NASADEM only 8781 meters for Mt. Everest?

[lihaijun]

Dear All,

I downloaded the tile of "NASADEM_HGT_n27e086" from Earthdata Search. However, the maximum elevation in this tile is only 8781 meters, which is not equals to the elevation of Mt. Everest (8848 meters). What causes the large difference here? If this is caused by the vertical uncertainty, how could we trust the vertical accuracy in other regions?

Thanks,
Haijun

[LP DAAC - jwilson]

We are forwarding your inquiry to our Subject Matter Expert for further assistance.

[LP DAAC - jwilson]

The user is right about what he observed w/ the NASADEM tile.

The NASADEM User Guide doesn't have any information on its vertical accuracy. It's based on SRTM (along with few other DEMs), which does full fill mission performance requirements of 16 m absolute vertical error. I don't know what's causing the large difference. We have reached out to our science experts for any guidance.

[lihaijun]

Are there any updates about this issue from the experts?

Thanks,
Haijun

[LP DAAC - jwilson]

Our Subject Matter Expert is still waiting a response from the PI.
We apologize for the delay.

[LP DAAC - jwilson]

Dear Haijun,
As mentioned in the previous reply it's based on SRTM (along with few other DEMs), which does full fill mission performance requirements of 16 m absolute vertical error. At this time the Private Investigator (PI) has not replied to our request for further guidance. We apologize for the inconvenience.

[LP DAAC - jwilson]

@lihaijun,
- Likely/possible contributing factors to apparent discrepancy:
o 30m pixel size with area-based sampling (Possibly Greatest Factor)
 SRTM and ASTER are both use area-based sampling grids where the elevation value of each pixel is an average of elevations over the area within that pixel or the elevation at the center of a pixel (Guth et. al., 2021). Meaning, the pixel that contains the Mt Everest summit also contains all other area within that which would then be an average of the elevations in that 30 m pixel area resulting in an elevation lower than 8848 m.

o For NASADEM, Mt. Everest is gap filled data from ASTER GDEM3
 Using the num file accompanying the hgt file for the NASADEM granule, it was determined that the area containing the Mt Everest summit have pixel values that indicate gapfill using GDEM3 data. More details about NASADEM NUM file index values can be found in Table 2 in the NASADEM v1.2 User Guide.
 ASTER GDEM v3 20m vertical accuracy (Fujisada et. al., 2005)

o Highest error over steep terrain/high relief for both SRTM and ASTER data
 Regarding the absolute error of 16m for the NASADEM dataset, when looking at how the NASADEM/SRTM and ASTER perform, high-relief regions such as the Himalayas tend to have the greater error (Farr et. al., 2007, Fujidasa et. al., 2005).

Farr, T. G., et al. (2007), The Shuttle Radar Topography Mission, Rev. Geophys., 45, RG2004, doi:10.1029/2005RG000183.

Fujisada, H., Bailey, G. B., Kelly, G. G., Hara, S., & Abrams, M. J. (2005). Aster dem performance. IEEE transactions on Geoscience and Remote Sensing, 43(12), 2707-2714.

Guth, P. L., Van Niekerk, A., Grohmann, C. H., Muller, J. P., Hawker, L., Florinsky, I. V., ... & Strobl, P. (2021). Digital elevation models: terminology and definitions. Remote Sensing, 13(18), 3581.