Waimakariri 0.05m Urban Aerial Photos Index Tiles (2021)

Data Acquisition:
The aerial photography for this project was captured within the 2020/21 flying season (September 2020 – April 2021).

Camera and Flying Height:
All photography was captured using Vexcel's digital UCE camera fitted with the 100 mm lens and flown at an altitude of approximately 3,155 ft (962 m) with the lowest ground GSD set at 0.05 m.

Capture Dates:
The aerial imagery was captured on 3 January, 4 January 2021, and 7 February 2021.

Sun Angle:
Imagery captured with a minimum sun angle of +40 degrees. The sun angle of the imagery used for the orthorectification ranged from 46.8 to 68.5 degrees.

Flight Planning:
The flight survey number for this project is SN14152. The flight plan is named Waimakariri Urban 5cm UCE and comprises 41 runs, 1,267 frames. All imagery captured in stereo: forward overlap 60%, min 54%; side overlap 35%, min 15%.

Environmental Specification:
Imagery captured with no cloud or cloud shadow within the area of interest.

Urban Building Displacement Specification:
Urban 0.05 m GSD imagery – using the UCE camera and by flying with 60% forward overlap and with 35% sidelap (standard stereo coverage) will achieve ±1.1 m building lean per 3 m height in the corners of the imagery used. Tall structures inwards of the corner of the frame will have less lean the closer to the nadir they are.

Ground Control:
A combination of existing control and LINZ benchmarks were used for aerial triangulation for this project.

Data Processing:
All aspects of the data processing from imagery processing to DTM creation and ortho production and product deliverables was undertaken in-house by Aerial Surveys staff.

Map Projection:
All spatial data for this project provided in terms of New Zealand Transverse Mercator 2000 map projection (NZTM2000). The ellipsoidal datum is New Zealand Geodetic Datum 2000 (NZGD2000). The airborne GPS and ground control GPS data was converted from ellipsoidal heights into orthometric heights using the LINZ NZGeoid16 separation model. For this project the orthometric vertical datum is New Zealand Vertical Datum 2016 (NZVD2016).

Image Processing:
Level-0 to Level-3 image processing carried out using UltraMap software. All imagery has gone through QA checks ensuring there is no cloud cover and cloud shadow. During aerial acquisition the aircraft on-board GPS navigation data and ground base station data collected and post processed. Level-3 imagery process provides model based colour correctness/radiometry and even tonal balance across each project area.

Aerial Triangulation (AT):
Aerial triangulation carried out using Intergraph with Hexagon ISAT software. The AT brings together the GPS data and imagery using a two part process which stitches the imagery together using tie point matching for the relative orientation phase and observing ground control points for the absolute orientation phase. A final report is generated to check RMSE values are within specification. AT in the project has been performed to make the expected accuracy of ±1 pixel in XY and ±2 pixels in Z.

DTM for Ortho Production
The digital terrain model used for this project was derived from LiDAR DTM data flown in 2020. Outside the LiDAR DTM a DTM was collected from the 0.05 m GSD stereo imagery using photogrammetric techniques, largely automated pixel matching and autocorrelation process. The DTM data was merged together seamlessly and accuracy checked to meet the ortho imagery specification. The DTM was edited suitable for the ortho production.

Ortho Rectification Process
Ortho imagery created using Hexagon OrthoPro software. The imagery is orthorectified using the stereo-edited digital terrain model (DTM) to provide a geometric accurate seamless ortho mosaic dataset. The generation of seamlines between frames follow natural physical features such as ridges, valleys, roads and rivers. The seamlines are used for the final ortho mosaic that stitches the imagery together using feather mosaicking techniques. The ortho imagery is extracted aligned to the LINZ 1:500 map sheet tile layout. Minimal colour adjustment was undertaken, generally over water areas and along coastal foreshore for colour matching and to minimise glare. Ortho accuracy was checked by observing a selection of Geodetic marks around the project area and the results are shown below. The ortho accuracy met the expected of +0.1 m @ 95% confidence level in clear flat open areas. The points that were in flat open areas were well within accuracy.