In this article, we will discuss everything you need to know about how to measure basal area per acre, how to properly use an angle gauge and prism gauge, and how to design your own cruise. We will even provide a free cruise sheet that you can download by clicking the image below:
What is Basal Area?
Basal area per acre is a commonly used measurement of wood density in a forest stand. But what exactly is basal area, and what does it represent? In short, basal area is the total amount of surface area occupied by wood on a horizontal plane. If we imagine a horizontal plane at DBH (4.5 feet off the ground) spanning an entire acre and intersecting all trees on that acre, what would be the total surface area of the log faces that were intersected by that plane? That’s basal area.
Basal area is a special measurement because it neither strictly measures density like trees per acre, nor measures mean tree size like DBH. Instead, it provides a mix of those two attributes and gives us a little information on both. Combined with other details like tree height, basal area per acre can be a reliable way of estimating total wood volume on an acre, making it a crucial measurement for inventory, management, and timber value analysis. Learning how to measure basal area is thus essential for understanding forest management.
The Basics of Measuring Basal Area Per Acre
The only way to know the true basal area of a given acre is to measure every single tree and sum the total area each tree represents. Clearly, that is far too impractical, so foresters developed means of estimating basal area through point sampling.
In point sampling, foresters use tools (an angle gauge or prism gauge) designed to simultaneously measure the distance and diameter of a tree to judge whether that tree is representative of a given amount of basal area per acre, known as the basal area factor or BAF. Each tree is determined to be “in” or “out” of a plot, and the total tally of trees “in” the plot are multiplied by the basal area factor. Thus, such a plot (also known as a variable radius plot) will follow these steps:
- Find plot center
- Go around the plot center using an angle gauge or prism gauge and count trees in the plot based on the parameters of the respective gauge (more on that later).
- Multiply the total tally by the BAF of the gauge used.
- Repeat this process for multiple plots and average the results.
That’s it. That’s the process. The rub, however, is in the proper use of the angle and prism gauges, deciding on a proper BAF, as well as designing the overall cruise. Let’s go into detail for each step
Choosing the Basal Area Factor
When it comes to choosing a proper BAF for a point sample, it is important to remember that a larger BAF will tally fewer trees. The goal is to be able to as few trees as possible while still getting a representative sample of the stand. Thus, a larger BAF can be used for a stand with larger trees and a smaller BAF can be used for a stand with smaller trees.
In the US northeast, where trees are generally smaller, 10 BAF is considered standard. However, 20 BAF is used for stands of larger saw logs. Out in the western US, where larger trees are the norm, even 40 BAF can be used. While there are no clear rules, a good rule of thumb is to choose a BAF that will tally around seven trees per plot. When in doubt, use a smaller BAF. It is better to tally too many trees than too few.
On an angle gauge, BAF can be chosen freely, as each instrument comes with usually four different slots for four different BAF measurements. On a prism gauge, however, it is limited to one BAF per prism, so there is less optionality.
Remember, the BAF you choose is the basal area per acre each tree represents in a plot, so if you tallied seven trees using 20 BAF, that plot would represent 140 BA/Acre.
Taking a Plot With an Angle Gauge
An angle gauge measures basal area per acre by measuring the angle a tree’s width relative to your eye. The narrower the angle, the less likely a tree is to be counted in a plot.
To use an angle gauge, start (as always) by finding plot center. Once found, stand directly on plot center. It is important to note when using an angle gauge that the person taking the measurements (the cruiser) acts as plot center, and while counting trees, the cruiser spins around from a single point.
To begin measuring, pick a starting point. It is best to use a starting point that can be easily remembered. I recommend tying a ribbon to a branch to mark where you start. After a starting point is determined, place the loose end of the gauge’s chain in your mouth and fully extend the gauge away from your eyes, as seen in the image below.
turning around in a circle, carefully get a clear sight picture of every tree around you and determine if each tree fits within the slot (of the specified BAF, as marked on the side) or extends past the slot.
If the tree extends past the slot, as seen in the picture below to the left, the tree is counted in. If, however, the tree fits within the edges, as seen in the photo below to the right, the tree is counted out.
Continue this process with every tree around you and stop at your starting point. Be sure not to double count trees! Take your total tally and multiply the number by your chosen BAF. That will give you the total basal area per acre that plot represents.
Taking a Plot With a Prism Gauge
Determining basal area with a prism gauge is similar to the process for an angle gauge, with a few key differences. Prism gauges measure basal area by creating an offset image of every tree bole. The narrower the tree, and the farther away it is, the greater the offset. Trees that are sufficiently offset are counted out.
Similar to the angle gauge, begin a prism gauge plot by finding plot center. However, unlike an angle gauge, the center of a prism gauge plot is the prism itself. It is necessary for the cruiser to rotate around the prism while holding the prism over plot center.
