Every cruise comes down to the same question before a single tape goes around a tree: how do you pick which stems get measured and which ones don't. Fixed radius and variable radius are the two answers that have run the industry for decades, and most cruisers have a strong opinion about which one they'd rather carry into the woods.
Fixed area plot cruise: the straightforward one
A fixed radius plot is exactly what it sounds like. You walk to plot center, swing a radius (1/5 acre, 1/10 acre, whatever the cruise design calls for), and tally every stem inside that circle regardless of size. A 6-inch pole and a 30-inch sawlog both count as one tree if they're both inside the string.
The math afterward is simple multiplication. Count the trees, know your plot size, blow the count up to a per-acre basis. New cruisers pick this up fast because there's no instrument calibration to screw up, just a tape or a laser and a boundary.
The tradeoff shows up in dense young stands. A 1/10-acre plot in a thick 15-year-old plantation can mean tallying 60+ stems before you move to the next point, and most of those stems are small diameter and low value. You're spending plot time on trees that barely move the volume estimate.
BAF plot cruising: weighting by size instead of area
Variable radius plots flip the logic. Instead of a fixed boundary, you stand at plot center with a prism or angle gauge and a basal area factor, commonly 10 or 20 BAF, and sight every tree around you. A tree "counts in" if its diameter, viewed through the prism, appears wider than the gap, which in practice means bigger trees count in from farther away and small trees only count in if they're close.
This is the appeal of BAF plot work: a stand's volume lives mostly in its larger stems, so a method that naturally weights toward those stems gets you to a usable basal area estimate with fewer tallies per point. A cruiser running a 20 BAF prism in a mature mixed stand might tally 8 to 12 trees per point instead of wading through 40.
The cost is technique sensitivity. Borderline trees, the ones right at the edge of counting in or out, need a careful check with a wedge prism or a tape-and-angle-gauge combo, and a sloppy sightline or an untrained crew member can bias a whole cruise without anyone noticing until the numbers come back strange. Slope correction matters more too, since BAF cruising assumes you're reading diameter at the right distance and angle, and steep ground throws that off if you're not correcting for it.
So which one fits your cruise design
Fixed area plots make sense for young, even-aged stands, regen surveys, or any job where stem count and species composition matter as much as volume. If you need to know how many crop trees per acre survived a thinning, a fixed plot gives you that number directly without the weighting.
BAF plots make sense for mature, merchantable timber where the goal is a basal area and volume estimate for a cut, and where field time per point is the limiting factor. Most industrial cruise crews default to variable radius for exactly that reason: fewer stems tallied per point means more points covered per day, and more points generally means a tighter confidence interval on the volume call.
Neither method solves the underlying problem, though, which is that both are still sampling. You're walking a subset of points and extrapolating a whole stand's stem count and size distribution from whatever landed inside your plots or your prism sweep, with all the point-placement error and borderline-tree judgment calls that come with it. If the cut estimate is riding on getting that extrapolation right, it's worth asking whether a sampled plot is still the right foundation. Tree Crown Mapping builds a stem-level crown count for the full stand from a sub-10cm aerial pass, so the basis for a cut estimate is a full count instead of a blown-up sample.
If you're scoping your next cruise design and want to see what a full-count alternative looks like, that's worth a look before you lock in your plot type.