The Photogrammetry Process

Aerial Photography

High quality aerial photographs are acquired by Terralink International partnering with some of Australasia’s top flying companies. We have staff with many years experience in planning and managing the process of capturing aerial photographs and readying them for the next step in the photogrammetry process.

Terralink International has the most comprehensive stock of orthorectified aerial images of New Zealand on hand, of varying scales and ages. We are constantly adding to our store of images are plan to have complete high resolution aerial imagery coverage of New Zealand.

Satellite Imagery

Satellite photos are used in the photogrammetry process in the same way as aerial photographs. As the cost of satellite imagery comes down, it is used more and more and has several distinct advantages over aerial imagery.

When acquiring aerial imagery it is necessary to schedule specially equipped aeroplanes to be in the area. The pilots may then have to wait for clear weather, as photos of clouds are not very useful. Satellites are constantly in the air so the scheduling dilemma is removed. Satellites can revisit a scene more frequently, taking pictures as often as three days apart. Satellites provide multi-spectral images which allow for very detailed analysis of the images produced. To find out how satellite imagery can be used take, a look at the Land Cover Database of New Zealand.

Scanning

Once aerial photographs have been acquired, they must be scanned into a digital format to allow manipulation and orthorectification. To do this Terralink International has two state of the art Lecia and ZI Photoscan 2000 scanners for both single frame and roll film scanning.

Scanned photographs can be supplied in colour or black and white, with resolutions up to 7um in the following file formats, TIF, JPEG, MrSID, Bitmap, GIF, Intergraph COT & RLE and Vitec.

Aerial Triangulation

Aerial triangulation is the calculation of true ground co-ordinates, used as the base reference in photogrammetry. By collecting a few surveyed ground control points and combining them with plate measurements, Terralink International calculates the co-ordinates of more control points and displays them in the projection required for stereo compilation. This allows aerial imagery to be aligned and geo-referenced to reflect their true positions on the Earth.

Stereo Compilation

By laying a pair of matched and overlapping aerial images alongside each other, a 3D view can be created that allows determination of ground elevations and building or cultural feature heights. The result of this step in the process is the creation of a digital model of terrain elevations and contours.