How to define a target signature for a satellite site search

Before anyone runs a country-scale image sweep, somebody has to answer a smaller question first: what does the thing you're looking for actually look like from above? That answer is the target signature. Get it wrong and the sweep comes back either empty or buried in false hits. Get it right and the output is a list you can route to a rep.

This is the part of the project that gets rushed, usually because it feels like the "easy" step before the real analysis starts. That step is what determines whether the rest of the project is worth running.

What a target signature is

A signature is the set of visual features that separate your target facility type from everything else sitting on the landscape at the same resolution. Not a text description, a decomposed list: footprint shape, roof form, yard layout, material staging, vehicle presence, and the spatial relationships between them. A scrap yard and a lumber yard can both read as "open yard with piles and trucks" at a glance. The signature is what tells them apart on the pixel.

For a site search run against RGB imagery in the 0.5-2 m range, you're working with shape, shadow, texture and color, not labels. No thermal signature, no spectral index to lean on. So the signature has to carry its weight on form alone: is the stockpile conical or windrowed, does the yard have a scale house and a single gated entrance, is there a rail spur running into the back of the lot, does the roof have the saw-tooth profile of a fabrication shed versus the flat slab of a warehouse.

Building the signature from known sites

Start from facilities you already know are real. Pull up five to ten confirmed sites of the type you're hunting and look at them side by side at the resolution your sweep will run at, not zoomed in past what the sensor delivers. Write down what's common across all of them and, just as important, what varies and shouldn't be treated as a requirement. If three of your ten reference sites have covered storage and seven don't, covered storage isn't part of the signature.

Pay attention to scale too. A pit that's four hectares and one that's forty hectares might both be "the right kind of pit" but a signature built only from the big ones will miss the small operators, and a lot of an unaddressed market is small operators. Note the size range you're willing to call a hit, not just the shape.

Then check the signature against near-neighbors, the facility types that get confused with your target. A quarry and a construction-debris yard can look alike from above. A tank farm and a water treatment plant can too. List the near-neighbors explicitly and write down the one or two features that separate them, because that's what keeps the sweep's output from needing a second pass of manual cleanup before it's usable.

Where signatures break down

Seasonal cover is the most common failure. A yard with exposed stockpiles in July can read very differently under snow or after a rain event has darkened the ground. If your sweep only runs once a year, decide up front which season the signature is calibrated for and don't assume it transfers.

Regional construction style is the second. A fabrication yard built to one country's shed conventions doesn't necessarily look like the same facility type built somewhere else. If the search area spans more than one region, sanity-check the signature against reference sites from each region before you commit to a full run.

And watch for signatures that are really just "big open lot with trucks," because that description fits half the industrial land in any country. If your signature can't rule out a used-car lot or a trucking depot, it isn't finished.

Getting from signature to census

Once the signature holds up against known sites and near-neighbors, it's ready to hand off as the spec for a sweep. That's the piece Facility Discovery runs on your behalf: an annual, country-scale pass over high-res RGB imagery built around the signature you've defined, turning it into a geocoded list of matching sites instead of a description nobody can act on.

If you've got a facility type in mind and need help turning it into something a sweep can run against, that's a conversation worth starting now.