Lab - Forestry
Forest Mensuration Techniques
For this lab you will need a tree, a yardstick (a measuring tape may also help), paper and pencil, this worksheet, and a stick of any sort.
If you do not have a tree (desert deployment, etc.) you may use any tall object - a building, flagpole, utility pole, etc. The principles remain the same.
You are to complete 3 exercises (I, II, & IIIa or IIIb)- each measures trees using variations on the same technique. Download this lab sheet to your computer and fill in the answers. Then use the worksheet to complete the Lab 2 assignment under Tests and Quizzes.
Answer the set of questions following each exercise.
If you do not get a sunny day where you are located, please use Excercise IIIb below as an alternative to Exercise IIIa. Otherwise, you MUST do Exercise IIIa.
Be SAFE and do not measure trees if thunderstorms are nearby.
USE THE SAME TREE FOR ALL THREE EXERCISES.
I. Tree Height Measurement by Michael Kuhns, Extension Forestry Specialist
You can measure heights of very tall objects such as trees by projecting a right triangle (one that includes a 90 degree angle) using your arm, a stick, and your line-of sight.
Material: A stick that is equal in length to the distance from your eye (cheekbone) to your fingers when your arm is fully extended in front of your face. Break off part of the stick or mark it at the correct length if you don't find one that is exactly right.
Procedure:
1. Grasp the stick by the tips of the thumb and index finger and hold it out in front of you with your arm fully extended. The stick must be held vertical.
2. Walk toward or away from the tree until the tip of the stick is visually lined up with the top of the tree and the bottom of the stick is lined up with the bottom of the tree. Your line of sight to the tree base should be as close as possible to horizontal. In sighting to the top and bottom of the stick rotate your eye rather than your head.
3. The distance from your eye to the base of the tree is equal to the height of the tree. Measure this distance with a measuring tape. If no long-distance measuring device is available, calibrate your step (the walking distance between your two feet--walk normal, don't stretch) or pace (walking distance for two steps) over a known distance (say 50 feet). Then measure the distance A-D in paces or steps and convert to feet, meters, etc.
Answer the following questions:
1. What was the length of the stick you used to help measure the tree?
2. What was the distance from you to the base of the tree?
3. How did you measure the distance from you to the base of the tree (e.g. paces, yardstick, tape measure, etc.)?
4. How tall did you calculate the tree to be?
5. What are possible sources of error with this technique?
II. Tree Height Measurement by Bizarre Labs:
The heights of trees (or any other tall object) can easily be found using a device called hypsometer. A hypsometer is basically a long stick divided into even units used to find height by triangulation. A yardstick or meter stick will work just fine.
Material: A yardstick
Procedure:
1. If you are using a yardstick, stand exactly 25 feet from the tree being measured. Hold the yardstick, with the zero end downward, 25 inches from your eye. Line up the bottom of the yardstick with the base of the tree. Without moving your head, look to the top of the tree. Where it crosses the yardstick, read off the measurement in inches. Each inch will equal one foot in the tree's height.
2. If the tree is taller than your hypsometer will measure, stand 50 feet away. Again hold it 25 inches from your eye, as before, only this time multiply your result by 2 to get the correct height. If it is taller still, then step back to 75 feet, multiplying your result by 3, or 100 feet, multiplying the result by 4, etc.
3. If you are using a meter stick, the procedure is basically the same. Stand 5 meters from the tree. Hold the meter stick 50cm from your eye. Each 10 centimeters will equal one meter of the tree's height. If standing at 10 meters, double the result; at 15m, triple it, etc. Granted, a five-meter tree isn't very tall, but it is a convenient scale to start with for the sake of mathematical progression.
Note: there are other types of hypsometers, all of which measure height or altitude. Another type of hypsometer measures elevation by noting what temperature water begins to boil; this boiling point decreases with an increase in elevation.
Answer the following questions:
1. How did you measure your distance to the tree?
2. How many inches tall on the yardstick did the tree appear to be?
3. How tall did you calculate the tree to be?
4. What errors may be made using this technique?
5. How far back did you have to stand to measure the height of the tree?
CHOOSE ONE OF THE TWO METHODS BELOW - FOR A TOTAL OF THREE METHODS FOR THIS ASSIGNMENT
III a. Tree Height Measurement by Bellnet
You will need a sunny day to do this exercise. If you are in a location where you do not have a sunny day to produce shadows, do Exercise IIIb below instead.
Materials: Yardstick, pencil, data sheet (see below), and calculator (optional)
Procedure:
1. On a sunny day hold a yardstick perpendicular to the ground and measure the length of its shadow. Repeat two times. Calculate an average. Record your results on your data sheet.
2. Measure the length of the shadow of the tree. Repeat two times and calculate an average. Record your results on your data sheet, including the dates and times.
3. Calculate the height for your tree using your calculated averages and the following formula.
Tree height = (ydstick length x ave. tree shadow length)/ ave. yardstick shadow length
Answer the following questions. (SHOW YOUR CALCULATIONS USING UNITS)
1. What is the average length of the yardstick shadow?
2. What is the average length of the tree shadow?
3. What is the height of the tree?
4. What are two major sources of errors associated with this technique (see data table for one source)?
5. Explain the variation between the three tree height measurement methods.
6. In your experience, which do you think was most accurate and why?
III b. Tree Height Measurement (Alternate to Shadow Lab.)
Use this exercise ONLY if you are in a location where you do not have a sunny day to produce shadows.
Loggers learned a great deal from people who lived in forests for generation. They might be cutting down trees that are over a certain height and it is important that they have an easy way to estimate which of the trees around them are to be cut. You can estimate the height of a tree or a building using their method.
Material: A partner and a pencil
Procedure:
1. Let your partner standt the base of the tree. Move a distance away from the tree then, holding your pencil at arms length, between your thumb and forefinger so that it brackets the height of your partner (the top and bottom of the pencil coincide with the top of the head and the bottom of the feet of your partner).
2. Now, use this length to step out the height of the tree in "pencil lengths". You can now measure the height of your partner and multiply this by the number of pencils high the tree is.
3. Repeat three times from different distances and calculate an average to improve the accuracy. Report the average of your three trials below in # 2.
Answer the following questions to be turned in for grading:
1. How did you measure the height of your partner? What was his or her height?
2. How many pencil heights did the tree appear to be (average of your three trials; show calculations)?
3. How tall did you calculate the tree to be? Use the average value from #2.
4. What are two potential sources of errors associated with this technique?
5. Explain the variation between the three tree height measurement methods.
6. In your experience, which do you think was most accurate and why?