Society of Photogrammetric Engineering and Remote Sensing for a new binary data
format standard for LIDAR data (Schuckman, 2003). Both ellipsoidal and orthometric
heights were requested for the final delivery of the LIDAR data in this study. The study
area was split into 1 km by 1 km tiles and two ASCII files were delivered for each tile,
one for ground points and one for non-ground points. Each ASCII file consisted of the
following fields:
x
UTM easting (m)
x
UTM northing (m)
x
height above WGS84 ellipsoid (m)
x
orthometric height above CGVD28 (m)
x
GPS time (s) from the start of each GPS week.
The addition of the GPS time stamp allowed us to analyze the data based on flight
lines, made it possible to extract data for times when the GPS constellation was poor,
and provided a time stamp for water level (from the tide gauge). Without times, it is
difficult to separate LIDAR returns between flight lines.
Terra Remote Sensing Inc. of Sidney, British Columbia, Canada, was contracted to
acquire the LIDAR survey data for two study areas: low-lying parts of the City of
Charlottetown and a coastal strip extending about 50 km along the central North Shore
of Prince Edward Island (Forbes and Manson, 2002). The data were acquired on 1-2
August 2000. The aircraft was positioned using phase kinematic GPS, referenced to a
geodetic ground monument north of the Charlottetown Airport. The LIDAR system was
a diode-pumped I/R YAG laser operating at a pulse repetition rate of 10 kHz (10,000
laser pulses per second) with a scanning mirror oscillation rate of 15 Hz and a scan
angle of 50
o . The LIDAR was a first-return system, so no subsequent returns were
measured in this case. Down-looking video was acquired simultaneously to assist in
interpreting the LIDAR data. With the aircraft at a flying height of 600 m, the LIDAR
ground swath was approximately 600 m wide and the ground spacing between LIDAR
points was less than 2 m. The technical specifications required the horizontal and
vertical accuracy to be 95% within 30 cm of measured GPS points.
2. Validation of LIDAR Elevation Models
Highly accurate DEMs of the coastal zone are required to predict flooding extent
from storm-surges of the order of 1 m. The accuracy of a DEM derived from LIDAR
data depends on the successful removal of systematic errors associated with the
acquisition system, and on a validation process to confirm that the specifications are
met. Filin (2003) provides an overview of systematic error types and treatment of these
errors in LIDAR systems. In order to ensure that the LIDAR data meet high vertical
accuracy specifications, independent ground validation data are required. However, in
the Charlottetown study area, the available topographic control at the time of the survey
was inadequate and a variety of approaches were used to test the LIDAR data.
During the initial quality assurance of the LIDAR data, water levels from the
Charlottetown tide gauge (Parkes et al., 2002) were used to assess the accuracy of water
surface hits in the LIDAR data set (Webster et al., 2002). During the LIDAR survey,
the winds were light, the harbour surface had no significant waves (although it may
have been rippled), and it was assumed that the water level at the tide gauge was a good
measure of water level throughout the harbour. A total of 16,515 LIDAR point hits on
162
Webster and Forbes
format standard for LIDAR data (Schuckman, 2003). Both ellipsoidal and orthometric
heights were requested for the final delivery of the LIDAR data in this study. The study
area was split into 1 km by 1 km tiles and two ASCII files were delivered for each tile,
one for ground points and one for non-ground points. Each ASCII file consisted of the
following fields:
x
UTM easting (m)
x
UTM northing (m)
x
height above WGS84 ellipsoid (m)
x
orthometric height above CGVD28 (m)
x
GPS time (s) from the start of each GPS week.
The addition of the GPS time stamp allowed us to analyze the data based on flight
lines, made it possible to extract data for times when the GPS constellation was poor,
and provided a time stamp for water level (from the tide gauge). Without times, it is
difficult to separate LIDAR returns between flight lines.
Terra Remote Sensing Inc. of Sidney, British Columbia, Canada, was contracted to
acquire the LIDAR survey data for two study areas: low-lying parts of the City of
Charlottetown and a coastal strip extending about 50 km along the central North Shore
of Prince Edward Island (Forbes and Manson, 2002). The data were acquired on 1-2
August 2000. The aircraft was positioned using phase kinematic GPS, referenced to a
geodetic ground monument north of the Charlottetown Airport. The LIDAR system was
a diode-pumped I/R YAG laser operating at a pulse repetition rate of 10 kHz (10,000
laser pulses per second) with a scanning mirror oscillation rate of 15 Hz and a scan
angle of 50
o . The LIDAR was a first-return system, so no subsequent returns were
measured in this case. Down-looking video was acquired simultaneously to assist in
interpreting the LIDAR data. With the aircraft at a flying height of 600 m, the LIDAR
ground swath was approximately 600 m wide and the ground spacing between LIDAR
points was less than 2 m. The technical specifications required the horizontal and
vertical accuracy to be 95% within 30 cm of measured GPS points.
2. Validation of LIDAR Elevation Models
Highly accurate DEMs of the coastal zone are required to predict flooding extent
from storm-surges of the order of 1 m. The accuracy of a DEM derived from LIDAR
data depends on the successful removal of systematic errors associated with the
acquisition system, and on a validation process to confirm that the specifications are
met. Filin (2003) provides an overview of systematic error types and treatment of these
errors in LIDAR systems. In order to ensure that the LIDAR data meet high vertical
accuracy specifications, independent ground validation data are required. However, in
the Charlottetown study area, the available topographic control at the time of the survey
was inadequate and a variety of approaches were used to test the LIDAR data.
During the initial quality assurance of the LIDAR data, water levels from the
Charlottetown tide gauge (Parkes et al., 2002) were used to assess the accuracy of water
surface hits in the LIDAR data set (Webster et al., 2002). During the LIDAR survey,
the winds were light, the harbour surface had no significant waves (although it may
have been rippled), and it was assumed that the water level at the tide gauge was a good
measure of water level throughout the harbour. A total of 16,515 LIDAR point hits on
162
Webster and Forbes