A method of using reference sites comprising: providing a plurality of reference sites that are referenced by geographic coordinates; obtaining, at a sensor, an image of a portion of the surface of the Earth, for which geographic coordinates of the surface portion covered by the image are known; identifying reference sites within the image based on geographic coordinates; determining which of the identified reference sites are visible in the image; and using the visible reference sites to compute one or more atmospheric parameters; wherein the reference sites are modeled atmospheric correction objects (MACOs) that are identified as relatively homogeneous, and being likely to change over time in a predictable manner; characterised in that: by detecting whether or not a MACO site is impacted by transient conditions, wherein the transient conditions include a shadow: predicting a surface reflectance for the particular MACO site; determining a radiance received at the sensor for the particular MACO site; and altering the radiance received at the sensor for the particular MACO site to compensate for shadow effects; wherein the one or more atmospheric parameters are computed by comparing the altered radiance received at the sensor for the particular MACO site with the predicted surface reflectance for the particular MACO site. providing a plurality of reference sites that are referenced by geographic coordinates; obtaining, at a sensor, an image of a portion of the surface of the Earth, for which geographic coordinates of the surface portion covered by the image are known; identifying reference sites within the image based on geographic coordinates; determining which of the identified reference sites are visible in the image; and using the visible reference sites to compute one or more atmospheric parameters; wherein the reference sites are modeled atmospheric correction objects (MACOs) that are identified as relatively homogeneous, and being likely to change over time in a predictable manner; characterised in that: by detecting whether or not a MACO site is impacted by transient conditions, wherein the transient conditions include a shadow: predicting a surface reflectance for the particular MACO site; determining a radiance received at the sensor for the particular MACO site; and altering the radiance received at the sensor for the particular MACO site to compensate for shadow effects; wherein the one or more atmospheric parameters are computed by comparing the altered radiance received at the sensor for the particular MACO site with the predicted surface reflectance for the particular MACO site. by detecting whether or not a MACO site is impacted by transient conditions, wherein the transient conditions include a shadow: predicting a surface reflectance for the particular MACO site; determining a radiance received at the sensor for the particular MACO site; and altering the radiance received at the sensor for the particular MACO site to compensate for shadow effects; predicting a surface reflectance for the particular MACO site; determining a radiance received at the sensor for the particular MACO site; and altering the radiance received at the sensor for the particular MACO site to compensate for shadow effects; wherein the one or more atmospheric parameters are computed by comparing the altered radiance received at the sensor for the particular MACO site with the predicted surface reflectance for the particular MACO site.