Fixed-roof vs floating-roof tanks in satellite imagery
If you're new to reading tank farms from above, the roof type is the first fork in the road. Fixed-roof and floating-roof tanks can look almost identical on a plot plan or in a ground photo. From a nadir satellite pass, they don't, and getting the call wrong wastes a day you don't have before the weekly report drops.
What separates them on the ground
A fixed-roof tank is a cone or dome welded straight onto the shell. It's a static structure, usually used for refined products, chemicals, or smaller-volume crude that doesn't justify the extra steel. Nothing inside it moves relative to the roof. A gauge hatch or a small pressure-vacuum vent might poke through the top, but the roofline itself is fixed the day it's built.
A floating-roof tank is different hardware entirely. The roof is a steel pan (or pontoon ring with a center deck) that rests directly on the stored liquid and rides up and down with the level. You'll see a rim seal where the roof meets the shell, sometimes a foam dam ring near the edge, and on external floating roofs a thin handrail circling the perimeter. These are the tanks terminals use for crude and condensate because the floating roof cuts vapor loss and lets operators pack more volume into a given footprint without a fixed vapor space.
What to look for in the imagery itself
The giveaway from above is the shadow. On a floating tank, the rim wall casts a crescent-shaped shadow onto the deck wherever the sun isn't directly overhead, and the width of that crescent changes as the deck rises and falls with fill level. More product, the deck rides higher, and the shadow gets thinner. A fixed-roof tank casts a shadow too, but it's a function of shape and sun angle alone. It looks the same in January and July, full tank or empty one, because nothing underneath it moves up or down.
A few more tells once you're looking at a frame:
- Rim detail. EFRs show a visible gap and seal line around the edge; fixed roofs meet the shell in a clean, continuous weld line with no gap.
- Surface texture. Floating decks often show faint circular marks from support legs when a tank is drawn down low enough that the roof is resting rather than floating, plus occasional pooled-water patches after rain near the drain. Fixed roofs shed water in a sloped, uniform pattern with no legs to mark.
- Diameter. The biggest-diameter tanks on a pad are typically floating-roof; crude and condensate storage tends to run large, while fixed roofs cluster smaller, on the products and chemical side of the yard.
One caveat worth flagging before you build a watch list: internal floating roof tanks (an IFR, a floating pan sitting inside a fixed cone shell) look exactly like a fixed-roof tank from directly above. The floater is hidden under the dome. There's no way to tell an IFR from a true fixed-roof tank on exterior imagery alone, so if a terminal's tank list mixes both types, don't assume every cone-topped silhouette is dead storage. Cross-check against terminal permit filings or prior disclosures where you can.
Why the roof call matters before you spend time on a tank
Only tanks with an exposed floating roof give you a daily geometric signal you can turn into a level estimate. Fixed-roof tanks don't expose anything about what's inside from above; you'd need a different data source entirely to know whether one is full or empty. Sort your tank list by roof type before you start, and spend analyst time only on the EFRs. That's the filtering step that makes a daily per-tank read across a whole terminal pad workable instead of a manual slog through hundreds of silhouettes.
Once you've got the floating-roof tanks separated out, the shadow geometry does the rest of the work, tank by tank, day over day. Oil Tank Fill runs that read on the floating-roof tanks in your area of interest so you're not the one squinting at crescents before the report comes out.