Field guide

Detecting Along the Westward Trails

Six routes carried a few hundred thousand people across the continent in twenty years. Most of the ground they crossed holds nothing. Knowing which stretches don't is the whole job.

In short Wagon ruts and swales are shallow linear depressions, which is exactly what a Local Relief Model is good at showing — but Lidarman's automatic flags are tuned for compact, structure-sized shapes and will not mark a swale. You read the trail yourself in the terrain layers. The flags are for what sat beside the trail: a station, a dugout, a corral. And the trail line itself is rarely the interesting part — a wagon rolling past leaves less behind than a camp that sat still for a night.

Want to see whether a route runs near your ground? Turn on the "Trails west" layer — try the free demo first, no account.

1. What actually survives

Between 1841 and 1861 more than 300,000 emigrants went west on these routes, a crossing that took about five months. The National Park Service puts the Oregon Trail at roughly 2,170 miles — of which about 300 miles of ruts remain. That ratio is the single most useful fact here: on seven-eighths of the route there is nothing left to see in the ground, and the surviving stretches are not evenly spread.

What survives comes in two forms. Ruts are the paired parallel depressions cut by wheels, and they last where wagons crossed rock or firm ground. Swales are the wider, shallower version: the NPS describes a swale as "a shallow ditch" and a "wide depression", formed where hooves packed the earth between the wheel tracks and the whole corridor sank. Grass grows over a swale, which is why they are easy to walk across without noticing and easy to see in elevation data.

Ground type decides which you get. At Scotts Bluff in Nebraska the ruts "have all eroded as the sandstone and siltstone that make up the ground through this area is soft and easily eroded", leaving a broad swale near Mitchell Pass. At the Oregon Trail Ruts State Historic Site near Guernsey, Wyoming, wagons crossed soft sandstone and cut ruts deep enough that they are still a landmark. California Hill in Nebraska is another stretch where the climb concentrated traffic into visible tracks.

Worth knowing A trail is not a line. Where the ground allowed it, wagons spread out abreast to avoid each other's dust and ruts, so a corridor can be a braided band of parallel traces a mile or more wide. If you find one swale, look sideways for its neighbours before assuming you have found the route.

2. Reading a trail in the terrain layers

A swale is a long, shallow, low-relief depression, which is the signature the Local relief style exists to isolate — it subtracts the broad shape of the landscape and leaves the small local deviation, rendering lower-than-surroundings ground blue. A swale running across a hillside shows as a thin blue thread that ignores the contours, which is what separates it from a natural drainage: water follows the slope downhill, and a wagon road cuts across it to keep a workable grade.

Slope is the useful second opinion. It lights up every edge regardless of which way it faces, so a shallow trough that is nearly invisible on hillshade often reads as two faint parallel lines. Use hillshade to get your bearings and to judge whether a feature is plausible for the terrain, then switch to local relief to decide whether it is really sunken. There is more on all four styles in LiDAR Hillshade Maps for Metal Detecting.

Be sceptical of straight lines. Field boundaries, old fence lines, buried pipelines, two-tracks and firebreaks all produce long shallow linear features, and most linear things you find near a trail corridor are younger than the trail. An emigrant route wanders with the ground; a pipeline does not care about the ground at all.

3. Why the auto-flags won't find the trail

This is worth being blunt about, because it is a real limit rather than a setting you can change. Lidarman's candidate detector scores each shape it finds by how rectangular or circular it is, and weights the result toward things that are plausibly structure-sized. A swale is neither — it is long, thin, and often runs off the edge of the search area entirely. It will not be flagged, and no radius setting will make it flagged.

So on a trail corridor the numbered markers are answering a different question than the one you are asking. Read the imagery yourself for the route. Let the flags do what they are good at: picking out the compact, structure-shaped things sitting near it.

4. The stopping places are the point

A wagon that rolled past dropped very little. A place where people stopped — because they had to — is where material accumulates. So the productive way to use a trail route is as a pointer to the places along it that forced a halt:

River crossings and fords. Traffic funnels to the one usable bank, waits its turn, unloads, and sometimes loses things in the water.
Water and grass. Springs and creek junctions set where a day's travel ended, which is why the same campsites were used year after year.
Climbs and descents. Hills where wagons had to be lightened, double-teamed or lowered on ropes — the same places that cut the deepest ruts.
Station sites. The Pony Express ran on relay stations spaced a hard ride apart, and those were built structures with foundations, corrals and rubbish behind them. These are the compact shapes the detector is built to flag.

All of which is a terrain question as much as a history one, and the terrain has not moved. The ford is still where the bank is low; the camp is still beside the spring.

5. Using the trails overlay

Lidarman's map has a Trails west layer in the control at the top right, showing six routes: Oregon, California, Mormon Pioneer, Pony Express, Santa Fe and Old Spanish. The lines come from the National Park Service's own congressionally designated alignments for those National Historic Trails.

Read this before you trust the line A designated alignment is an approximate corridor, not a survey of the ruts. It is drawn at national scale and generalized further to keep the map quick, so treat it as "the route came through about here", not as "the wagons were on this pixel". The ruts are what you find in the terrain data; the line only tells you where to go looking.

The workflow that makes sense: switch the layer on, zoom to your area to see whether a route passes near it, then turn the layer off and search the terrain properly. Start with the places from section 4 rather than a random point on the line.

Land status along these routes is genuinely mixed — private ground, federal land, state land and protected sites can follow one another within a mile, and the rules are completely different on each. That is its own subject, covered in Permission and Legal Basics Before You Dig.

More guides

Finding Ghost Towns With LIDAR — What a vanished town leaves in the ground, where the list of 16,525 of them comes from, and why the dot is only a starting point.
Detecting Near Civil War Battlefields — The protected core is off limits — the study area around it often isn't. How to tell the two apart before you knock.
Finding Old Mining Camps — 44,677 abandoned metal mines, what the camp left behind as opposed to the workings, and why the two are a short walk apart.
Finding Old Homestead Sites With LIDAR — What a vanished foundation, root cellar or filled-in farm road looks like in elevation data, and how to tell them from natural ground.
LiDAR Hillshade Maps for Metal Detecting — Hillshade, slope, Local Relief Model and sky-view openness, and a practical workflow for reading them together.
Historical Maps for Metal Detecting — Where to get old maps free, what the symbols mean, and how to line a century-old sheet up with the ground you are standing on.
Permission and Legal Basics Before You Dig — Private land, public land and protected sites are three very different situations. General ground rules, not legal advice.

Got a stretch of trail in mind? See what the ground still shows.

Try the free demo

← Back to guides · Next: LiDAR Hillshade Maps for Metal Detecting →