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Fixed Plot Sampling

Fixed Plot Sampling. Cruise Design Measurement Computations. Sampling Intensity. Determined by Value of product(s) Variability within the stand Budget limitations. Application. Trees in the population are selected with equal probability

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Fixed Plot Sampling

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  1. Fixed Plot Sampling Cruise Design Measurement Computations

  2. Sampling Intensity • Determined by • Value of product(s) • Variability within the stand • Budget limitations

  3. Application • Trees in the population are selected with equal probability • Normally plot locations are allocated on an unbiased grid pattern • Works best for large areas with relatively uniform density and species distribution

  4. Potential Problems • If highly variable stand, large plots and/or large numbers of plots may be needed to get accurate estimates.

  5. Sampling Intensity • USFS Way

  6. Sample Selection – from Precruise data • Determine the sampling error for the sale as a whole. (set to 10%) • Subdivide (or stratify) the sale population into sampling components as needed to reduce the variability within the sampling strata. • Calculate the coefficient of variation (CV) by stratum and a weighted CV over all strata. (this will be covered more later in the statistics lectures) • Calculate number of plots for the sale as a whole and then distribute by stratum.

  7. Number of Plots Value of t is assumed to be 2 Error is set at 10%

  8. Distribute Plots by Stratum • For each stratum, the calculation would look like this: • n1 = (17.6 * 185) / 67.9 = 48 plots • n2 = (7.7 * 185) / 67.9 = 21 plots • n3 = (7.2 * 185) / 67.9 = 20 plots • n4 = (35.4 * 185) / 67.9 = 96 plots • Which totals to the 185 plots for the sale.

  9. Tree Expansion Factor • 1 divided by the fixed plot size times the number of plots

  10. Sample Error - step 1 (Calculate Standard Error)

  11. Sample Error – Step 2 36.2% is a bit larger than the level we set to begin with (10%) – Implications?

  12. Sampling Intensity • Without Precruise data (cheaper) • Assumes fairly uniform distribution of species, sizes and grades • Can work for even-aged or uneven-aged stands if assumptions are met

  13. Fixed Area Plot Sampling Plot Cruising Plot cruise planning: 160 acre tract 10% cruise (sample) 1/5 acre plots Given 1. Area to be sampled:

  14. Fixed Area Plot Sampling Plot Cruising Plot cruise planning: 160 acre tract 10% cruise (sample) 1/5 acre plots (0.2 ac.plots) area sampled = 16.0 ac. 2. No. of plots needed:.

  15. Fixed Area Plot Sampling Plot Cruising Plot cruise planning: 160 acre tract 10% cruise (sample) 1/5 acre plots area sampled = 16.0 ac. 80 plots needed 3. Samples represent how much area?

  16. Fixed Area Plot Sampling Plot Cruising Plot cruise planning: 160 acre tract each represents 2 acres (20 sq. chains) 10% cruise (sample) 1/5 acre plots area sampled = 16.0 ac. 80 plots needed 4. Plot spacing? Distance between plots

  17. Fixed Area Plot Sampling Plot Cruising Plot cruise planning: 160 acre tract each represents 2 acres (20 sq. chains) 10% cruise (sample) 5 chain spacing between lines 1/5 acre plots area sampled = 16.0 ac. 80 plots needed 5. Plot spacing? Distance between lines

  18. Fixed Plot Cruising Office Procedures

  19. Number of Plots • Tract size times percentage sampling intensity gives you total sample area needed • Example: 23 acre tract X 10 percent sample = 2.3 acres • Area needed times plot area gives plots needed. • Goal: Select plot size that yields around 4 to 8 trees per plot • Example 1: 2.3 acres X 5 (fifth acre plots per acre) = 11.5 plots (make it 12) • each represents 2 acres • Example 2: 2.3 acres X 10 (tenth acre plots per acre) = 23 plots • each represents 1 acre

  20. Operational Features • Plot center is marked • Circular plot radius measured • Often 10 percent sampled • Unless huge forest with limited budget • Or highly variable forest • Or high valued forest

  21. Fixed Plot – 1/10th acre

  22. Plot Allocation Can be unbiased yet systematic by randomizing location of start point and bearing of sample lines.

  23. Example of Systematic Layout

  24. Field Procedures Fixed Plot Cruising

  25. Procedure • Mark plot center • Start checking trees for tally clockwise from the North for consistency • Mark each finished tree to prevent tallying the same tree twice

  26. Slope Correction

  27. Boundary Plots • If entire plot doesn’t fall within the survey area, don’t simply shift the plot center so it fits into the tract. This results in fewer edge trees sampled.

  28. Boundary Plots – Half plot • If entire plot doesn’t fall within the survey area, double count the half that does.

  29. Boundary Plots – Quarter plots • can be used in corners where all tally trees are counted 4 times.

  30. Boundary Plots - Mirage Plots (preferred) Imagine folding the plot back on itself

  31. Boundary Plots - Walkthrough plots If a line the same distance from the plot center to the tree, when projected through the tree crosses the boundary, count it twice.

  32. Tally Sheet • Date = 8/19/2013 Size = 1/10th acre Forest Cover Type = • Stand Size and Condition = Stand Age = Stocking or Density Class = • Site Index = Slope = Soil Classification = • Understory Vegetation = Treatments needed = • Plot = 1 • Tree Species DBH Form Class Height Grade Crown Class Notes • 1 RO 14 78 1.5 2 • 2 SP 16 80 2 2 • 3 YP 18 82 2.5 1 • Plot = 2 • Etc.

  33. Computations • Can be preformed by hand, in Excel or in Two Dog • Trees per acre – by species and/or by size • Basal Area per acre – by species and/or by grade • Expansion Factor equals percent area sampled

  34. Combined Stand and Stock Table • No. of trees Cubic-foot volumes------------------------------------ ---------------------------------DBH Per acre Per acre(in.) Tract total average Tract total average------------------------------------------------------------------------------------------- 5 1195.5 11.2 1730.4 16.1 6 1432.6 13.4 3767.6 35.1 7 1455.1 13.6 5667.8 52.9 8 1150.6 10.7 6294.4 58.7 9 1128.0 10.5 8379.0 78.2 10 1082.9 10.1 10622.4 99.1 11 823.4 7.7 11142.7 103.9 -------- ----- ---------- -------Total 8268.1 77.2 47604.3 444.0 Stand table gives estimated number of stems by size (in this case) and per acre Stock table gives total volume and per acre volume (could be board foot or cubic foot) 107.2 acre tract of small mixed hardwoods

  35. Cruise Summary (by species not sizes)

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