Click here to download the point cloud data for the North Shore coastline
DATA ACQUISITION
Airborne Data Acquisition
An airborne laser scanner survey was conducted over the North Shore, from North Head to Long Bay
(approximately 22.5 km following the shoreline). Operations were undertaken on 19th June 2019 in good flying
conditions. Data were acquired using a Riegl VUX-1LR lidar system, mounted on an EC120 helicopter, operated
by Christchurch Helicopters. The laser survey was based on the following parameters:
Parameter
Parameter
Scanner
Riegl VUX-1LR
Pulse Repetition
820 kHz
Flying Height
50-80 m above ground
Swath Overlap
75-100%
Scan Angle
180 degrees
Aircraft speed
45 knots
Scan Frequency
170 Lines per second
Nominal pulse density
50 pls/m2 (p/flightline)
The scanner-IMU was mounted on a front facing boom extending below the cockpit with an unobstructed
240-degree field of view, with a GNSS antenna mounted on the cockpit.
Survey operations were conducted from North Shore Aerodrome, with each survey comprising a sequence of short,
linear flightlines aligned to the coast. Flightlines were acquired north-south, and then south-north, to
account for the effects of occlusion during a single overpass. Each return sortie too approximately 70 mins
of flying time (not including travel time to and from a regional base). Following the first sortie, all
instrumentation was powered down and dismounted, before being remounted and reinitialized. This approach
mimics exactly the procedure that would be followed between two widely separately surveys in time.
Global Navigation Satellite Systems (GNSS) Base Station Data
GNSS observations were recorded at a 3rd order (2V) LINZ geodetic mark (GSAL) to correct the roving
positional track recorded at the sensor. This is a continuous operating reference station (CORS) operating
as part of Global Surveys Leica Geosystems SmartFix network, recording observations at 1 s. The details of
the reference station are as follows:
LINZ
Benchmark Code:
GSAL (Albany Triton)
Benchmark Position:
Latitude:
36° 44' 27.51079" S
Longitude:
174° 43' 23.50966" E
Ellipsoidal height
(m):
88.262
Antenna:
Leica AS10
A further ground survey of check point data was acquired using Leica GS15 GNSS systems operating using
network RTK GNSS based on the Global Survey SmartFix network. >300 observations were acquired from
across the survey area, classified by land-cover to include hard surfaces (roads); and short grass pasture.
Note: network RTK GNSS have typical absolute accuracies of 4-6 cm over the baseline lengths used here (15-25
km).
Real Time Kinematic GNSS Checkpoint Data
A distributed network of 351 checkpoints were acquired as checkpoints to evaluate the vertical accuracy and
precision of the survey data. All points were collected using network-derived RTK GNSS observations based
on the Leica Geosystems SmartFix network of broadcasting referencing stations. Measurements were acquired
with a Leica GS16 receiver on the 24th January 2020, and acquisition settings that enforced a 3D standard
deviation of < 0.025 m for each observation. To capture any broad scale patterns of georeferencing
error, the checkpoints were collected in four regional surveys at Browns Bay, Mairangi Bay, Milford and
Narrow Neck, as shown in Figure 6 overleaf.
DATA PROCESSING
Trajectory Modelling
Lidar positioning and orientation (POS) was determined using the roving GNSS/IMU and static GNSS observations
acquired using Waypoint Inertial Explorer Software. The resulting solution maintained attitude separation
of less than +-2 arcmin and positional separation of less than +-1 cm. Trajectories were solved
independently for each of the two surveys.
Lidar Calibration
Swath calibration based on overlap analysis was undertaken using the TerraScan and TerraMatch software
suite. Flightline calibration was undertaken to solve for global and flightline specific deviations and
fluctuations in attitude and DZ based on over 100,000 tie-lines derived from ground observations. The
results of the calibration, based on all used tie-lines is shown in Table 2 below:
Survey
Initial mean 3D
mismatch (m)
Calibrated mean 3D
mismatch
1
0.055
0.014
2
0.044
0.011
Point Cloud Classification
Data were classified using standard routines into ground, above ground and noise.
For Survey 1, the point density over the entire area is 97.5 points/m² for all point classes and 44.2
points/m2 for only ground points.
For Survey 2, the point density over the entire area is 55.7 points/m² for all point classes and 30.9
points/m2 for only ground points.
The difference between the two datasets reflects trimming of Survey 1 to incorporate only the coastal fringe,
while Survey 2 extends inland by typically 300 m to provide a demonstration of the potential wider coverage
observable from the flightpath. On the beach areas and along the cliff sections, typical densities are in
excess of 100 points/m2 in both surveys. The final point cloud classification for each survey is shown in
Table 3:
Surface Type
Classification Code
Point Class
Survey 1
Observations
Survey 2
Observations
Unclassified
1
Off-Ground
204,644,243
226,749,086
Ground
2
Ground
143,160,406
182,111,679
Total Points
347,804,649
408,860,765
