Small-footprint Airborne Laser Scanner Data for Validating Marine Geoid Models
Gruno, Anti¹; Vain, Ants²; Liibusk, Aive³; Ellmann, Artu¹; Oja, Tõnis²; Jürgenson, Harli^3
1Tallinn University of Technology, ESTONIA; ²Estonian Land Board, ESTONIA; ³Estonian University of Life Sciences, ESTONIA

Airborne Laser Scanners (ALS) have been widely used for collecting topographic data over large areas. The main use of ALS data has been digital elevation models, but it has proven to be a very useful in many other applications. For instance, two wavelengths ALS can be used for bathymetric measurements in coastal waters. This study however, utilizes one wavelength ALS system for validating gravimetric geoid models over marine areas. This is made possible by the ALS ability to registrate echoes from the water surface, mostly in the nadir range.

For validation case was taken a new high-resolution (1'x2') regional gravimetric geoid model GRAV-GEOID2011, which is a combination of the ESA gradiometric satellite GOCE data with regional terrestrial gravity datasets. Note that over the marine areas, sea bottom gravity measurements from 1966-1967 alongside with a recent airborne gravity campaign data were included into the geoid modeling. Several campaigns on the ice cover of Gulf of Riga during the winter 2010 were also carried out with Scintrex CG5 gravimeter to validate the current airborne and sea bottom gravity data. The limitations of the sea ice measurements are the transportation issues on ice, the time that takes to cover large areas and the ice cover itself which may vary from winter to winter in a great extent. The resulting geoid model covers the entire area of Estonia and surrounding waters of the Baltic Sea. Using GNSS/levelling data within the Estonian dry land revealed 1-2 cm accuracy for the geoid model. Note that such a validation cannot proceeded over marine areas.

Therefore the ALS method was used to evaluate the GRAV-GEOID2011 accuracy over marine areas. Two test areas (the Väinameri Basin and the Gulf of Riga) at the eastern shores of the Baltic Sea were selected for regional geoid validation due to the remarkable changes in the geoid undulations in those areas. The vertical position (with respect to the GRS-80 reference ellipsoid) of the sea water level, which follows roughly the equipotential surface of the Earth's gravity field, was determined by the Leica ALS50-II laser scanner. Measurements were carried out under calm weather conditions in May 2012 at the altitude of 2400 m and included 60-100 km long flight route in three different locations. From detected ALS echoes a 100 m wide corridor in nadir was selected. Accurate tide gauge data were added to the ALS results to correct the heights of the point cloud. After primary point cloud processing, data was filtered with Averaging Time Filter to remove sea wave oscillation. The remaining long wavelength signal was compared with GRAV-GEOID2011 model. The comparison revealed that the detected discrepancies between the filtered ALS measurements and the geoid model do not exceed +/- 10 cm. This study revealed that ALS can be used as an alternative method to validate gravimetric geoid on open water areas.

Keywords: ALS, marine geoid, GOCE, mean sea level