Terrain Data – which to use?

      Tuesday 20 March 2018 |  by


I often get asked what’s the best FREE! Digital Elevation Model (DEM) to use and how to access it. Here’s a run-down with an explanation and Google EarthEngine download scripts

Medium resolution (30m):

Easiest/global coverage:


Download here  (USGS registration needed)

OR Google EarthEngine script here (registration required)

Harder, more accurate/not quite global coverage (Expanded on below):

ALOS World 3D – 30m (AW3D30)

Browse here / Download here (Quick ALOS registration needed. Google Chrome issues but works with Firefox)

OR Google EarthEngine script here (registration required)

Harder to download highest resolution near-global (12.5m):

12.5m Alaska Satellite Facility DEM

Near-global 12.5m DEM tiles can be downloaded from the Alaska Satellite Facility. The model is based on the 30m SRTM or NED elevation models, downscaled using ALOS PALSAR’s 10m surface elevation model data. You can read more about the processing here. If downloading multiple tiles, we recommend bulk ordering and downloading with the supplied python script (data not available on Google Earth Engine and Firefox’s DownThemAll plugin is obsolete at time of writing).

Download here (Earthdata registration needed). Video tutorial here

Fine resolution (1m+) models *see below for DEM/DSM

UK (near-complete):

LiDAR DSM Composite

Browse / Download here

USA (1m & 3.3m sparse, 10m near-complete):


Browse / Download here

DEM, DSM, DTM difference?

Without getting into pedantics:

  • Digital Terrain Models (DTM) are the height of the earth’s surface in relation to other points
  • Digital Elevation Models (DEM) are the height of the earth’s surface specifically in relation to a standard global/regional elevation (normally sea level). They are a subset of DTMs
  • Digital Surface Models (DSMs) are the height of the surface including objects (buildings, vegetation, etc). See below for clarification:

Medium resolution datasets explanation:

Section summary: AW3D30 is currently the most accurate DEM, though a new NASA DEM product is expected imminently (time of writing: Mar 2018).

There are currently 3 main open-source DEMs in use globally: AW3D30, NASA DEM (the improved iteration of SRTM) and ASTER Global DEM Version 2 (GDEM2). These all have different versions, and it is important to briefly summarise their histories to explain their differences and future implications. All 3 datasets were initially charged for, but over time, their transition to being freely available has influenced their roles.

SRTM was collected over 11 days in 2000, though lacked global coverage (~80% coverage) and had “voids” (black spots where no direct line-of-sight existed). Version 2 (AKA “finished” product ~2006) “filled” these voids by interpolation and using other elevation data. The dataset has subsequently been reprocessed with improved models, the latest of which is now referred to as ‘NASA DEM’

Though ASTER was producing scene-scale DEMs from 2003, the global GDEM V1 was only released in 2009 with a 99% land coverage (though ASTER data then had a charge associated). This had the same resolution as SRTM data (30m over US, 90m for security reasons over rest-of-world). GDEM V2 which incorporated more ASTER data and dealt with a number of issues was later released 2011 (at 30m resolution?). In 2012, SRTM data was made freely available.

ASTER data however was only later freely distributed 2016. As such, SRTM was updated to V3/SRTM ‘Plus’ in 2013, fundamentally improving via enhanced void-filling (using GDEMs amongst other datasets). However, in 2014, the US announced they would roll-out 30m (1 arc second) DEM data, with the increased 30m resolution being adopted in SRTM V3 from 2014-2015. The data was further improved with more accurate “unwrapping” of the original interferometry data, the improved product now being referred to as NASA DEM.

The ALOS World 3D (AW3D) product development began in 2014, with their 30m AW3D30 product released freely in 2016 (a similar time to ASTER data).

Being based upon their native 5 x 5m satellite product (the fee-based AW3D5) vs 30 x 30m of the alternatives, AW3D30 unsurprisingly appears to have the highest elevation accuracy. Santillana & Makinano-Santilla‘s direct comparison yielded the following vertical Root Mean Square Errors:

  • AW3D30  5.68m
  • SRTM V3  8.28 m (Note: NASA DEM will likely be better than this)
  • GDEM2  11.98m

As mentioned above, AW3D30 does not provide a global coverage. In addition, NASA are currently very near completion of a new global (free) DEM making use of NASA DEM, GDEM2 and multiple other DEM products.

Upon investigation, Kemen provides a good summary of the initial data, pointing to a number of issues which might account for the delay. At the time of writing (Feb 2019), no announcement has been made as to when the final product will be made available.

Finally, it should be noted that other fee-based high-resolution DEMs/DTMs exist down to 0.5m pixel resolution. For reference, some indicative prices can be found here for e-geos and here for WorldDEM

This article will be updated in the near future

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