How digital elevation models supply route height

Line meets grid

Software turns horizontal route positions into an ordered series of terrain heights.

A raster digital elevation model divides an area into cells with stored elevation values. To create a route profile, software chooses positions along the route, locates those positions in the model and assigns a height from nearby cells. It may use the nearest cell or interpolate between several cells. The method is a product decision.

The samples are paired with cumulative distance along the route. Distance forms the horizontal axis of the profile and terrain height forms the vertical axis. Summary values such as highest point and total ascent are then derived from the ordered series, usually after further handling of missing values and noise.

Route vertices and height samples need not be the same thing. A line may contain sparse planning points yet be sampled at regular intervals. A dense recording may be resampled less often than its original GPS observations. Either approach can be reasonable, but it changes which local features enter the profile.

What the cells represent

Surface definition and cell spacing set the detail available to the route.

USGS commonly uses DEM as a general term for gridded elevation and normally treats an unspecified DEM as bare-earth terrain. A digital surface model can instead include the tops of buildings or vegetation. A route planner needs to know which surface it has sampled because both can be internally valid while describing different things.

Raster resolution is the linear size of each cell. Features smaller than that cell cannot be represented explicitly. Ordnance Survey describes OS Terrain 50 as a low-resolution model with 50-metre post spacing, intended for broad applications. It can reveal large landforms without resolving every embankment, hollow or narrow ridge crossed by a route.

Resolution is not the same as accuracy. A small cell can contain an inaccurate value, and a coarser product can be well controlled for its intended scale. Source measurements, processing, vertical datum, update date and quality metadata remain separate questions.

Where local errors appear

A correct grid can still give the wrong route feature when the line is misplaced.

A route drawn a few metres to one side of a path can sample a different cell. On gentle ground the difference may be negligible. Beside a cliff, cutting, ravine or narrow ridge, the same offset can create a conspicuous height error. Horizontal route accuracy is therefore part of every terrain-derived profile.

Interpolation gives a value between stored samples. It does not discover an unseen feature. Adding more route samples can make the resulting graph appear continuous, but every value still comes from the same underlying surface.

• BridgeBare-earth terrain may supply the valley floor or water height rather than the deck.
• TunnelThe route travels below the represented surface, so terrain height describes the land above.
• CuttingA generalised model may not preserve the excavated road or railway level.
• Water edgeHydro-flattening and shoreline generalisation can affect nearby samples.

Before using the result

Inspect the model metadata and the suspicious section together.

1. Surface

   Confirm what the model represents.

   Look for bare earth, surface, bathymetric or another defined elevation product.

2. Grid

   Read cell spacing, datum and stated accuracy separately.

   None of these properties can stand in for the other two.

3. Line

   Check the route against the mapped path at the mismatch.

   An offset line may explain a spike that the terrain data itself does not contain.

4. Context

   Identify structures the terrain cannot represent.

   Keep bridge, tunnel and cutting corrections explicit rather than smoothing them without explanation.

The profiles in the beacen Library are route-planning views.

When a precise height matters, trace the displayed value to the named terrain product, its datum and its processing history. Interpolation can estimate between stored samples, but it cannot turn a modelled height into a direct measurement of the route.