Christchurch Post-Earthquake 0.1m Urban Aerial Photos Index Tiles (24 February 2011)

Data Processing
In order to expedite their production these orthophotos were produced using a number of shortcuts that would not be followed for a fully specified orthophoto project. While the orthophotos are fit for use by experienced geodata users they are not suitable for general distribution without a suitable disclaimer.

The aerial photos position and orientation (POS) were determined using the POS observations collected at the GPS base station and in the aircraft. This data was processed using NZGD2000 reference system. A coordinate for the base station was computed using single baseline processing and data supplied by to NZAM by GNS, from the GeoNet station MQZG. Given the magnitude of the earthquake it is likely that the location of MQZG has changed. However, as no information is available yet it had to be assumed that the coordinate for MQZG had not changed.

For the orthophoto generation NZAM used DTM sourced from within our archive. This DTM was not edited or checked for change.

Automated mosaic seam line placement was used during the orthophoto production. NZAM chose to use a simple ‘most nadir ‘algorithm for their placement. This selects the most central portion of each available photo and thereby helps minimise the amount of perspective view lean on buildings. The seam lines can be clearly seen on some of the photography.

Data Supply
The geodata is all in terms of New Zealand Transverse Mercator map projection.

The Orthophotos are tiled into NZTopo50 1:1000 tiles and named using the convention NZTopo50PrimeAABB (e.g. BX0302). The orthophotos are supplied in GeoTIFF file format.

Quality Exceptions
The orthophotos have only had a cursory review. Given that the DTM was not updated and checked it is likely that the orthophotos will include areas were the imagery appears smeared or is out of position.

While NZAM endeavoured to collect cloud free photography there is the odd puff of cloud and cloud shadow in the imagery. This would not be acceptable for a fully specified orthophoto project, but they do not significantly impact on the useability of the photography for the earthquake response work.