New approaches to extensive forest inventory in Western Australia using large-scale aerial photography

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Abstract
With the publication of its 1987 Timber Strategy, the Western Australian Government called for a new strategic-level inventory over 1.3-million hectares of the state's jarrah forests by 1991. The design for this inventory uses two phase or double sampling; the first phase involving intensive systematic sampling by large scale aerial photographs (1:1200) to obtain gross bole volume estimates for the forest and its various subunits, the second phase involving resampling a proportion of the first phase plots on the ground to obtain detailed information for adjusting the first phase photo-estimates and for partitioning gross bole volumes into product classes of various types. Large-scale photographs improve sampling efficiency partly because of their use to obtain precise tree measurements independent of ground control. One method available for such photography uses sequential exposures with a single camera and requires an accurate altimeter and tip/tilt recorder to determine scale and camera-orientation respectively. A second approach, which was selected for the jarrah inventory, uses simultaneous exposure of twin cameras mounted at each end of a fixed base, usually a boom mounted on a helicopter. Calculation of photo-scale with this method relies on the known orientation and separation of the cameras. A global positioning system which provides position data from satellite signals is used during photography for the jarrah inventory to navigate to sample points and is linked to a laptop computer which fires the cameras and records the geographic co-ordinates of sample points for subsequent use in a geographic information system (GIS). The GIS is then used to produce maps for locating the second phase subsample of photo plots on the ground for remeasurement. The paper reviews requirements for estimating timber volumes from aerial photographs and discusses the large-scale photography methods being used for the jarrah inventory.