Lecture 9FORE 3218

Forest Mensuration IIForest Mensuration II

Lectures 9Lectures 9Inventories with Point SamplesInventories with Point Samples

Avery and Burkhart,Avery and Burkhart,Chapter 11Chapter 11

Lecture 9FORE 3218

What Is Point Sampling?

A method of sampling based on a tree’s size, rather than its frequency of occurrence (probability to size)

The probability of tallying a tree depends on – The sighting angle of a prism or

angel gauge– DBH of the tree– Distance between sampling point

to the tree

Lecture 9FORE 3218

Selecting a Sighting Angle

Basal Area Factor (BAF)– Related to the angle size of

the prism used in point sampling

– Chosen to provide a tally of 5 - 12 trees per sample point Eastern Canada: 2 Western Canada: 5

Lecture 9FORE 3218

Plot Radius factor

The ratio of sighting intercept to plot radius

Plot Radius factor – Shall a boundary tree be

tallied?= Plot radius factor times tree

dbh in cm gives limiting distance in m

For example, when using a BAF 2 prism, the ratio (tree dbh to plot radius) is 1/35.4 and plot radius factor is 0.354

Sighting intercept

Plot radius

Lecture 9FORE 3218

How Point Sampling Works

Imaginary zones proportional to stem basal area and encircling each tree determine which tress will be tallied at a given point

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Equality of Tree Basal Area on a Per-Unit-Area Basis

Why each tallied tree represents the same basal area on a per-unit-area basis?

Proof

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Implementing Point Sampling

The Stick-Type Angle Gauge

The Wedge Prism

Lecture 9FORE 3218

Corrections for Slope

When wedge prism is used, on-the-ground compensation for slope can be made by titling the top edge of the prism through estimated slope angle-at right angels to the line of slope

For stick-type angle gauges, the sighting angle can be reduced by making the intercept narrower or the sighting base longer

Check all doubtable trees by measuring dbh and horizontal distance

Lecture 9FORE 3218

Calibration of Prisms or Angle GaugesCalibration of Prisms or Angle Gauges

Set up a target of known width (Set up a target of known width (WW) (e.g., 1 ) (e.g., 1 ft) against contrasting backgroundft) against contrasting background

Back away from the target until the target Back away from the target until the target exactly fills the sighting angle (exactly fills the sighting angle (DD))

BAF = 2500 (BAF = 2500 (WW//DD))22; where ; where W W and and D D in min m

Lecture 9FORE 3218

Calibration of Prisms or Angle GaugesCalibration of Prisms or Angle Gauges

Lecture 9FORE 3218

Calculating basal area (BA) from a point cruise

BAFpointsofnumbertalliedtreesTotalBA

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Calculating stand density (trees per ha)

pointsofnumber totalfactor)conversionha(pertallied)trees(no.TPH

Therefore

p

ii

n)(TF)(n

TPHExample

where the per-ha conversion factor (tree factor)

per treeBA BAF

Lecture 9FORE 3218

Volume Calculation 1 -Calculating volume per ha (VPH) by the volume-factor approach (VF)

Prior to conducting a timber inventory, one must select an appropriate volume table, e.g., Horner’s standard volume equation

p

ii

nVFn

VPH ))((

))(( iVTFVF ii For each given tree tallied

Lecture 9FORE 3218

Volume Calculation 2 -Calculating volume per ha (VPH) by the volume/basal area ratios approach

i

ii BA

vVBAR

Lecture 9FORE 3218

Volume Calculation 2 (con’t)

haper BA n

r

haperBAtreesofno.ratiosofsumhaperVolume

i

Compute volume per ha using volume to basal area ratios (VBARS):

Where: ri is volume to basal arearatio for a tree of a certaindiameter and height

Example

Lecture 9FORE 3218

Point SamplingAdvantages1. No need to establish a fixed plot

boundary, thus greater cruising speed

2. Large high-value trees are sampled in greater proportions than small stems

3. BA and volume may be derived without direct measurement of dbh

4. When volume-per-ha conversion factor are developed in advance, volume determination can be made in a minimum time

DisadvantagesDisadvantages1.1. Heavy underbrush reduces Heavy underbrush reduces

sighting, therefore efficiencysighting, therefore efficiency2.2. Because of small size of Because of small size of

sampling unit, errors in tally sampling unit, errors in tally can be very serious after can be very serious after expansionexpansion

3.3. Slope compensation causes Slope compensation causes problemsproblems

4.4. Edge-effect bias when Edge-effect bias when sampling very small tracts or sampling very small tracts or long narrow tractslong narrow tracts