Submitted to: Environmental Monitoring and Assessment
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/14/2005
Publication Date: 1/24/2006
Citation: Booth, D.T., Cox, S.E., Berryman, R.D. 2006. Precision measurements from very large-scale aerial digitial imagery using imagemeasurement, laserlog, and merge software applications. Environmental Monitoring and Assessment. 112:293-711. Interpretive Summary: Land managers are often faced with measuring things like stream widths or diameters of shrub and small-tree canopies. Accurate assessments of average width, length, or diameter depends on making many measurements evenly distributed over or among the management units, plant communities, or stream reaches being monitored. Conventionally, such measurements are made by crews working on the ground with tape measures. This is expensive and measurement-distributions are often biased by ground access. It is now possible to obtain very high resolution aerial photography that clearly shows features land managers need to measure. This paper describes three software applications we developed that capitalize on the potential to obtain accurate measurements from high-resolution digital aerial photography. We found that by using the software, we could make one measurement per image and complete four images a minute - that is equivalent to ground crews driving or walking to four separate sites and making a measurement. Measurements obtained using the software were accurate to tenths of a foot with an error less than 10%. We believe the software will help reduce the cost and increase the sample adequacy of natural-resource monitoring efforts.
Technical Abstract: Very large scale aerial (VLSA) imagery has sensor resolutions as fine as 1 millimeter per pixel and provides new opportunities for all types of feature dimension measurement. Dimension measurement is an important aspect of resource monitoring and accurate measurements can be easily obtained from VLSA imagery using the three new software programs described here. VLSA images have small fields of view and are used for intermittent sampling across extensive landscapes. Pixel-coverage among images is influenced by small changes in airplane altitude above ground level (AGL) and orientation relative to the ground, as well as by changes in topography. These factors affect the object-to-camera distance used for image-resolution calculations. 'ImageMeasurement' offers a user-friendly interface for accounting for pixel-coverage variation among images by utilizing a database. 'LaserLOG' records and displays airplane AGL measured from a high frequency laser rangefinder and displays the vertical velocity. 'Merge' sorts through large amounts of data generated by LaserLOG and matches precise airplane altitudes with camera trigger times for input to the ImageMeasurement database. We discuss application of these tools, including error estimates. We found measurements from aerial images (5 ' 26 mm / pixel GSD sensor resolution) using Imagemeasurement, LaserLOG, and Merge, were accurate to centimetres with an error less than 10%. We recommend these software packages as a means for expanding the utility of aerial data.