mean wind speed values with respect to the surface of the sampling plane (S). The obtained value of the fugitive gas flow (Q) is corrected by a corrective factor (a) obtainable by comparing concentration values obtained by direct measurements and values calculated by dispersion models.