Choose elevation data for terrain analysis

Guide Updated 24 October 2026

Choose elevation data by the surface, place, date and height reference your analysis requires, then compare cell spacing and stated accuracy separately. Two products can look similar on a shaded map while representing different surfaces or measuring heights from different reference levels.

Name the surface you need

Begin with the question the elevation values must answer. Bare-ground terrain, the tops of buildings and vegetation, and water depths are different targets. USGS distinguishes terrain models from surface models in its product descriptions; the latter can include above-ground features.¹

Do not decide from the abbreviation DEM alone. Open the product documentation and record how the surface was produced. A labelled surface field in the elevation comparison is a route to that evidence, not a replacement for the product’s technical description.

The elevation question selector points to selection, download, mosaic and older-format workflows. Use it to choose the next check without making the product decision for you.

Separate grid spacing from accuracy

Cell spacing describes how the raster is sampled horizontally. Accuracy describes agreement with a reference under a specified assessment. A dense grid can still have uncertain heights, and resampling a grid more finely does not create new source observations.²

Read the accuracy statement with its metric, reference, tested population and product edition. Keep an unspecified statement unspecified. Do not compare a maximum-error claim directly with a root-mean-square statistic as if they measured the same thing.

For USGS products, the 3DEP overview distinguishes source-resolution data, project-based products and seamless products.¹ Those categories help explain why a statewide mosaic and a local project may have different acquisition histories even when they are supplied through the same service.

Record two reference systems

Horizontal coordinates locate the cells. A vertical datum defines the reference for their height values. Recording a map projection alone does not complete the elevation record.

NOAA explains that inconsistent vertical datums can introduce artificial discontinuities when elevation sources are combined.³ Before a mosaic or comparison, write down the source references and the intended target reference. A required datum transformation is a separate operation with its own coverage and uncertainty; it is not accomplished by changing a label.

Keep horizontal and vertical units explicit as well. Do not infer that elevation values are in degrees because the grid’s coordinates are geographic, or that every height is in metres because a neighbouring product uses metres.

Match the data’s history to the question

Distinguish acquisition dates from publication and retrieval dates. A newly downloaded file may represent an older survey, while a seamless product can combine several source periods. For work involving terrain change, those distinctions are part of the result rather than administrative details.

Follow the download workflow to keep metadata and tile identifiers together. USGS publishes both textual and spatial metadata for 3DEP products.¹ Use those records to identify the relevant source area and period rather than taking the portal’s page date as the survey date.

Before merging tiles, compare references, pixel grids and NoData definitions. If an exercise depends on an older transfer format, use the legacy workflow and document any replacement explicitly. Matching the old filename is not the same as reproducing the old input.

Not covered on this page

• Survey-grade or construction suitability.
• A site-specific vertical transformation.
• Independent accuracy testing of the listed products.