Otago - Dunedin and Mosgiel LiDAR 1m DEM (2021)

Data Acquisition:
Airborne Laser Scanner (ALS) data was acquired from a fixed wing aircraft on 24 June 2021, using AAM's Optech Galaxy Prime 473 LiDAR system.

Survey Specification:
 Scanner: Optech Galaxy Prime 473
 Flying Height: 1070 m AGL
 Scan Angle: ±23 degrees
 Scan Frequency: 66 Hz
 Pulse Rate: 600 kHz
 Swath Overlap: 30%
 Swath Points Per M2: 8

Data Processing:

RTX processing was utilised to calculate the GPS trajectory. Trimble CenterPoint® RTX™ is a proprietary GPS, GLONASS, BeiDou, and QZSS enabled technology that provides high-accuracy GNSS positioning worldwide without the use of traditional local base stations or a VRS network. By combining real time data from a global reference station infrastructure with innovative positioning and compression algorithms, Trimble RTX technology computes centimeter-level positions based on satellite orbit and clock information. Trajectory processing provides the accurate position and orientation of the sensor, essential for georeferencing the dataset.

All initial processing was undertaken in UTM60S/ITRF2008. Reprojection and application of the
NZGD2000 deformation model converted the data to NZTM/NZGD2000 using concord software. To convert the point cloud heights from ellipsoidal to NZVD2016 heights, a geoid adjustment was performed using the NZGeoid2016 separation model.

AAM uses proprietary automated ground classification routines in TerraScan, based upon algorithms tailored for major terrain/vegetation combinations, to initially classify the laser strikes into ground / non-ground classification and to generate an accurate ground surface. The ground classification is then manually reviewed and edited to reach ICSM level 2 standards. Following this process, non-ground classes are then classified using automated routines and macros. Water was also manually reviewed to ensure correct presentation in the downstream elevation products (DEM and DSM). The definition of the ground points may be less accuracte under vegetation.

Classification of the point cloud follows the classification scheme below:
1 - Unclassified
2 - Ground
3 - Low Vegetation
4 - Medium Vegetation
5 - High Vegetation
6 - Buildings
7 - Low Noise
9 - Water
17 - Bridge Deck
18 - High Noise

WSP NZ Ltd undertook a field survey to acquire ground test points, assumed to be error-free, to validate the accuracy of the LiDAR data and derivative products. The test points were uniformly distributed across the area and located on clear ground. Comparison of the field test points with elevations interpolated from measured data, yielded the following accuracy assessment:

 Test point sites: 6
 No. points: 393
 Mean difference: 0.007 m
 St. Deviation: 0.025 m
 Standard Error (RMS): 0.026 m

The mean elevation difference was removed from the supplied data. The Fundamental Vertical Accuracy is therefore 5 cm (RMS x 1.96), which are within the RMS specification of 20 cm at 95% confidence.

The expected horizontal accuracy was calculated by Flying Height Above Ground(1070 m)/10,000 therefore is 21 cm at 95% CI for this dataset, which is within the specification of 100 cm.

The DEM was generated from the ground points to define the bare earth ground surface. Hydroflattening was performed through breaklines that were drawn manually and the coastline was clipped to a polygon that was derived from the water points boundary, to give a clean presentation along the coast.

The DSM was generated using first return ground and non ground classes, excluding high and low noise.

The deliverables to LINZ were:

1m gridded bare earth digital elevation model (DEM)
1m gridded digital surface model (DSM)
Classified point cloud

All product deliverables supplied in terms of NZTM map projection and NZVD2016 vertical datum.