A mapping system comprises a sensor unit which is mobile and receives magnetic audio frequency transmission from a site and positioning data with respect to the site to be mapped. A processing unit is operationally coupled with the sensor unit and determines at least one parameter of the magnetic transmission, forms positions of the sensor unit on the basis of positioning data, associates positions and the at least one parameter together, forms a quality map graphically showing a distribution associated with the at least one parameter with respect to positions on the site and outputs said quality map.