An investigation into the feasibility of forest inventory by means of stereo satellite imagery employing digital photogrammetry technology

[摘要] ENGLISH ABSTRACT:The aim of the study was to extract elevation information (such as tree height) fromstereo satellite imagery (IRS-I C), to scrutinise the performance of the DTM (DigitalTerrain Model) tools as provided by the LH (LeicalHelava) Systems' softcopysystem, and subsequently to perform a feasibility study on the application of apractically viable forest inventory design.A softcopy photogrammetry workstation (LH Systems DPW 770), IRS-I C stereopanchromatic satellite imagery, and digital aerial photography at a scale of 1:30000(scanned at 15 micrometers) was used. The study was conducted over various sites inthe Sabie area (province of Mpumalanga) in South Africa, where extensive man madeforests with pine and eucalypts are to be found. The extraction of stand parameterssuch as tree height was performed manually, semi-automatically, and automatically.In addition, the compartment area was determined using a GIS tool. The DigitalSurface Models (DSM), representing the canopy structure of the stands, was extractedfrom the IRS-I C imagery and validated through a comparison of the resultingcontours with the corresponding contours generated by aerial photogrammetricmethods.Due to the coarse spatial resolution of the IRS-IC imagery (5m) and the suboptimalBIH (BaselHeight) ratio (0.57), only objects featuring a height exceeding 20m couldbe manually measured with confidence. Furthermore, only the edges of thecompartments proved to be suitable for the determination of tree heights (i.e. with asufficiently large parallax difference and image contrast).The manual determination of tree heights in the IRS-I C imagery yielded accuracies ofabout 95% compared to the height values of the aerial photographs and the grounddata. The application of image enhancement techniques had severe effects on theaccuracy of the IRS-IC stereo model, resulting in deviations of about -57m from the'true' value. It was observed that image matching was only a problem where featureschanged their appearance (e.g. clearfelled or burnt areas) during the acquisition periodof the stereo pair of the satellite imagery.LH Systems' Adaptive Automatic Terrain Extraction (AATE) tool performed verywell for the creation of digital terrain and surface models when using digital aerialphotography with a high scanning rate. In contrast, the automatic creation of canopysurface models from various forest compartments did not yield any useful resultswhen applied to IRS-l C imagery. AATE could not model the canopy structureproperly. The coarse spatial resolution of the satellite imagery in conjunction with thesparse post spacing (20m) and matching errors are most likely to be responsible forthis poor performance.Two-phase sampling and the Hugershoff method were chosen for automaticallyderived height values to be evaluated for possible application in forest inventory.Unfortunately, neither for the determination of the regression estimator for the firstmethod, nor for the calculation of timber volume after application of the Hugershoffmethod could any useful result be obtained. This is mostly due to the fact that imagematching errors and blunders (resulting in tree heights of -885m) were not properlyaccounted for in the terrain extraction software. However, the outcomes for themanual measurement of tree heights performed on the satellite imagery show thatunder optimal conditions accuracies can be achieved similar to those for the heightdetermination in small scale aerial photographs, but at lower cost. The obtained heightvalues can then be used for the calculation of timber volume according to Eichhorn'slaw.Keywords: AATE, blunders, digital photogrammetry, DPW770, forest inventory,Hugershoff IRS-l C, matching error, remote sensing, satelliteimagery, two-phase sampling

[发布机构] Stellenbosch University