Find your starting point (marking it, ideally) and view every tree around you through the prism, such as in the photo below. Note that the distance of the prism from your eye is irrelevant, though a matter of personal preference and visibility.
As mentioned previously, The prism will create an offset image. Look carefully at the bole (stem) of each tree. If the offset is still within the edges of the real position of the tree, as seen in the image to the left below, the tree is counted in. If however, there is no intersection between the offset image and the real tree, and the edges do not meet, such as in the image to the right, the tree is counted out.
As with an angle gauge, this process is repeated for every tree around you, and the final tally is multiplied by the BAF of the prism. Once more, be careful not to double-count trees!
Which Is Better, an Angle Gauge or a Prism Gauge?
Both are perfectly acceptable to use. Prisms can be easier to see through, but in low light conditions, it can be more difficult than an angle gauge. Prism gauges are also limited to a single BAF, whereas angle gauges have more options, as previously mentioned. Ultimately, it comes down to preference.
For beginners, I recommend an angle gauge to start, if only because it is more intuitive to treat yourself as plot center than to circle around the gauge.
Designing a cruise
Now that you have an understanding of how to conduct a single plot, is important to understand how to best conduct an entire cruise, which is what we call a system of forest measurement.
The design of a cruise is going to be crucial for the accuracy of your numbers. If one is not careful, it is easy to unintentionally pick and choose the location of your plots. This will greatly bias your data and give you inaccurate results, likely over-estimating the basal area per acre. Particularly if a cruise has management or financial consequences, it is crucial to create a cruise that is as unbiased as possible.
To that end, one must pick a system for picking plots. Here we will focus on two popular systems: random sampling and systematic sampling.
In random sampling, plots are chosen (you guessed it) randomly prior to beginning the cruise and plotted on a map. Plots must then be navigated to manually with help of a GPS, map, and compass.
In systematic sampling, plots are chosen using a specified system, such as walking along northerly lines and taking a plot every 250 feet.
Examples of both systems can be seen in the visuals below.
How Many Basal Area Plots Should Be Taken?
How many plots you incorporate in your cruise depends on how intense you want the sampling to be. However, a good rule of thumb is to do one plot for every five acres or five plots minimum.
Recording Your Data
Along with defining a cruise system, it is important to develop a system for recording and tallying your data. If all you need is a gross measure of basal area per acre, averaging your totals from each plot will work fine. However, it may be wise and useful to break down the data further and tally trees by species and, if your experience allows, product class. Take, for example, this sample cruise data broken down by species using a basal area factor of 20.
| BAF=20 | Red Spruce | Sugar Maple | Yellow Birch | Total BA |
| PLOT 1 | 2 | 0 | 3 | 100 |
| PLOT 2 | 3 | 2 | 2 | 140 |
| PLOT 3 | 4 | 1 | 1 | 120 |
| Average BA | 60 | 20 | 40 | 120 |
By breaking down the basal area per acre by species, we can get a more clear picture of the composition of the stand. This can be crucial for cruises for purchasing property or preparing a timber sale. If one only needs to know stocking levels, or if a stand is largely dominated by a single species, such detail may be unnecessary.
Some Notes
The following are some finer details to keep in mind before you go out to the woods.
Understand the Units You Are Using
In the United States, where I leave, we measure basal area in terms of square feet of basal area per acre. In Canada, Europe (and really the rest of the world) basal area is measured in terms of square meters per hectare. Both are perfectly fine to use, but angle gauges and prisms from both countries are calibrated to those specific units. They are not interchangeable. I personally worked on the border of the US and Canada, and it could get confusing. Know what tool you are using!
Dealing with Borderline Trees
Regardless of whether you decide to measure basal area with a prism gauge or angle gauge, situations will present themselves (probably often) where a tree is just on the border of being in or out, and it can be difficult to know which. Technically speaking, every tree has a specific “limiting distance” based on its diameter. If you find yourself unsure of a trees status, you can measure its diameter and distance from plot center. If its distance exceeds the limiting distance, it’s out.
However, a much more expedient (and in my experience, fairly accurate) method of dealing with borderline trees is to count every other borderline tree in. However, one must be able to keep track. A simple note sheet can work well for this.
Using Basal Area Practically
Once you learn how to measure basal area per acre and practice, it is a skill that can open up a lot of useful data and broaden your understanding of your forest. For example, basal are per acre is the prime measurement used to estimate cords per acre, and it is the basis of most hardwood stocking guides in the northeast. If you become competent with measuring it, I recommend you get reference materials such as volume tables for your region and species mix to better utilize the data. For now, however, get out there and practice!