Composite (default) — a blend of several signals into one image. Raised edges like walls and foundation outlines tend to show up warm and bright; sunken spots fade toward blue/purple. A solid all-around view.
Hillshade — looks like sunlit terrain, the easiest one to read at a glance. Great for getting your bearings, though it can occasionally underplay a feature if it happens to face the "wrong" way relative to the simulated sun.
Slope — lights up every edge and change in steepness in yellow, no matter which direction it faces. Best for not missing anything — but on its own it can't tell you whether an edge is a bump or a dip.
Local relief (red/blue) — the clearest view for sorting bumps from dips: red = higher than the ground around it, blue = lower. This is the same layer the automatic candidate-detector itself is looking at.
What to use for what:
🛤️ Sunken paths & old roads — Local relief, look for a blue trail. Slope is a good backup (shows as two parallel yellow edges where the path dips in and climbs back out).
🕳️ Basements & cellars — Local relief (a blue rectangle), and check Slope too — steep cellar walls often show up as a bright yellow "picture frame" outline around a darker, flatter floor.
🧱 Foundations (old wall stubs) — Local relief or Composite, look for a thin red outline.
🪨 Walls, berms & mounds — Composite or Local relief — these are raised, so they read warm/red on both.
Quick workflow: start with Slope to catch every edge in view, switch to Local relief to figure out if what you found is raised or sunken, then Hillshade as a gut-check that it looks like real terrain.
Nobody keeps a list of old dumps — the ones worth digging were informal, and most closed before anyone was writing them down. So you find them the way you'd find anything else here: on an old sheet, and in the shape of the ground.
Read the words on the topo sheet. Turn on the historic map and hunt for Dump, Disposal, Sewage Disposal or Incinerator printed in plain text. They're marked far more often than people expect, and a label on a 1940s sheet with nothing there today is about as good as this gets.
Then think like the town. Refuse went downhill, downwind and just out of sight: the head of a ravine, a hollow behind the cemetery, a worked-out gravel pit or quarry, the outside bend of a creek. Rarely more than a quarter-mile from the last house.
What it looks like in the terrain. A ravine that stops being a ravine. Ground should keep falling and instead goes flat or lumpy — that's fill. On the relief view it reads as a bulge where the slope ought to be smooth. The scan names that shape when it finds it: "mounded and irregular — possible dump or spoil heap", drawn as a green circle so you can pick the dumps out of a scan at a glance without reading every tag.
Work out from a ghost town. Switch on Ghost towns, pick one, and scan the ground just downhill of it. A settlement that lasted forty years put its rubbish somewhere, and it isn't far.
Two honest warnings. Not every lump is a bottle dump — spoil heaps, field-clearance piles, forestry brash and natural slumps all read the same way, which is why that label says "possible" and stops there. And a modern landfill is a different thing entirely: capped, monitored, and full of things you don't want to put a shovel through. If it's on today's map as a landfill, it isn't what you're after.
Permission is the same as anywhere else on this map — an old dump still sits on somebody's ground.
Every USGS topographic sheet ever printed — about 180,000 of them, going back to 1884 — laid over the map at whatever opacity you like. Press the button and you get the sheets that cover wherever the map is currently centred, oldest first.
Why this matters more than any other layer here. Lidar tells you the ground has a rectangular dip in it. A 1902 topo sheet with a black square drawn on that exact spot tells you it was a building. One is a candidate; the two together are a target. This is the cheapest confirmation step available, and it costs nothing.
Things worth hunting for on an old sheet: solid black squares, which are buildings; a School, Ch, P.O., Mill or Cem label with nothing there today; roads that no longer exist; and anything the surveyor took the trouble to print Ruins beside. Conventions shifted between eras and map series, so read the sheet's own margin legend rather than assuming.
Or skip the squinting. USGS already wrote down about 100,000 of those labels by name — schoolhouses, post offices, churches, mills, stores and stations that are gone now. Switch on Old schoolhouses & mills and they show up as dots you can tap.
Coverage. Each sheet only covers its own quadrangle — a rectangle, outlined on the map when a sheet is on. Pan outside that rectangle and the overlay stops; press the button again to list the sheets for the new spot. Older maps come at coarser scales (1:125,000 or 1:62,500), so zoom out a little if a very old sheet looks blurry — that is the paper, not the screen.
Source: the USGS Historical Topographic Map Collection. TIF next to a sheet downloads the original full-resolution georeferenced scan — several megabytes, and the thing to take if you want it offline or in QGIS.
The first three colours are the shape of what the scan found. Most circles are one of these.
Rectangular — the shape closely fills its own bounding rectangle. Best match for building foundations, walls, or other rectilinear structures.
Circular — the shape closely matches a circle. Best match for mounds, pits, or other circular features.
Irregular — doesn't clearly match either. Lowest-confidence tier: could be a real irregular feature (a bend in an old path), or just terrain noise/shadow artifacts. Worth a look, but verify before trusting it.
The last three are bigger, thicker circles, and they override the shape colour — the scan has something to say about this one beyond its outline, so it stops being just a shape.
Structure — the floor profile holds up: a cellar hole, dug foundation or building platform. These are rare, a handful per scan, and they're the ones to walk to first.
Dump — raised, lumpy and shallow: a possible dump or spoil heap. Same strength of evidence as a structure, different find — see Finding bottle dumps above.
Off limits — a dashed red ring. The candidate sits on protected land the scan could name; the colour inside it is still its shape or reading. Read the popup before you plan anything around it.