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Double sampling, also called subsampling, is a cruising methodology in which only some of the ‘in’ trees are measured. When we use the term 'sampling' (not double sampling), we refer to the use of Plots (or sample points) for assessing a subset of trees on a stand. Sample plots can be fixed area or variable radius plots, and are in contrast to 100% tally cruises in which all trees are assessed at the stand level without the use of plots. Double sampling takes this approach a step further by measuring only some trees that are visited (or assessed), and simply counting the others. The goal of double sampling is to reduce the effort (time, cost) of cruising by reducing the number of field measurements that must be taken. Double Sampling is typically combined with plot-based sampling but can also be used with 100% tally cruises (see Sample Tree Ratios below). MobileMap currently supports 4 different methods of double sampling: Point Count Measure, First Tree North, Big BAF, Sample Tree Ratios. See also InventoryManager: Double Sampling.
A key concept in double sampling is the definition of 'sample groups'. Sample groups are typically defined by species, or by a combination of species and products. They may also be defined by broader tree groups, like hardwoods vs softwoods. Measurements for 'measure' trees typically include, at a minimum, the species and DBH of that tree, along with enough information to calculate tree volume using the selected volume calculations for the cruise. Even for measure trees, it is common for height information to be collected only on a subset of trees, and interpolated, or 'dubbed', using known or calculated diameter to height ratios for the remaining trees. Tally trees, by contrast, do not have any measurements and the only information that is recorded is what is required to determine their sampling group (e.g., species and product) as well as their quantity (count).
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