Log stacker vs knuckleboom loader: telling them apart from overhead imagery

What's actually different on the ground

A log stacker is a self-propelled machine, usually rubber-tired, built around a vertical mast with forks or a grapple that lifts and carries logs much like a forklift handles pallets. It moves between the deck and the infeed, stacking tiered piles and squaring them off. A knuckleboom loader works a fixed or semi-fixed spot instead. The boom folds at a knuckle joint and swings from a pedestal, a trailer, or the back of a truck, reaching out to grab logs with a grapple and swing them onto a deck or a truck bed without the machine itself traveling far.

From the ground the difference is obvious: one drives, one swings. From 500 mm GSD overhead, you lose the motion and you're reading shape, shadow, and position instead.

Reading shape and shadow from directly above

A stacker shows up as a compact rectangular chassis with a mast casting a thin, tall shadow that shifts with the sun angle but stays roughly centered over the machine. Tire tracks fan out around it in the yard dirt or gravel, and you'll often catch it mid-pile, forks buried in a stack with logs visibly displaced on one side.

A knuckleboom loader reads differently. The boom itself, even folded, throws an angular, jointed shadow unlike anything a straight mast produces. If it's pedestal-mounted, the base sits in one fixed spot pass after pass, usually at the edge of a deck or alongside a rail spur or truck lane, with no tire tracks leading up to it. A truck-mounted knuckleboom carries that same jointed-arm shadow on a flatbed chassis, parked at the yard's load-out point rather than threaded between piles.

The other tell is what's touching the ground. A stacker's footprint includes visible wheels or tracks under a boxy cab. A knuckleboom's footprint is either a small concrete or gravel pad with no visible undercarriage at all, for the pedestal versions, or a long flatbed trailer shape for the truck-mounted versions. At 0.5 m resolution those outlines hold up well enough to call even without the boom fully extended.

Why the distinction is worth making on a quarterly pass

Mixing these two up on a count means missing what's actually changed at a yard. A sawmill adding a second stacker is scaling its deck-handling capacity, usually a sign of more inbound volume than one machine can square away. A yard adding a knuckleboom at the load-out is gearing up truck turnaround, which points at outbound shipping pressure, not necessarily more logs coming off the trucks. Same equipment count, different story, if you can't tell a mast from a boom.

For a rep working sawmill and timber yard accounts, that distinction answers a different question than a raw machine count does. It tells you whether the yard needs a replacement unit for aging stock, a second machine for added throughput, or neither, because what grew was outbound handling and not deck volume.

A few more things worth knowing

Can a knuckleboom also stack logs into a deck pile? Yes, a pedestal-mounted unit near a deck will build and square piles the same way a stacker does. The giveaway from overhead is still the fixed base and jointed-arm shadow, not what it's doing with the logs at the moment of the pass.

Does GSD matter for telling these apart? At coarser resolution than about 1 m, both machines blur into similar blob shapes near a deck. Under 0.5 m, the mast-versus-boom distinction and the wheeled-versus-fixed-base footprint both hold up reliably enough to count by type, not just by total.

What about a loader mounted on a wheeled carrier instead of a truck or pedestal? Those show tire tracks like a stacker but still carry the jointed-boom shadow. Footprint and shadow together, rather than either alone, is what settles it.

If sorting out which yards are adding stackers versus knucklebooms by eyeballing imagery every quarter sounds like more squinting than your week has room for, Timber Yard Equipment runs that count and reads the deck volume alongside it, so you get the equipment type breakdown without doing the shadow-reading yourself.