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use_of_wedge_prism [2012/04/05 19:05] ddrummond created |
use_of_wedge_prism [2012/07/12 19:44] (current) 128.192.48.132 |
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| ====== Use of Wedge Prism ====== | ====== Use of Wedge Prism ====== | ||
| + | The prism is a piece of a wedge-shaped rectangular glass which causes a | ||
| + | distortion of light, displacing the image towards the thinner edge. | ||
| + | It is used to estimate the basal area of a stand. | ||
| + | |||
| + | ====Basal Area==== | ||
| + | |||
| + | 1. Hold prism (not your eye) over the selected point at a comfortable distance from | ||
| + | the eye, with the long side horizontal. Hold prism with right hand by lower part of | ||
| + | the thicker edge. | ||
| + | |||
| + | 2. With one eye closed, point with the upper part of the prism so as to divide the | ||
| + | tree in question at breast height. Refraction of light through the prism will cause | ||
| + | the portion of the tree below breast height to appear separated. Count as 1 all of | ||
| + | the trees whose figures are superimposed, as 1/2, or measure for accuracy those which | ||
| + | touch only at the edge. If they do not touch each other they are not counted. | ||
| + | {{ :use_of_wedge_prism1.jpg |}} | ||
| + | |||
| + | 3. Turn in a circle, checking each visible tree, making sure not to count the same tree | ||
| + | twice. | ||
| + | |||
| + | 4. The number of trees counted, multiplied by basal area conversion factor of prism gives | ||
| + | us the basal area per stand acre. | ||
| + | | ||
| + | BA/Acre = (Total number of trees counted X BAF of prism)/(Number of samples) | ||
| + | | ||
| + | ====Basal Area Factor.==== Set up a target exactly one foot wide. Fro an approximate 10 BAF prism, | ||
| + | sight about 33 feet from target. Move toward or away from target until right edge of target | ||
| + | (as seen in prism) jibes with the left edge of target (just above prism). Measure exact | ||
| + | distance prism to target. Take several readings and use average.{{ ::use_of_wedge_prism2.png|}} | ||
| + | |||
| + | Calculate BAF by formula:\\ | ||
| + | BAF = (43560)/[(1+4)(D/W)<sup>2</sup>]\\ | ||
| + | D is distance prism to target\\ | ||
| + | W is width of target\\ | ||
| + | or: BAF = (43560)/[(1+4)(D)<sup>2</sup>]\\ | ||
| + | Where target "W" is exactly 1 foot wide\\ | ||
| + | or: BAF = 10890(W/D)<sup>2</sup> \\ | ||
| + | A close approximation\\ | ||
| + | |||
| + | ====Plot Radius Factor==== | ||
| + | The distance from a sampling point to the borderline tree equals DBH times PRF. For a BAF 10.0 prism | ||
| + | the PRF is 2.75. A 10-inch tree times 2.75 gives a critical distance of 27.5 feet, a 14-inch tree | ||
| + | is 38.5 feet, etc. The number of trees per acre that each tallied 10-inch tree represents 18.35; | ||
| + | each 14-inch tree is 9.35, etc. | ||
| + | The basic formula: PRF = √(75.625/BAF) | ||
| + | ====Borderline Trees==== | ||
| + | Always measure borderline trees more than a casual check on basal area. A short cut method counts | ||
| + | borderline trees as 1/2. | ||
| + | 1. Measure horizontal distance from sample point to center of tree at BH. | ||
| + | 2. Get DBH. | ||
| + | 3. Multiply DBH by prism PRF. When this product is more than taped distance, the tree is "In". | ||
| + | | ||
| + | Example: (PRF is 2.75, DBH 10.0) | ||
| + | 2.75 X 10.0 = 27.5 feet, taped distance is 27.2 feet; tree is "In". | ||
| + | | ||
| + | ====Precautions==== | ||
| + | 1. Prism must be in center of plot, not the eye. Walk around the prism; do not stand in one | ||
| + | place and move the prism around you. | ||
| + | 2. Always hold the face of the prism at right angel to eyesight, horizontally and vertically. | ||
| + | (Except for leaning trees and when correcting for slopes). | ||
| + | 3. For leaning trees move prism forward then backward on its vertical axis according to the | ||
| + | inclination of the tree trunk. | ||
