the harbour surface in eight areas showed a mean offset of -0.85 m. A bimodal
distribution with five areas revealed a mean offset of -0.93 m, while three others gave a
mean of -0.66 m (Webster et al., 2002). The standard deviation of the LIDAR
elevations on the water surface in the eight sample areas ranged from 0.14 to 0.31 m for
sample sizes between 1,013 and 3,634. The disagreement in the LIDAR elevations and
water level data was later interpreted as possibly relating to tidal hydraulics in the
Yorke and Hillsborough River arms of the harbour (Webster et al., 2004).
A separate comparison was made between a Canadian Hydrographic Service
(CHS) benchmark CHTN 1-1963 on the Coast Guard wharf near the tide gauge at the
foot of Queen Street in Charlottetown and LIDAR hits on the wharf surface within a
radius of 3 m. The ellipsoidal and geoidal (CGVD28) elevations of the benchmark were
determined as part of the vertical datum control survey reported in King et al. (2002).
Initial comparison with presumed ground (bald-earth) points in the LIDAR data gave
an offset of -2.2 m, but it turned out that the classification algorithm had erroneously
identified water surface points as ground points and wharf deck points as non-ground.
Subsequent identification of points on the wharf deck in the vicinity of the benchmark
showed an offset of -0.92 m ellipsoidal. On the basis on these results and consultation
with the data acquisition contractor in which no systematic errors were identified, an
adjustment of +0.9 m was applied to all data points prior to incorporation in the DEM.
This produced realistic flooding levels for a simulation of the 21 January 2000 storm
surge (Webster et al., 2002, 2003), whereas the original DEM without adjustment had
suggested much more extensive flooding.
There was some question whether the 0.9 m adjustment would be equally
applicable in the North Shore study area some distance from Charlottetown. Therefore,
validation work was undertaken in that area as well, consisting of surveying cross-shore
profiles using real-time kinematic (RTK) differential GPS techniques with horizontal
and vertical resolution better than 5 cm. These data were available at eight monitoring
sites maintained by the Geological Survey of Canada in the study area (Forbes and
Manson, 2002). Comparison of gridded LIDAR points (processed using GRASS as
described below) with RTK survey points along one such transect at Brackley Beach
(Figure 4) showed good correspondence. This profile crossed two dune ridges and
revealed that the crest elevation of the high narrow dune crest was somewhat
underestimated in gridded LIDAR topography. Reflection from the tops of rose and
bayberry plants was also evident in an overestimation of ground elevations in the
depression between the two dune crests. Otherwise, this example showed successful
validation and indicated that the 0.9 m offset adjustment was appropriate in the North
Shore study area.
To further test the validity of the 0.9 m adjustment, a high-precision GPS campaign
was carried out within the Charlottetown survey area in the summer of 2001 (Fraser,
2001). Carrier phase static GPS measurements were collected and processed at 15 sites
throughout the city so that a more detailed analysis of the height differences could be
done (Figure 5). The GPS sites were selected based on a variety of factors including:
spatial distribution throughout the LIDAR study area; GPS satellite geometry to
minimize obstructions (e.g. away from large buildings where multipath could be a
problem); flat smooth areas of dense LIDAR point coverage such as grass fields in city
parks; and critical waterfront features such as wharf decks. Local GPS base stations
were established using the provincial geodetic control network. Baselines between the
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Airborne Laser Altimetry
distribution with five areas revealed a mean offset of -0.93 m, while three others gave a
mean of -0.66 m (Webster et al., 2002). The standard deviation of the LIDAR
elevations on the water surface in the eight sample areas ranged from 0.14 to 0.31 m for
sample sizes between 1,013 and 3,634. The disagreement in the LIDAR elevations and
water level data was later interpreted as possibly relating to tidal hydraulics in the
Yorke and Hillsborough River arms of the harbour (Webster et al., 2004).
A separate comparison was made between a Canadian Hydrographic Service
(CHS) benchmark CHTN 1-1963 on the Coast Guard wharf near the tide gauge at the
foot of Queen Street in Charlottetown and LIDAR hits on the wharf surface within a
radius of 3 m. The ellipsoidal and geoidal (CGVD28) elevations of the benchmark were
determined as part of the vertical datum control survey reported in King et al. (2002).
Initial comparison with presumed ground (bald-earth) points in the LIDAR data gave
an offset of -2.2 m, but it turned out that the classification algorithm had erroneously
identified water surface points as ground points and wharf deck points as non-ground.
Subsequent identification of points on the wharf deck in the vicinity of the benchmark
showed an offset of -0.92 m ellipsoidal. On the basis on these results and consultation
with the data acquisition contractor in which no systematic errors were identified, an
adjustment of +0.9 m was applied to all data points prior to incorporation in the DEM.
This produced realistic flooding levels for a simulation of the 21 January 2000 storm
surge (Webster et al., 2002, 2003), whereas the original DEM without adjustment had
suggested much more extensive flooding.
There was some question whether the 0.9 m adjustment would be equally
applicable in the North Shore study area some distance from Charlottetown. Therefore,
validation work was undertaken in that area as well, consisting of surveying cross-shore
profiles using real-time kinematic (RTK) differential GPS techniques with horizontal
and vertical resolution better than 5 cm. These data were available at eight monitoring
sites maintained by the Geological Survey of Canada in the study area (Forbes and
Manson, 2002). Comparison of gridded LIDAR points (processed using GRASS as
described below) with RTK survey points along one such transect at Brackley Beach
(Figure 4) showed good correspondence. This profile crossed two dune ridges and
revealed that the crest elevation of the high narrow dune crest was somewhat
underestimated in gridded LIDAR topography. Reflection from the tops of rose and
bayberry plants was also evident in an overestimation of ground elevations in the
depression between the two dune crests. Otherwise, this example showed successful
validation and indicated that the 0.9 m offset adjustment was appropriate in the North
Shore study area.
To further test the validity of the 0.9 m adjustment, a high-precision GPS campaign
was carried out within the Charlottetown survey area in the summer of 2001 (Fraser,
2001). Carrier phase static GPS measurements were collected and processed at 15 sites
throughout the city so that a more detailed analysis of the height differences could be
done (Figure 5). The GPS sites were selected based on a variety of factors including:
spatial distribution throughout the LIDAR study area; GPS satellite geometry to
minimize obstructions (e.g. away from large buildings where multipath could be a
problem); flat smooth areas of dense LIDAR point coverage such as grass fields in city
parks; and critical waterfront features such as wharf decks. Local GPS base stations
were established using the provincial geodetic control network. Baselines between the
163
Airborne Laser Altimetry