Wairoa 0.1m Urban Aerial Photos (2014-2015)

Data Acquisition:
The aerial photography for this project was captured within the 2014/15 flying season (September
2014 – April 2015) on 22 December 2014 and 5 January 2015.

All photography was captured using Vexcel's digital UCLp camera and flown at 3850ft (1173m) flying height.

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 or Cyient staff.

Map Projection:
All spatial data for this project provided in terms of New Zealand Transverse Mercator (NZTM) map
projection. The datum is New Zealand Geodetic Datum 2000. The height datum is orthometric
Napier 1962 (sea level). The products are tiled into NZTopo50 map sheet tiles as noted below.


Image Processing and Aerial Triangulation:
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. Imagery processed to level 3 and checked for colour correctness/radiometry and
even tonal balance across each project area. The aerial triangulation 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. LINZ control, 9th order horizontal and 3rd order vertical and other existing control from Aerial
Surveys control data base were used to strengthen the block adjustment or as independent checks on
position during final QA of the ortho imagery. A final report is generated to check RMSE values are
within specification.

DTM Creation:
The digital terrain model used for this project was derived from LiDAR DTM data flown in 2003. The
DTM was then updated in any areas of change. Outside the existing data the DTM creation for the
urban areas was collected from stereo imagery using photogrammetric techniques, largely automated
pixel matching and auto-correlation process that creates mass points of the terrain surface with
further manual editing to remove points on water bodies and breaklines added around water bodies
and along all ridges, valleys and areas of steep terrain change, such kerbs, retaining walls, drains. In
areas of dense vegetation form lines are collected. The final DTM took the form of breaklines and
mass points. A Triangulated Irregular Network (TIN) was then created and used for the orthorectification
process. DTM Accuracy: ±0.2m @ 95% confidence level in clear open areas (2 sigma).

RGB and Ortho Rectification Process: Ortho rectification is the process of removing (from the image) the effects of camera tip/tilt and displacement caused by terrain relief. During this process each frame is 'draped' over the terrain model and the photograph then becomes 'scaled' and 'levelled' in terms of true ground coordinates. 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 then aligned to LINZ sheet tile layout.