| + | 4. Correct for slope by rotating prism to the same amount of slope between the prism and the | ||
| + | tree, but at right angles to the eye. | ||
| + | 5. If there is an object between you and the tree to be checked, move one step sideways, | ||
| + | keeping a constant distance between you and the tree. | ||
| + | 6. In dense strands be careful not to confuse or incorrectly associate the trunks. (If a tree | ||
| + | is "In" above the brush, it is "In" at BH). | ||
| + | 7. Remember that each diameter size has its own plot radius, the radius varies directly with | ||
| + | the tree diameter. Therefore, the largest and most valuable trunks are usually sampled more | ||
| + | intensely than the smaller trunks. | ||
| + | --The prism helps to train the eye to estimate the basal area of stands, use it. | ||
| + | ====Volume Per Acre==== | ||
| + | __Volume:__ Volume is related to basal area and merchantable or total tree height. The average | ||
| + | basal area per acre in each height class multiplied by the volume factor gives us the volume per | ||
| + | acre. The factors can be for any scale- board feet by any rule, cubic feet, or cords. If the | ||
| + | available factors cannot be relied upon, they can be developed as necessary. | ||
| + | ====Volume Factors==== | ||
| + | Estimate the volume per acre by multiplying the number of trees measured in each length class | ||
| + | by the following factors:* | ||
| + | ^Pulpwood ^^^Sawlogs ^^^^| | ||
| + | ^Merchantable height^Cubic Vol.(w/bark)^Cords(w/bark)^Merchantable height1/^Int.1/4^Scribner^Doyle^Cubic(w/o bark)| | ||
| + | |10 |7 |.08 |1 |7 |6 |4 |1.2 | | ||
| + | |20 |14 |.16 |2 |13 |11 |8 |2.0 | | ||
| + | |30 |20 |.22 |3 |18 |16 |12 |2.7 | | ||
| + | |40 |26 |.29 |4 |23 |20 |15 |3.4 | | ||
| + | |50 |31 |.34 |5 |28 |25 |21 |4.0 | | ||
| + | |60 |36 |.40 | | | | | | | ||
| + | |70 |39 |.43 | | | | | | | ||
| + | ^v = 10 (sum of products/number of points ^^^v = 100 (sum of products/number of points) ^^^^| | ||
| + | |||
| + | * For a 10 factor prism\\ | ||
| + | 1/ Merchantable height is in 16 foot logs | ||
| + | |||
| + | ====Local volume factors==== | ||
| + | Pulpwood factors are fairly consistent. Sawlog factors are not very reliable in different localities. | ||
| + | The procedure to prepare local factors is as follows: | ||
| + | 1. Write down the sample trees according to diameter class and number of logs. (The total trees | ||
| + | tallied divided by the number of points or samples.) | ||
| + | 2. Obtain the number of plots per acre from the table at page 54. | ||
| + | 3. Multiply in order to obtain trees per acre. | ||
| + | 4. Write down volume for each tree. | ||
| + | 5. Multiply to obtain volume per acre. | ||
| + | 6. Total volume per acre for each length class. | ||
| + | 7. Write down the basal area per acre (trees per plot for each length class multiplied by the | ||
| + | basal area factor). | ||
| + | 8. Volume factor is the volume per acre divided by basal area per acre. | ||
| + | Refer to following example | ||
| + | |||
| + | ===Basal Area Factor: 10=== | ||
| + | ^Dbh Class ^Trees per point ^^^Plots per acre ^Trees per acre ^^^Volume per tree^^^Volume per acre^^| | ||
| + | ^ ^1-log^2-log^3-log ^ ^1-log^2-log^3-log^1-log^2-log^3-log^1-log^2-log^3-log| | ||
| + | |10 |0.136|0.136| |18.349 |2.5 |2.5 | |40 |60 |70 |100 |150 | | | ||
| + | |12 |0.785|0.628|0.157 |12.739 |10.0 |8.0 |2.0 |50 |90 |120 |500 |720 |240 | | ||
| + | |14 |0.428|0.534|0.107 |9.355 |4.0 |5.0 |1.0 |80 |130 |170 |320 |650 |170 | | ||
| + | |16 |0.140|0.419|0.140 |7.163 |1.0 |3.0 |1.0 |100 |170 |230 |100 |510 |230 | | ||
| + | ^Total ^1.489^1.717^1.404 ^ ^Boardfeet per acre ^^^^^^1020 ^2030 ^640 | | ||
| + | ^ ^^^^^Basal Area per Acre ^^^^^^14.89^17.17^4.04 | | ||
| + | ^ ^^^^^Volume Factor ^^^^^^68.5 ^118.2^158.4| | ||
use_of_wedge_prism.1333652745.txt.gz · Last modified: 2012/04/05 19:05 by ddrummond
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