System for measuring at least one physical quantity representative of the quality of the air in an observation zone, the system comprising: - a mapping of the observation zone, comprising a set, denoted V, of modelled values representative of the physical quantity; - means for measuring the physical quantity, possessing a number N of positions or a number N of trajectories in the observation zone, the N positions or the N trajectories being intended to exhibit a spatial distribution, denoted S opt, in the observation zone; - means for calculating the spatial distribution S opt, configured to: • construct a mesh of the observation zone, the mesh comprising a number G of points; • calculate, for a given spatial distribution, denoted S, of the N positions or of the N trajectories, an estimator of the set V, denoted V̂, for each of the G points of the mesh according to the formula: V ^ j = ∑ i = 1 N V i w ij ∑ i = 1 N w ij, j ∈ 〚 1, G 〛 where: - V i are the modelled values of the set V which are extracted for the positions or for the trajectories of the measurement means exhibiting the given spatial distribution S, - w ij is a transfer function going between s i and m j, where s i are the positions or the trajectories of the measurement means exhibiting the given spatial distribution S, and where m j are the positions of the points of the mesh in the observation zone; • calculate a cost function, denoted φ, representative of the difference or of the likelihood between V̂ and the modelled values, denoted V, of the set V which are extracted at the G points of the mesh; • extract the spatial distribution S opt which minimizes or maximizes the cost function depending on whether the cost function is representative of the difference or of the likelihood between V̂ and V. - a mapping of the observation zone, comprising a set, denoted V, of modelled values representative of the physical quantity; - means for measuring the physical quantity, possessing a number N of positions or a number N of trajectories in the observation zone, the N positions or the N trajectories being intended to exhibit a spatial distribution, denoted S opt, in the observation zone; - means for calculating the spatial distribution S opt, configured to: • construct a mesh of the observation zone, the mesh comprising a number G of points; • calculate, for a given spatial distribution, denoted S, of the N positions or of the N trajectories, an estimator of the set V, denoted V̂, for each of the G points of the mesh according to the formula: V ^ j = ∑ i = 1 N V i w ij ∑ i = 1 N w ij, j ∈ 〚 1, G 〛 where: - V i are the modelled values of the set V which are extracted for the positions or for the trajectories of the measurement means exhibiting the given spatial distribution S, - w ij is a transfer function going between s i and m j, where s i are the positions or the trajectories of the measurement means exhibiting the given spatial distribution S, and where m j are the positions of the points of the mesh in the observation zone; • calculate a cost function, denoted φ, representative of the difference or of the likelihood between V̂ and the modelled values, denoted V, of the set V which are extracted at the G points of the mesh; • extract the spatial distribution S opt which minimizes or maximizes the cost function depending on whether the cost function is representative of the difference or of the likelihood between V̂ and V. • construct a mesh of the observation zone, the mesh comprising a number G of points; • calculate, for a given spatial distribution, denoted S, of the N positions or of the N trajectories, an estimator of the set V, denoted V̂, for each of the G points of the mesh according to the formula: V ^ j = ∑ i = 1 N V i w ij ∑ i = 1 N w ij, j ∈ 〚 1, G 〛 where: - V i are the modelled values of the set V which are extracted for the positions or for the trajectories of the measurement means exhibiting the given spatial distribution S, - w ij is a transfer function going between s i and m j, where s i are the positions or the trajectories of the measurement means exhibiting the given spatial distribution S, and where m j are the positions of the points of the mesh in the observation zone; • calculate a cost function, denoted φ, representative of the difference or of the likelihood between V̂ and the modelled values, denoted V, of the set V which are extracted at the G points of the mesh; • extract the spatial distribution S opt which minimizes or maximizes the cost function depending on whether the cost function is representative of the difference or of the likelihood between V̂ and V. - V i are the modelled values of the set V which are extracted for the positions or for the trajectories of the measurement means exhibiting the given spatial distribution S, - w ij is a transfer function going between s i and m j, where s i are the positions or the trajectories of the measurement means exhibiting the given spatial distribution S, and where m j are the positions of the points of the mesh in the observation zone; • calculate a cost function, denoted φ, representative of the difference or of the likelihood between V̂ and the modelled values, denoted V, of the set V which are extracted at the G points of the mesh; • extract the spatial distribution S opt which minimizes or maximizes the cost function depending on whether the cost function is representative of the difference or of the likelihood between V̂ and V.