Lecture 9 FORE 3218 Forest Mensuration II Lectures 9 Inventories with Point Samples Avery and...
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Transcript of Lecture 9 FORE 3218 Forest Mensuration II Lectures 9 Inventories with Point Samples Avery and...
Lecture 9FORE 3218
Forest Mensuration IIForest Mensuration II
Lectures 9Lectures 9
Inventories 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
Lecture 9FORE 3218
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
Lecture 9FORE 3218
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
BAFpointsofnumber
talliedtreesTotalBA
Lecture 9FORE 3218
Calculating stand density (trees per ha)
pointsofnumber total
factor)conversionha(pertallied)trees(no.TPH
Therefore
p
ii
n
)(TF)(nTPH
Example
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
n
VFnVPH
))((
))(( 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 Sampling
Advantages1. 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 efficiency
2.2. 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