Terrain science and data lineage

DTED is a tiled binary format for regularly spaced elevation posts. It describes how values are organized and referenced; it does not by itself identify the sensor, age, processing history, or real-world accuracy of those values.

DTED levels and post spacing

Level	Nominal angular spacing	Approximate equatorial spacing	DTED.org role
DTED0	30 arc-seconds	900 m	Lower-resolution fallback
DTED1	3 arc-seconds	90 m	Supported enhanced level
DTED2	1 arc-second	30 m	Current public state overlay
DTED3	Higher density	Source-dependent	Supported by the release format, not derived from SRTM1

The linear distance represented by an angular interval changes with latitude. “30 m” and “90 m” are useful approximations, not fixed ground sample distances everywhere.

Post spacing is also not horizontal or vertical accuracy. A dense grid can contain systematic bias, void-fill artifacts, surface returns, or registration errors.

Current public source: SRTM

Public state overlays currently use the USGS SRTM 1 Arc-Second Global collection in its native DTED 1-arc-second product. The Shuttle Radar Topography Mission flew aboard Space Shuttle Endeavour from February 11–22, 2000 and used single-pass radar interferometry to produce a near-global elevation model.

USGS documents the source as:

• Geographic projection and 1° × 1° tiles.
• WGS84 horizontal datum.
• EGM96 vertical datum, with elevation values in meters.
• Nominal one-arc-second posting, approximately 30 m.
• Near-global land coverage between roughly 60° north and 56° south.
• A void-filled product in which some source tiles can still contain voids.

See the USGS product record and SRTM Collection User Guide for the authoritative product description.

Independently published source alternatives

DTED.org can also publish source-specific products. They use separate endpoints because the inputs do not represent the same physical surface:

• Copernicus GLO-30/GLO-90 are global DSM products referenced natively to EGM2008. Operational releases record the geoid transformation to EGM96.
• USGS 3DEP 1/3 arc-second is a U.S. bare-earth DTM referenced to NAVD88. DTED2 publication down-samples the denser source and records both horizontal and vertical transformations.
• NASADEM is an improved SRTM-derived, one-arc-second EGM96 surface model.
• ALOS AW3D30 is a photogrammetric DSM; masks identify cloud, snow, and complemented elevations.
• ASTER GDEM V3 is a stereo-derived DSM. Its companion NUM layer and known artifacts make it a secondary source rather than an unqualified replacement.

Each alternative is discoverable through the product API with its source ID, version, terrain model, native reference, output reference, processing profile, licence, and attribution. DTED2 describes post spacing; it does not make the underlying sources equally accurate.

For sources that require a datum transformation, the selected operation is not left to per-pixel runtime discovery. The converter asks the pinned PROJ database for the best locally available non-ballpark operation over the region, records its description and full pipeline, and supplies that same pipeline to each GDAL warp. This prevents a nominally successful warp from selecting an unavailable regional grid and emitting only nodata.

Reflective surface versus bare earth

SRTM measured radar reflections. Vegetation and structures can contribute to the returned height, so the result should not automatically be interpreted as a modern bare-earth terrain survey. Land-cover change since 2000 can also matter.

This distinction is important for clearance, obstacle, canopy, urban, and precision-engineering uses. DTED.org preserves the source format; it does not convert SRTM into a new bare-earth model.

Horizontal and vertical references

SRTM positions use WGS84 latitude and longitude. Heights are referenced to the EGM96 geoid, a modeled mean-sea-level surface, rather than directly to the WGS84 reference ellipsoid commonly used for raw GNSS ellipsoidal height.

Ellipsoidal and geoid-referenced heights are different quantities. A client or workflow combining GNSS, survey, lidar, or another DEM must know each source's vertical reference and apply an appropriate transformation when required. NGA's WGS84 and Earth Gravitational Model resources provide the underlying geodetic context.

DTED.org release lineage

For public state releases, DTED.org:

1. Acquires the selected USGS collection and native DTED product with a receipt.
2. Normalizes 1° cell names and retains source hashes and provenance.
3. Records any tightly bounded same-source boundary-post repair and one-meter duplicated-edge reconciliation performed during raster conversion.
4. Audits expected boundary coverage, DTED headers and matrices, land voids, shared seams, elevation limits, and representative control points.
5. Builds an immutable release. Canonical products may use the documented lower-resolution baseline; source alternatives explicitly do not.
6. Publishes coverage, quality, source, and client-configuration artifacts.

An active release can be validated, published for evaluation after a failed gate, or unverified when no quality result is attached. Those states are kept visible because publication status alone must not imply certification.

Appropriate use

Use the map and release metadata to answer four separate questions:

• Coverage: Is an enhanced cell present at the area of interest?
• Density: What DTED level is the highest available there?
• Evidence: What source, audit, and client result accompanies the release?
• Suitability: Is that evidence adequate for this specific decision?

For the complete system flow, see the OV-1 operational view.