Volume and Decay
| Net Volume Adjustment Factor |
|---|
Net Volume
Adjustment Factor (NVAF) sampling collects data on a number of
selected trees to account for errors in the estimates of net tree
volume. The NVAF is calculated from the ratio of actual to estimates
of sample tree volumes and is applied as a correction to VRI ground
sample volumes. This data, used in conjunction with the original
ground sampling data, provides an unbiased estimate of the net
volume in the project area.
The Net Volume Adjustment Factor (NVAF) is an
integral part of the Vegetation Resources Inventory (VRI). The NVAF
is one of the components that must be completed in order to provide
VRI sample data that meets all of the Ministry of Forests and Range
(MOFR) inventory standards. The NVAF is an adjustment factor that is
used during sample compilation to produce unbiased estimates of net
merchantable tree volume. It works to adjust the combined estimates
of gross volume produced by the taper equation and decay and waste
losses estimated by the cruiser through the net factoring process.
NVAF sampling involves detailed stem analysis of sample trees,
calculation of actual net volume, and calculation of the ratio
between actual net volume and estimated net volume (where estimate
net volume is obtained from net factoring and taper equations). The
NVAF adjustment is needed for the following reasons:
The cruiser estimated net volumes are obtained when a cruiser
applies standard rules to determine the percentage of sound wood in
a tree after detecting physical indicators of decay on a tree. The
process of applying the decay estimation rules is known as net
factoring. If no decay indicators are detected, a tree is
considered to be 100% sound. However, a tree may have decay even if
there are no visible indicators, or a cruiser may miss the
indicators. Therefore, net volume may be over-estimated.
The rules used to associate decay indicators with amount of decay
are based on expert knowledge, and past experience. As would be
expected, the rules may over-estimate or under-estimate the amount
of decay on the average for a ground sample project. For more
information on how the rules are applied, please refer to the Vegetation Resources Inventory Ground Sampling Procedures
manual
(available on the website
http://www.for.gov.bc.ca/hts/vri/standards/gs_vri.html.
The BEC-based tree taper equations may
over-estimate or under-estimate close utilization volume when
applied to local areas.
Standards for NVAF can be found at: http://www.for.gov.bc.ca/hts/vri/standards/nvaf.html
For more
information on NVAF, contact Will Smith at: 250-356-6853,
Will.Smith@gov.bc.ca