Venlo glasshouse vs tunnel house: reading roofline shape from above

Two roof shapes, two signatures

When you're sizing a block across a county or a province, you're not walking every range. You're reading a pass of high-res imagery and trying to tell a Venlo glasshouse from a tunnel house before you've ever stood under one. The two roof types resolve differently once you know what the ridgeline is telling you.

A Venlo roof is pitched glass, cut into narrow bays, usually somewhere in the 3.2 m to 4 m span range, repeated side by side across the whole block. From above that reads as a tight, regular ridge pattern, almost like corduroy, with a grid of glazing bars running perpendicular to the ridges. Glass reflects more evenly than plastic film, so a Venlo range tends to show up bright and uniform in RGB, with sharp, straight ridge lines you can trace block to block.

A tunnel house is arched, not pitched. The curved cover means sunlight hits each span at a changing angle across its width, so instead of a flat bright panel you get a soft gradient: light on the crown, darker toward the eaves. Line up several tunnels and that gradient repeats, giving the block a banded, almost corrugated look that's nothing like the hard grid of a glasshouse roof.

Gutter-connect or standalone

Span width tells you the structure type. Whether the bays are gutter-connected tells you how to size it.

Gutter-connected ranges, Venlo or multi-span tunnel alike, share a continuous ridge and gutter system across the whole block. There's no gap between bays, so the covered footprint reads as one unbroken polygon no matter how many spans sit underneath it. A forty-bay gutter-connect range counts as one structure on the map, not forty.

Standalone tunnels are the opposite. Each hoop is its own unit, with a service alley or drainage gap running to the next one. Those gaps show up as thin dark lines along the length of the block, breaking what would otherwise look like one continuous cover into a row of distinct footprints. Miss that distinction and you'll either undercount a tunnel farm by treating it as a single block, or overcount a gutter-connect range by splitting it where there's no actual gap.

NIR helps more here than RGB does. Plastic film and glass both reflect visible light reasonably well, but they don't behave the same in the near-infrared band, and the alley gaps between standalone tunnels tend to show bare soil or gravel, which separates more cleanly from the cover material in NIR than it does in true color.

Why the roofline matters for sizing

A region's protected-cropping inventory needs structure type and footprint area, not just a count of shapes on an image. An input supplier planning glass-specific volumes doesn't want tunnel houses folded into the Venlo total, and a planner tracking gutter-connect expansion against standalone hoop additions needs the two kept apart from one pass to the next. Getting the roofline right at the classification stage is what makes the rest of the inventory usable.

Greenhouse Mapping builds that annual structure inventory from satellite passes, classifying each footprint by type as part of the same pass so a Venlo range and a tunnel block never end up counted the same way.

If you're sizing a region's protected-cropping territory and want that roof-shape classification done at the imagery stage rather than argued over afterward, get in touch.