CoastGIS'99: geomatics and coastal environment
Density of the scanned points
The density of the scanned points is an important characteristic of a DTM
generated from laser scanning. The clients specify the required point
density based on the use of the elevation information. The laser scanner
parameters and the flying characteristics are determined in function of
the required point density. For the 1994 project, the average point
density asked by the GWA was 1 point per 12 m
2
(3.5 m * 3.5 m) while
the PWN required 1 point per 4 m
2 (2 m * 2 m) (Han, 1995). For the
1996 survey of the Dutch coast, the Survey Department asked for a point
density of 1 point per 16 square metres. After analysis ofthe processed
data, it came out that the point density was inappropriate to dune
areas. Dune tops were removed and valley bottoms were flattened. The
spatial resolution was too low to model the terrain elevation variations. To overcome this problem, the Survey Department decided to
increase the point density to 1 point per 6 m
2 , in order to get proper
coastal zone modelling.
Data processing
Data processing, which is widely automatic, consists in determining
the co-ordinates of the scanned points and classifying the points as
being ground points or else.
A first step in data processing consists to determine the position of
the plane during the survey using the dGPS measurements. The high
reliability of the plane position determination is due to the use of
several multiple GPS base stations m the field during the survey (Looman, 1997). Once the plane position along the flightline is determined,
the position of the laser scanner is interpolated along this flight line
considering the attitude changes of the laser scanner during the flight.
Then X, Y, Z, co-ordinates of the scanned points can be computed using
the position of the laser scanner, the direction of the laser beam and
the distance between the laser scanner and the scanned points. The
resulting point co-ordinates set includes data points resulting from
objects like houses and from vegetation in dense vegetation areas (Looman, 1997). Making use of the spatial relation between neighbouring
points, they are classified either as ground points or as object/vegetation
points.
Once the co-ordinates of the scanned points are generated and classified, the next step consists in performing quality controls and applying
corrections.
Quality control and achieved results
The quality controls of the DTM are performed in two ways. First an
mternal quality control is performed in analysing elevation differences
in overlapping areas of adjacent strips and using a transversal control
strip. This overlaps analysis consists m making sure that elevation
154
Density of the scanned points
The density of the scanned points is an important characteristic of a DTM
generated from laser scanning. The clients specify the required point
density based on the use of the elevation information. The laser scanner
parameters and the flying characteristics are determined in function of
the required point density. For the 1994 project, the average point
density asked by the GWA was 1 point per 12 m
2
(3.5 m * 3.5 m) while
the PWN required 1 point per 4 m
2 (2 m * 2 m) (Han, 1995). For the
1996 survey of the Dutch coast, the Survey Department asked for a point
density of 1 point per 16 square metres. After analysis ofthe processed
data, it came out that the point density was inappropriate to dune
areas. Dune tops were removed and valley bottoms were flattened. The
spatial resolution was too low to model the terrain elevation variations. To overcome this problem, the Survey Department decided to
increase the point density to 1 point per 6 m
2 , in order to get proper
coastal zone modelling.
Data processing
Data processing, which is widely automatic, consists in determining
the co-ordinates of the scanned points and classifying the points as
being ground points or else.
A first step in data processing consists to determine the position of
the plane during the survey using the dGPS measurements. The high
reliability of the plane position determination is due to the use of
several multiple GPS base stations m the field during the survey (Looman, 1997). Once the plane position along the flightline is determined,
the position of the laser scanner is interpolated along this flight line
considering the attitude changes of the laser scanner during the flight.
Then X, Y, Z, co-ordinates of the scanned points can be computed using
the position of the laser scanner, the direction of the laser beam and
the distance between the laser scanner and the scanned points. The
resulting point co-ordinates set includes data points resulting from
objects like houses and from vegetation in dense vegetation areas (Looman, 1997). Making use of the spatial relation between neighbouring
points, they are classified either as ground points or as object/vegetation
points.
Once the co-ordinates of the scanned points are generated and classified, the next step consists in performing quality controls and applying
corrections.
Quality control and achieved results
The quality controls of the DTM are performed in two ways. First an
mternal quality control is performed in analysing elevation differences
in overlapping areas of adjacent strips and using a transversal control
strip. This overlaps analysis consists m making sure that elevation
154