Ashburton 0.075m Urban Aerial Photos (2021-2022)

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 area was flown with a minimum of 40 % side overlap as limited by the Nadir sensor. Ashburton CBD area, as defined by ECAN, was flown with 80 % side overlap. This was done to mitigate the effects of elevated feature displacement in an urban area.

Ground control
A total of 102 ground control points were observed for use with the aerial triangulation and bundle adjustment. 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.

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 were carried out to ensure accuracies of less than 1 x GSD value. This was achieved with all adjusted RMS residual values at less than 7.5 cm, ensuring excellent overall quality and spatial accuracy.

Digital Terrain Model (DTM)
XPro uses Semi-Global Matching (SGM) for high-resolution DSM computation. A ratio of 1:1 was used for generation resulting in a 7.5 cm GSD resolution point cloud. Semi-Global Matching 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
The following accuracy requirements have been met for this DTM:
• Vertical Accuracy ≤ 1 metre (@ 90% confidence)
• Horizontal Accuracy ≤1 meter (@ 90% confidence)
• Grid spacing = 1 meter cell size

Orthorectified aerial photography
Individual radiometric profiles were used 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.