1、FW2051 Field TechniquesIntroduction to Point SamplingObjective: To learn basic concepts of point sampling, how to use a prism, and common sources of error.Materials: Prism, angle gauge and loggers tape.Lets start out in room 143.Point sampling Probability Proportional to Size (PPS) or point sampling
2、 has been widely used in forest inventory. It is the method of selection of trees based on their size rather than their frequency of occurrence. In our two previous labs we used fixed area plot sampling and measured all trees within a given fixed area to estimate trees/acre and BA/area. These values
3、 were calculated by diameter and species and based on the frequency of which each tree was measured. In point sampling, inclusion of a tree in a plot depends on the basal area (cross sectional area of a tree at breast height) of the tree and its proximity (distance) to the sample point. With point s
4、ampling, you are most likely to sample big trees. Thus, in other words it is called probability proportional to size (PPS). Point sampling is known with different names angle count sampling, plotless cruise, prism cruising, variable-plot sampling, variable-radius plot and most commonly, point sampli
5、ng. The procedure to sample trees in the forest stand is simple and easy to use in the field, therefore it became popular among natural resource managers and ecologists. The instruments used in point sampling are angle gauges, wedge prisms and Spiegel Relaskops (we will briefly touch on Relaskops la
6、ter in the term). The basal area factor (BAF) is the number of units of basal area per acre (or per hectare) represented by each tree tallied in point sampling. In point sampling; Do not need direct measurement of either plot boundaries or tree diameters to figure out which trees are in and what bas
7、al area per acre is Use a fixed angle to determine which trees are in Each tree has its own plot with a radius proportional to its diameter. It means the probability of tallying a tree depends on its cross-sectional area (diameter) and the sighting angle used The imaginary tree plots vary with tree
8、size, such that big trees have big plots and small trees have small plots. Plot size is such that each “in” tree represents the same amount of basal area per acre. The smaller the sighting angle of prism or angle gauge, the more trees included in the sample. Instruments used in point sampling Angle
9、gauges: Angle gauges are one method used to determine whether or not to count a tree at a point sample. The tree is counted if it is wider than the opening in the gauge for the BAF we are using. The angle gauges we use in class have four different BAFs built in, 5, 10, 20 and 40 BAF. For todays lab
10、assignment, we will use the 10 BAF width on the gauge. With an angle gauge your eye is the center of the point.Wedge prisms: Wedge prisms are used to determine whether or not to count a tree at a point sample. The prisms we have for class are 10 BAF prisms. Trees are counted as in, if the offset ima
11、ge at breast height viewed through the prism and the actual image of the tree have some overlap. If there is no overlap, the tree is not tallied. With a prism the PRISM is the center of the point and must be held over your point center marker.Borderline trees: For borderline trees, measure the dbh a
12、nd the distance to the tree. The distance should be measured to the “center” of the tree, NOT THE FACE of the tree. If the tree is equal to or closer than the limiting distance for its dbh, then it is tallied. Limiting distances for 10 BAF prisms and gauges are listed in the table below. The table g
13、oes to 10ths of an inch. For an example, the limiting distance for a 15.4 in dbh tree is 42.35 feet . Any 15.4 in dbh tree 42.35 ft or closer to the sample point is counted. If you forget your table, but have a calculator, you can determine the limiting distance in feet for a tree by multiplying the
14、 diameter (in inches) by 2.75 (for BAF 10 sampling only). For instance, a 10.0” tree can be 27.5 feet away,Common sources of error:1. Not measuring borderline trees. Do not count every other one. Most people are biased one way or the other when they determine borderline trees, especially when they j
15、ust start out.2. Not holding the prism over plot center.3. Missing “hidden” trees. If a tree is behind another tree, carefully step to the side of your point center while keeping the same distance from the tree. You should then be able to see if its in, out or borderline.4. Missing far away big tree
16、s. Its important to look out in the distance for larger trees. A 30.9” white pine can be 84.97 feet away and still be in!5. Using a different prism or other measuring device than that which you normally work with. Table 1: Horizontal limiting distance in feet for BAF 10 point-sampling instruments If
17、 using an angle gauge on a slope you must correct for the slope in the direction you are looking. That means up and down slope you correct, looking side slope you do not correct. The way to slope correct with a prism is by turning the prism at an angle parallel to the slope along which you are looki
18、ng and sight to DBH. Again, when in doubt, measure the distance to the tree and compare to a table of limiting distances. Figure 3. Illustration of how to slope correct when using a prismhttp:/oak.snr.missouri.edu/nr3110/topics/vps/vps.htmlCalibrate your prism:1. Select a target2. Measure “d”, the t
19、arget diameter to the nearest tenth of an inch.3. Sight through the prism and adjust your distance from the target until a borderline image is obtained. Be as precise as possible.4. Measure the distance from the prism to the targets center. This is “A”. Take this measurement 5 times and calculate th
20、e average distance.5. Use the following formula to calculate your actual basal area factor:BAF = 43560 * (d/24A)2“d” = _“A1” = _“A2” = _“A3” = _“A4” = _“A5” = _A = sum(A1:A5)/5 = _BAF = _Calibrate your thumb: Any instrument or object that allows a fixed angle to be subtended can be used in point sam
21、pling, however its basal area factor (BAF) needs to be calibrated. To calibrate an instrument with unknown BAF or object or your own thumb, move back from a target tree of known dbh until the tree appears to be exactly the same angle (or size) as your instrument or object or thumb. To calibrate your
22、 thumb, hold your thumb straight out and walk away from the tree until the dbh appears to be the same size as your thumb. Measure the distance (A) from the centre of the tree to your eye and apply the following formula to calculate the BAF of your thumb. It is advised to use perfectly circular tree in flat ground (avoid slope area). BAF=43560*d/24A2Where, BAF denotes basal area factor of the thumb d is diameter at breast height to nearest 10th of an inch of targeted tree A is horizontal distance from tree to observers eye
