Application of the polynomial adjustment to the aeromagnetic survey of theSpanish Mainland; Requirements and shortcomings

Polynomial surfaces are a very useful tool in order to represent the geomag netic field over small area of the Earth's surface. Nevertheless, the metho d has not always been applied with enough precision, nor its limitations ar e considered in detail. This work analyses the conditions of applicability of this method to data from the aeromagnetic survey of the Spanish mainland accomplished in 1986, and the fulfilment of the necessary conditions to en hance a merely descriptive model to produce another of inferential characte r. The best statistical adjustment with only descriptive character, valid f or the whole Spanish mainland, is a third degree model. The study proves th at the conditions of validity for inferential models are met using polynomi als of second and third degree in restrictive areas, but not over the whole Iberian Peninsula. In this case, the model of second degree is proposed as the best option because of its simplicity and the extension of its region of validity. To a great extent, the restrictions to enhance the descriptive models are due to the own nature, specially to their geographical dependen ce, and can also affect the analysis of other kinds of geophysical data.