Linz releases stunning post-quake aerial photos of Christchurch

Land Information New Zealand (Linz) has publically released aerial photos of Christchurch taken two days after the devestating February 22 earthquake.

The photos can be accessed through Linz website here (a version of the "orthophotos" for Microsoft's free-download Silverlight has amazing resolution; an alternative Javascript viewer was slow and buggy when NBR tried it. UPDATE: there's also a fast, annotated viewer at eqviewer.co.nz).

Civil Defence National Controller, John Hamilton, commissioned the aerial images to help inform the work of the Christchurch Response Centre (CRC). 

“It’s important to remember that this imagery is a snapshot of the way Christchurch looked a couple of days after the earthquake. It’s good that this information is now available to all of New Zealand and the world.” said Geoff Howard, New Zealand’s Chief Topographer.

The imagery was collected by New Zealand Aerial Mapping (NZAM) flying at 1600m above the ground using a Vexcel UCXp large format digital aerial camera. 

ABOVE: A tiny section of the aerial image, seen in Silverlight. Click to zoom.

Extra for experts
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.

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 to NZAM by GNS Science, 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 was available at the time the photos were ortho rectified (made into a photo map) it was assumed that the coordinate for MQZG had not changed.

For the orthophoto generation NZAM used a Digital Terrain Model (DTM) sourced from within its 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.