GRS Densitometer

The GRS densitometer, when used with line-point transect sampling, combines horizontal and vertical vegetation sampling thereby enabling the collection of resource information across the landscape (horizontally) at different canopy levels (vertically). This technique provides estimates of cover for any element in a vegetated environment such as: trees, shrubs, herbaceous, down woody material, fuels, snags etc. This method of sampling has been shown to be accurate, objective, and repeatable between observers.

The cover estimates generated using the GRS densitometer are based on an evaluation of cover data collected at sample points evenly spaced along transects. For example, trees are determined to be "cover" trees by sighting through the densitometer and determining whether any portion of a tree crown intersects the vertical line of sight through the densitometer. The vertical line of sight is obtained by levelling both of the densitometer's bubble-levels and then sighting through the instruments so that the sighting marks are aligned. The GRS Densitometer can be oriented to sight either up or down, to provide canopy and/or ground sample.

Transects of several different forms and lengths may be used, depending on the characteristics of the sample area. If the sample area is too small to effectively place a straight-line transect within, then a diamond-shaped or triangular shaped intersect can be used. The transect layout should avoid any potential bias and error that may result from the systematic orientation of the transect with respect to physiographic (terrain) features.

The point-transect sampling methodology provides estimates of the cover contribution of the different characteristics measured at each point along the transect. Estimated distributions of species composition, tree size, age, crown diameter, and any other sampled characteristics are easily generated by dividing the number of points tallied for each recorded characteristic by the total number of sample points. For example, if 100 points are sampled along the transect and 29 of the sample points are covered by Douglas-fir trees, then the estimated cover of Douglas-fir is 29 percent. If 17 of the 100 sample points indicate cover is provided by trees equal to or greater than 36" dbh then the estimated cover of the 36" + size class is 17 percent. If 74 sample points indicate conifer cover and 9 points indicate hardwood cover then the total tree cover estimate is 83 percent ((74+9)/100) and the estimated percent conifer composition is 89.2 percent (74/83).