Laserfiche WebLink
Title 190-Forestry Inventory Methods Technical Note <br />Figure 2: Example of a Systematic Line -Plot Sampling Layout <br />I <br />I <br />o Another sampling protocol that limits bias involves using random distances and <br />azimuths to locate plots. Note that it is possible, even when plots are located <br />correctly using random or systematic protocols, that the resulting sampling design <br />may not represent all parts of a stand equally. Sample enough plots that these random <br />effects will not significantly affect stand -level summary data. <br />• Using the Prism <br />o A wedge prism is an angle -cut glass at a given basal area factor (BAF). Trees are <br />sighted through the prism at 4.5 feet above ground, or diameter at breast height <br />(DBH). Trees are counted as either "in" or "out." If the bole displacement as viewed <br />through the prism overlaps, the tree is in (fig. 3). Include every other borderline tree. <br />At each sample point, keep the prism over the plot center and rotate around the prism <br />in one direction only. <br />o Note that a relatively small tree will need to be close to the plot center to be counted <br />in, while a large tree can be quite far away. <br />• Using an Angle Gauge <br />o An angle gauge works on the same principle as a prism. Unlike a prism, the eye is <br />kept at plot center and the gauge is moved in a circle. An angle gauge such as the <br />cruz-all has several BAFs in one tool. A tree is considered out if the bole is narrower <br />than the sides of the chosen BAF (fig. 4). <br />o A desirable number of trees per plot is between 5 and 12. If the number of trees in <br />the plot tends to be fewer than 5, choose a prism or angle guage with a smaller BAF; <br />if the plots routinely have more than 12 in trees, a larger BAF is appropriate. A stand <br />of large trees requires a higher BAF and a stand of small trees a lower BAF. Keep <br />the same BAF throughout the stand to simplify calculations later. As a <br />recommendation, prior to conducting a forest cruise take a quick walk through the <br />stand, using a prism or angle gauge occasionally to determine the appropriate BAF. <br />(Technical Note No. 190-FOR-01, Jul 2018) <br />6 <br />