If you learned cruising the way most of us did, a prism cruise was probably the first variable-plot method you ran on your own. It's fast, it's been the industry default for decades, and it gets you a defensible volume number without walking every acre of the stand. A full stem count is a different animal entirely: instead of sampling a handful of points and expanding the numbers, you end up with every tree in the stand accounted for. Here's how the two compare in the field.
How a prism cruise works
A prism cruise, also called point sampling or variable radius plot cruising, uses an angle gauge or wedge prism to decide which trees around a plot center get tallied. You stand at a fixed point, sight through the prism at breast height on each tree around you, and the prism's basal area factor (BAF) determines whether that tree counts as "in" or gets left out. A tree whose displaced image overlaps the trunk is in; one that shows a clean gap is out. Borderline trees, where the image just touches the edge, get a tape check to confirm.
The BAF you pick sets how many trees you'll tally per point. A 10 BAF prism in a dense young stand pulls 15 to 20 in-trees per point; the same prism in open old growth grabs only 4 or 5. Cruisers pick the BAF to match stand density so plot counts stay manageable and the crew isn't standing at each point for twenty minutes.
Because each in-tree represents a known amount of basal area per acre, you can expand the tally from a sample of plots to a stand-level volume estimate. A relatively small number of plots, spread through the stand on a grid or random layout, stands in for the whole area, which is the math behind every cruise report you've signed off on. The accuracy depends on plot count, how uniform the stand is, and how carefully borderline trees get checked. More plots tighten the confidence interval. Fewer plots save field days and cost more in the margin of error you're carrying into the cut estimate.
Where a full stem count differs
A full stem count doesn't sample anything. Every tree crown in the stand gets identified and measured, so what you get back is a complete inventory, not a per-acre figure built up from plot averages. There's no BAF to pick, no borderline tree to argue about at the water cooler, and no expansion factor standing between what you observed and what's standing on the tract.
The tradeoff cruisers already know: a prism cruise is built around a crew covering ground on foot, which is exactly why sampling exists in the first place. Walking every stem on a section isn't realistic with cruise crews and plot sheets. A full stem count only becomes practical when something other than a boot on the ground does the counting, which is the whole reason that approach has been rare until recently. Cruisers have used it anyway because point sampling was the only way to get volume data at a cost that made sense for routine inventory work.
That's also where the two methods split on what question they're answering. A prism cruise estimates basal area and volume per acre for a stand you're treating as one statistical population. A full stem count gives you per-tree size for every tree in that population, stem by stem, which is a different kind of answer. One tells you what the stand averages. The other tells you what's standing there, tree by tree, which matters when a buyer wants a size breakdown rather than an average diameter.
If you're weighing whether your crew's next cruise plan needs another round of point sampling or whether the stand would be better served by a complete, per-tree crown map, the Tree Crown Mapping approach is built around that second option: one flight, every crown segmented, no expansion factor standing between the flight and the cut estimate. Worth a look before your next cruise season gets scheduled.