Near-global coverage of 1 arc second SRTM is becoming available

The near-global SRTM digital elevation model has been an extremely valuable data set for many years now. That data set has a resolution of 3 arc seconds, equalling roughly 90 m. A higher resolution (1 arc second or approximately 30 m) had been availbale for the USA, and higher resolution data had been available for small areas. For many years, researchers around the world have been hoping and waiting for a higher resolution near global or DEM to become available. The ASTER global DEM, with a nominal resolution of 30 m, is great because it does offer global coverage (in contrast to the SRTM DEM which does not cover regions north of 60° North and south of 58° South). However, the ASTER DEM suffers from data artefacts, and quality assessment showed that (despite a pixel size of 30 m) the effective spatial resultion is only approximately 120 m. In late 2012 I was told at a conference that a 1 arc second SRTM DEM was forthcoming within a year or two.

Now, NASA has begun to release the near-global (i.e. limited to between 60° North and 58° South) 1 arc second (approximately 30 m) SRTM data set. At least Africa, Europe and South America are now available through the USGS’s Earth Explorer. Already at first glance, this new data set is in moast areas far superior to both the 3 arc second SRTM DEM and the ASTER global DEM. Of course, as in the 3 arc second SRTM DEM, desert sand dunes (at least in hyperarid deserts) are recorded poorly because such surfaces simply did not backscatter enough of the radar signal. No-data areas in the 1 arc second STRM DEM are even slightly larger than in the 3 arc second SRTM DEM. The ASTER DEM still is the superior data set for such surfaces.

3 arc seond SRTM DEM. Note extensive data gaps in the dune field.

3 arc seond SRTM DEM (shaded relief image) of the Dunas Pamps Blanca – Palpa region. Note extensive data gaps in the dune field.

ASTER global DEM (ASTER GDEM is a product of METI and NASA). Note good representation of dune field and intense elevation noise on flat terrain in the northwestern portion of the image.

ASTER global DEM (ASTER GDEM is a product of METI and NASA). Note good representation of the dune field and intense elevation noise on flat terrain in the northwestern portion of the image.

1 arc second SRTM DEM. Note extensive data gaps in the large dune field and good representation of flat terrain in northwestern portion of the image.

1 arc second SRTM DEM. Note extensive data gaps in the dune field and good representation of flat terrain in northwestern portion of the image.

To assess elevation accuracy, I compared the 1 arc second SRTM DEM with the state-wide high-resolution (lidar-based) DEM of Germany’s federal state Baden-Württemberg (35,752 sq. km or 56.4 million SRTM 1 arcsec data pixels). That data set has a reolution of 1 m, so it had to be resampled to match the 1 arc seond resolution of the new SRTM data. I used both the DTM (digital terrain model, “bare earth model”) and DSM (digital surface model, including vegetation) and also looked at land use types (forest, agricultural land and grassland).

Shaded relief images of a small subset of Baden-Württemberg comparing lidar-based DTM and DSM with SRTM 1 arc second DEM.

Shaded relief images of a small subset of Baden-Württemberg comparing lidar-based DTM and DSM (both resampled to 1 arc second) with SRTM 1 arc second DEM. Lidar data (c) Landesamt für Geoinformation und Landentwicklung Baden-Württemberg.

Elevation difference between SRTM 1 arc second DEM and lidar-based DTM. The SRTM DEM is approximately 1 m too low in this region. Comparison between different landuse types clearly shows that the radar-based SRTM DEM records vegetation canopy.

Elevation difference between SRTM 1 arc second DEM and lidar-based DTM.

On average, the SRTM DEM is approximately 1 m too low in Baden-Württemberg. Comparison between different land use types clearly illustrates that the radar-based SRTM DEM records vegetation canopy rather than ground surface. This is also (but less clearly) visible in the shaded relief images. These images also show that the lidar-based DSM has an overall clearer appearance and shows more detail. Effective spatial resolution of the 1 arc second SRTM DEM does therefore not equal 1 arc second.

Elevation difference between SRTM 1 arc second DEM and lidar-based DSM for open land grid cells (open land grid cells here defined

Elevation difference between SRTM 1 arc second DEM and lidar-based DSM for open land grid cells (open land grid cells here defined as those for which the difference between lidar DSM and DTM is less than 0.5 m).

Absolute difference between lidar-based DEM and SRTM 1 arc second.

Absolute difference between lidar-based DEM and SRTM 1 arc second.

Because the SRTM DEM corresponds roughly to vegetation canopy, its apparent accuracy in forested areas is relatively low when compared with a lidar-based DTM (e.g. 10% of all SRTM grid cell elevations differ more than 15 m from the lidar DTM). However, when looking at open land grid cells only (i.e. those grid cells where the difference between lidar DSM and DTM is very low), almost two thirds of all SRTM grid cell elevations are within 2 m and more than 90% are within 7 m of the lidar DSM elevations.

All in all, the new SRTM DEM is a big step forward in the field of near-global elevation data sets.

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