Christchurch 0.075m Urban Aerial Photos Index Tiles (2020-2021)

Data Acquisition

The Camera
Imagery for this schedule was acquired using the Leica ADS100 line scanning camera with SH100 sensor head. The camera was fitted to a Leica PAV100 HP high performance gyro-stabilised mount.

Flight Planning
Careful consideration was given during flight planning to geographic location, terrain, topographical characteristics of the area, acquisition efficiency, final output resolution and meeting the various requested orthophoto quality specifications.

Environmental Specification
All imagery was captured without the presence of cloud or cloud shadow within the specified areas of interest. Where affected, all coastal, harbour, and tidal regions were captured during low tide periods, this being defined as the period 2 hours either side of low tide. Imagery covering coastal areas extends from the tide line during low tide periods out to 100 meters. Imagery of harbour areas extends out to 100 meters from the perimeter of the harbour. Rivers were captured during times of appropriate flow/stage height/rainfall restrictions as specified by the client. These criteria were set to avoid periods of high river flow (flooding), rather than to target abnormally low river flows.

Elevated Feature Displacement
The Christchurch area was flown with a minimum of 40% side overlap as limited by the Nadir sensor. Wherever practical, high side overlap was used to minimize the effects of elevated feature displacement in urban areas.

Ground Control
A total of 224 new ground control points were observed for use with the aerial triangulation and bundle adjustment of Schedule E areas. Data from Global Survey’s and LINZ CORS sites, along with several existing LINZ benchmarks, were used in Leica Infinity software to process and check the ground control.

Data Processing
Landpro partnered with Hexagon Geosystems for image processing.

Software
GNSS/IMU processing: Novatel Inertial Explorer
AT, SGM/Orthophoto Generation: Leica Xpro, DTM Filtration, QS2 & LAS Tool Environment
DTM Preparation: TerraSolid, Global Mapper
Seamline Generation: QS2 & Image Manager, Sweep and seamline editing, QM2, Inpho Orthovista
DTM Editing: DTM Editor Tool, Tile Extraction, QS2 Delivery Manager

Map Projection and Datum
All spatial data for this project is supplied in the New Zealand Transverse Mercator (NZTM2000) projection in terms of the New Zealand Geodetic Datum 2000 (NZGD2000).

Aerial Triangulation (AT)
No abnormalities or complications were encountered during this phase. All aerial triangulations are carried out to ensure accuracies in the region of 1xGSD value. This was achieved with all adjusted RMS residual values in the desired region of ±7.5cm, ensuring excellent overall quality and spatial accuracy

DEM for Orthophoto

Point Cloud Generation
XPro uses Semi-Global Matching (SGM) for high-resolution DSM computation. A ratio of 1:1 was used for generation resulting in a 7.5cm GSD resolution point cloud. Point clouds are output as RGBN encoded LAZ files.

Point Cloud Filtration
SGM filtration was done in Batch Converter and consisted of two processes following one after the other:
• Lasground - Filter of input data and extract ground points from input point cloud
• Lasclip – Removal of ground points inside polygons with buildings

DEM Preparation
The resulting point cloud was used to produce a 32bit floating raster DEM for use in orthophoto generation. The following accuracy requirements have been met for this DEM:
• Vertical Accuracy ≤1 metre (@ 90% confidence)
• Horizontal Accuracy ≤2 meters (@ 90% confidence

RGBN Orthophoto Production
Individual radiometric profiles were checked for each strip ensuring optimal results across the final ortho-mosaic. No complications were encountered during orthophoto generation, mosaicking and tiling. Various quality checks including accuracy, colour, contrast, sharpness, seamline positioning, refraction and generation artifacts were performed and found satisfactory on the final orthophoto tiles.

Peripheral Imagery
Peripheral imagery has been orthorectified with the same DEM used to generate the primary AOI orthophotos. As such, these DEMs may not cover the entire area of peripheral imagery and associated artefacts may be present.