Defect is the percentage of gross volume that does not meet net volume standards. Though accepted as standard practice, this concept of defect can only approximate a real world net loss of volume because defect can be hidden from view and different products have different levels and types of acceptable defect. Assisi can record other kinds of defect such as general damage, bark, grade, quality or risk codes, but it is only defect that discounts gross to net volume.
Perhaps because of this, defect is found measured in many different ways. Defect can be stated by whole tree or by log or by segment. It may be estimated directly or by length and diameter deductions. It may be measured in the field or defaulted. Defaults can be by amount or as additional to measured. Assisi supports all of these methods, but there are subtleties to how defect gets summed, averaged and grown.
I'll try and explain these in this Study Hall. But in the Assisi library there is no need to be quiet! So grab a pint, speak up and I'll try and fill in the gaps as you comment. Remember to follow this post so you get emails when comments or updates occur!
Total, Gross and Net Volume
Total volume is defined as the volume from stump to tip. The portion of total that is marketable is called gross volume. The portion of gross that meets product standards is called net.
Total gross and net volume is calculated in both board and cubic units. Board units simulate the volume of milled dimensional lumber. Board often uses equations and tables that over the years have been accepted as approximating the volume in actual dimensional lumber. However, that approximation has gotten long in the [saw]tooth as milling has become more efficient. The board equations and tables have largely remained the same anyway so that a common unit of board volume can be used for discussion. Cubic on the other hand is generally needed to be an accurate measure of volume including tapered volume of stems minus bark. Cubic is important for products such as chip, pulp, carbon, cord wood, weight among others.
Board & Cubic Defect
This is all great, but you can see already that defect needs to reflect whether the volume is in board or cubic units so that the valuable net volume truly reflects the product its being quoted for. And so we start with defect being stated in both board and cubic terms.
Whole Tree, Log and Segment Defect
Next we come to what dimensional element defect measures actually describe. Is it a whole tree, a log or a random length ("segment" in Assisi).
Whole tree defect values can be entered for both board and cubic units. Tree level defect will propagate to volumes whether from whole tree volume equations or tables or from log level volumes.
Log level defects imply dimensional lumber which in turn implies the need for only board defect. But other products such as poles may be created and these might benefit from cubic defect values. So Assisi currently uses a single defect field for log or segment level defect that is applied to both board and cubic volume for logs and segments. Furthermore measured log level defects will override tree measured or defaulted tree level defects.
Length & Diameter Deductions
Gross volume is discounted by defect. But at the log or segment level, defects can be described as length and diameter deductions as well. Length and diameter deductions can be useful to discount volume from burn, infection or physical surface damage or from section length damage that hasn't already been tagged as cull.
Length and Diameter deductions are added to any percent defect when calculating net volume. Specifically, net volume is first calculated using the deducted dimensions and then a percent defect applied to that. Gross volume on the other hand has neither defect nor deductions included.
Measured, Default and Additional Defect
We have been talking about defect values as measured in the field, but Assisi can also apply default defects.
Default defect is applied when defect is not measured in the field. Defaults are set on a species basis and act as tree level defect when used. Default defect allow a base level of defect to be applied while still measuring outliers as they occur. Defaults defect will require a zero be entered if a tree is desired to specifically have zero defect otherwise the default value will be used.
An "additional" defect can also be set along with a default that adds to the measured or defaulted value. Additional defect is useful for accounting for the effect of recent damage events when field data were gathered prior to the event.
During compilation, measured defect overrides default defect and additional defect is added to either measured or default defect. Furthermore, measured log level defect overrides tree level defect even if tree level was measured. Length and diameter deductions are accounted for by calculating the difference between volume with and without the deductions.
After gross and net volume are calculated, the percent difference in volume is recorded as the net overall defect for both trees and logs. Defect can come from many sources plus dimensional deductions so in the end it is the difference in gross to net volume that determines overall defect and is stored to the database as compilation results.
Defect and Growth
Defect does not attenuate or advanced during growth. That is, what was measured or defaulted to at cruise remains in effect for entire growth cycles whether growth is to an inventory date or in a long term simulation.
Furthermore, defect remains fixed to the lengths it was originally called on. For example, if log defects are recorded they will apply to the same length throughout tree growth. Same thing goes for length and diameter deductions.
Note that defect for whole trees will change as trees increase in volume if defects at the log level were the original source. Defect at the log level may remain constant however if length and diameter deductions were not originally used.
Inventory Growth and Simulation Growth
Growth in Assisi comes in two flavors. Inventory growth estimates growth from time of cruise to a certain date, usually today's date. Simulation growth estimates growth from inventory to a long term time horizon.
Processing defect during inventory growth is straightforward because inventory growth always starts with cruise data. Simulation growth however starts with inventory trees which themselves likely have already been grown. Because log lengths called in the field retain their lengths, they also retain their defects so Assisi can refer back to cruise data for defect even though trees have already grown to new dimensions. It gets tricky for inventory that has been expanded from some other source and for trees that have been tripled, but the process works the same.
I will stop here and see if anyone has additional questions. Just post them as comments and I will edit this post as needed. You can also add "Edit Notes" but I don't think they can be tagged to specific paragraphs. I will try... :)