Boom position and the photogrammetry height read on a ship-to-shore crane
If you've ever pulled a height figure off a satellite image and then checked it against a spec sheet, you've probably hit a number that looks wrong. That mismatch is the crane photographed in a different configuration than the one the spec sheet describes.
A ship-to-shore crane isn't one fixed shape. The boom tips up for stowage and down for reach over the vessel, the trolley moves along the boom, and the whole headblock assembly sits at a different elevation depending on what the crane is doing the moment the image was captured. Photogrammetry reads the shape that's in the frame and reports whatever height and reach that shape produces, independent of the crane's rated air draft or catalog spec.
Stowed vs. operating height, and why the gap is large
Most OEM spec sheets quote a few reference heights: height under spreader, height under boom (stowed), outreach, backreach. These numbers describe the crane at specific, named positions. A satellite pass doesn't wait for the crane to be in any of them.
At rest between vessel calls, a crane is usually stowed, boom raised to its travel position, roughly vertical or near it. That's the tallest the structure gets, and it's the configuration closest to the "height under boom" spec. Mid-operation, with the boom lowered to reach a vessel and the trolley run out over the ship, the top of the structure sits meaningfully lower and the silhouette is longer, not taller. Two photogrammetry reads of the same crane, taken on different days, can disagree by several meters for this reason alone, and both can be correct for the moment they were taken.
A terminal-by-terminal inventory sorts cranes into size classes relative to their neighbors, and boom position is one of the things that has to get accounted for before that sorting means anything.
Reading boom angle from the image itself
A height-and-reach read from VHR imagery works backward from the geometry that's visible: the boom's apparent length and angle against the known pixel resolution (GSD), cross-checked against the crane's shadow where the sun angle allows it, and against the rail gauge and sill beam as a scale reference. Boom angle shows up directly in that geometry. A boom lying close to horizontal, reaching toward a berthed vessel, reads as a long, low shape. A boom pitched up toward vertical reads as a short, tall one. Get the angle wrong and the derived height is off by more than rounding error, because you're reading a different triangle, not just scaling the right one incorrectly.
Note which configuration the crane was in when the pass caught it, and read the dimension that configuration gives you, instead of trying to guess the boom angle and correct for it. A stowed boom gives a usable, close-to-spec height figure. A working boom, mid-vessel, gives a usable reach figure and a rough sense of air draft at that specific trolley position, far short of the crane's rated maximum. Both are legitimate data points for an inventory. Treating them as interchangeable is the mistake.
What this means for a fleet-wide inventory
A terminal-by-terminal count stays useful when the dimension column on each crane record carries the configuration it was read in, not just a bare number, and anyone pulling comparisons across terminals checks whether they're comparing stowed heights to stowed heights or mixing in a few working passes. For maker attribution and crane type, boom position barely matters. For the height figure specifically, it's the first thing to check before trusting a cross-terminal comparison.
Crane Fleet Mapping builds its annual read with this distinction in mind rather than averaging over it, which is part of what makes a terminal-by-terminal inventory worth trusting instead of just a tidy spreadsheet of numbers.
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