Update README.md

[bradh]

Note that SRTM is first-return data, but DTED spec actually requires surface.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[mkrupczak3]

True, according to this article from the NGA the DTED spec originally was for Digital Surface Models: https://www.nga.mil/news/With_50-year_legacy_Digital_Terrain_Elevation_Data.html

It seems to imply that in recent years DTED derived from SRTM has been used, even though this means this data will be first return 'treetop' height.

This is corroborated by article 3.10.9.5 in the MIL-PRF-89020B spec for DTED:

[mkrupczak3]

I imagine that nowadays US Gov may have better digital surface model DTED available derived from the TanDEM-X mission, as mentioned in that NGA article.

Qualitatively, most of the civilian DTED we've tested so far seems to produce inferior results compared to SRTM GeoTiffs however. Same for the 'new and improved' NASADEM, which in testing seemed to produce inferior results to SRTM data

[mkrupczak3]

@bradh thanks for taking the time to open a pull request, we appreciate having another set of eyes on the code and documentation.

The Contributor License Agreement is not a hard requirement for documentation fix PR's, though it is recommended.

Lmk when you're ready for this to be merged 👍

[mkrupczak3]

Fixed in a1472eb. This PR may be closed.