Vegetation Resources Inventory
Overview on Adjustment Factor Development
1. Purpose of Attribute Adjustment
The main concern of the attribute adjustment process is to reduce or eliminate bias in
some key timber attributes.
In principle there are two phases in the VRI data collection. In the first phase, data is
collected on all polygons in the population using photo interpretation. It is held that the
photo interpreted data are mere estimates of the true values, and that they may contain
biases resulting from the nature of the photo interpretation process.
The second phase of the data collection is the selection of a sample of polygons from the
population for the purpose of ground sampling. The ground data is subject to sampling
error due to the fact that only a few polygons (50 to 200) from a population consisting of
thousands of polygons, and within the sample polygons, a random location is selected to
supply data for analysis. In addition to sampling error, the ground sampling is subject to
measurement error. Generally, it is held that measurement error is so small that it is
negligible. Past experience suggests that it is in the range of 1% to 3%.
In spite of the sampling and measurement error, it is generally accepted that the ground
data, on the average, is a more accurate representation of the true attribute values as
compared to the photo interpretation data. This belief is based on the fact the ground data
is based on careful measurements and undergoes rigorous quality assurance, while the
photo interpreted data is based on guesses and opinions of a photo interpreter.
2. Adjustment process
Generally, the attribute adjustment may utilise either ratio or linear least squares
regression or geometric mean regression estimators. Currently, the ratio estimator is the
most common choice in the “Fraser Protocol”, which will be described below. However,
the baseline attribute adjustment methodology, which is under development, will include
combinations of all three estimators.
The data preparation process and analysis consists of following steps:
1.
2.
3.
4.
5.
6.
Assemble the polygons in the target population
Select sample polygons to be visited on the ground
Collect Ground data from the sample polygons
Merge photo interpreted and ground data
Screen the merged data for data entry and other extraneous errors
Develop adjustment factors for adjusting photo interpreted data
Overview on Adjustment
Factor Development
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July 2003
7. Report the results of the data analysis
8. Apply the adjustment factors to the photo interpreted attributes in the VRI
database.
3. Attributes to be adjusted
All continuous timber attributes with corresponding ground measurements will be subject
to adjustment. The list of attributes includes the following:
1) Height of 1st species – rank 1 layer.
2) Ht of 2nd species – rank 1 layer.
3) Age of 1st species – rank 1 layer.
4) Age of 2nd species – rank 1.
5) Site index of 1st species.
6) Site index of second species
7) Estimated site index
8) Species composition @7.5(?) – all layers combined.
9) Number of stems/ha @ 7.5cm utilization – all layers & species combined.
10) Basal Area @7.5cm utilization – all layers & species combined.
11) Basal Area @12.5cm utilization – all layers & species combined.
12) Lorey Ht @7.5cm utilization level – all layers & species combined.
13) Whole Stem volume @7.5cm – all layers & species.
14) Whole stem volume @12.5cm – all layers & species.
15) Volume net top & stump (CU) 12.5cm – all layers & species combined.
16) Volume net top, stump (CU + decay) & decay 12.5cm– all layers & species
combined.
17) Volume net top, stump (CU +decay & waste), decay & waste 12.5cm – all layers
& species combined.
18) Adjusted volume, net decay, waste & breakage @12.5cm utilization
19) Adjusted volume, net decay, waste & breakage @17.5cm utilisation level.
There are biological and logic linkages between many of the attributes listed above. For
instance, stand height and age are closely linked to stand volume. Therefore the
adjustment of the three attributes has to occur in such a way as not to produce adjusted
values which are illogical. An example of biologically inconsistent adjusted values would
be a stand with an adjusted height of 30meters, adjusted age of 250 having an adjusted
volume of 10cu.meters/ ha.
An example of logical relationships within the 19 attributes is the requirement that
adjusted whole stem volume be larger than or equal to close utilization volume (volume,
less top and stump) for all polygons in the population. If the adjustment of the volumes at
the different utilisation levels and for different net categories is not harmonised,
situations could occur where adjusted whole stem volume would be less than close
utilization volume. Such situations would not be acceptable during VDYP 7 inventory
projection.
Overview on Adjustment
Factor Development
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July 2003
At the beginning of the VRI implementation, it was determined that producing adjusted
values for 19 timber attributes, which were harmonized to maintain biological and logical
relationships, would require considerable effort. However, in order to facilitate the
utilization of incoming ground sample data for short term timber supply analysis
purposes, a simpler adjustment process commonly referred to as the “Fraser Protocol”
was developed.
4. Fraser Protocol
In the Fraser Protocol only 3 of the 19 attributes listed above are adjusted. To maintain
the biological and logical relationships, a three stage adjustment process is used.
Stage 1: Derive adjustment factors for Height and Age of the first species. Here the
dependent variables are the ground measured Height and Age, and the independent
variables are the photo interpreted Height and Age
Stage 2: Use Adjusted Height and Age to generate interim stand net volumes (net decay
waste and breakage) using VDYP version 6. Currently, unadjusted species composition
and old FIP stocking class values are used to facilitate the production of interim VDYP
net volume.
Stage 3: Derive adjustment factors for net volume, with the ground net volume as the
dependent (y) variable and the interim VDYP net volume generated in Stage 2 as the
independent variable (x).
In this three stage process, the effects of adjusting Height and Age are linked with the
adjustment volume via the interim VDYP net volume. This way, awkward
inconsistencies in the adjustments are avoided.
In most cases ratios are used in the adjustment of the three attributes.
A detailed description of the attribute adjustment procedures based on the Fraser Protocol
are provided at the following web site:
http://srmwww.gov.bc.ca/tib/veginv/publications.htm
5. Baseline Attributes Adjustment Procedures
The 19 attributes listed in section 3, constitute the minimum number of attributes that
should be adjusted in order to allow the initialisation of VDYP 7 projection. No VDYP 7
projection will occur for a given polygon unless the adjusted attributes are harmonised to
ensure that biological and logical relationships are maintained.
A procedures manual with detailed descriptions of the harmonisation process and the
stages of developing adjustment factors will be provided in the near future. The release of
these procedures will coincide with the release of VDYP 7.
Overview on Adjustment
Factor Development
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July 2003