InventoryManager: Double Sampling

Owned by Kerry Halligan
Last updated: Feb 04, 2022
3 min read

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 determined to be ‘in’, 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. For more information on collecting stand, plot and tree information using double sampling, please see the detailed MobileMap documentation on this topic at https://masonbruce.atlassian.net/wiki/spaces/MD/pages/3145817

Note that double sampling can also be used to enforce subsampling of some measurements, rather than subsampling of all measurements. For example, double sampling could be used to determine which trees will receive measurements for height, crown ratio, form class, etc.

In InventoryManager, double sampling typically involves selecting appropriate values for plot type, plot size, sampling type (e.g. First Tree North), etc.  These fields will typically be required fields, and InventoryManager forms (e.g., create new stand, plot layout, etc.) will require that values are selected prior to saving.  In most cases these fields will use Coded Value Domains and thus will have dropdown lists for selecting values.  One particular field which is unique is Sample Tree Ratios.  

  • Sample Tree Ratios (STR)  - Sample Tree Ratios use predefined ratios to select which trees should be measured.  Sample Tree Ratios are established for each sample group and applied across the stand (rather than at the plot level like in FTN or Big BAF).  An example ratio is 'DF-S:5' which indicates that Douglas fir sawtimber should be sampled at a ratio of 1:5, meaning that one tree should be measured out of every 5 assessed (the other 4 would be tallied).  MobileMap will always flag the first tree in a sample group as a measure tree, then will use a random component when flagging additional trees.  In the previous example, it would flag tree 1 as measure, then 1 tree between trees 2 and 6, 1 tree between trees 7 and 11, etc. STR can be used with fixed area plots, variable radius plots and 100% tally cruises.

  • Editing Sample Tree Ratios (STR) - If sample tree ratios have already been defined for a stand, but need to be edited, select the stand and begin editing as you would for any other attribute field.  Clicking the edit (pencil) icon to the right of the current STR value will open the STR edit interface.  In this interface, enter a line for each STR by entering the species, produce and ratio value.  Use the + to add a new ratio and the - to remove a ratio.