The following dissertation partially gathers the research carried out within two projects of the DGICyT (Dirección General de Investigación Científica y Técnica, Spanish Government Department for Scientific and Technical Research) in the National Park of Doñana. The main objectives of those projects were to establish a reference frame for positioning within the Park and to determine an accurate geoid. This would allow obtaining orthometric heights derived from GPS observations, which are very useful in those studies regarding the hydrology and geohydrology of the area. The absence of the geoid model would lead to errors when determining the orthometric heights through ellipsoidal ones, due to the great geoid undulation gradient in this zone.

The National Park of Doñana is a protected area due to its great ecological value. It is located in the SW coast of Spain, between Huelva and Sevilla, and it has an extension of 500 km2 approximately. Almost half of its territory is a wetland, covered with water during the humid season. Additionaly, there are several restricted areas. These circumstances difficult the field observations.

The first part of the work focuses on the analysis of geodetic and levelling networks already established in the area. It is important to know the conditions and accuracy of these networks, since they will become the base for the network instilled in the inner zone of the Park. The network was used initially when adjusting the different geoid models and later when computing a new model. The local network was linked to the Spanish national network (REGENTE) and to the highly precision levelling network (NAP).

The adjustment of the gravimetric geoids IBERGEO95 and EGG97 in the zone was checked.
Both were adjusted by means of GPS/levelling/gravimetric observations at some selected points, in two axis showing a N-S and E-W direction approximately, in order to control any directional variation.

Checking the adjustment of IBERGEO95 and EGG97 allows to deduce that there is no guarantee of centimetre accuracy within the Park. Thus, a geometric geoid was computed by means of the remove-restore technique, using for that purpose: the global model CG01, obtained from the combined solution of earth data and data coming from space missions CHAMP and GRACE; and the digital terrain model obtained by merging the MDT25 model (from the Instituto Geográfico Nacional, National Geographic Institute of Spain) and the bathymetric data of the zone (from the Instituto Hidrográfico de la Marina, Marine Hydrogeographic Institute of Spain)

The field data adquired within this work, as well as the geoid model obtained (namely Doñana 2005) will be of great help for the different studies regarding the superficial hydrology of the marshland and the tectonic and hydrogeological structure of the Park. These studies are currently being carried out and clearly show the inter-relationship among geodetic, geological and hydrogeological disciplines.