TransDEM Forum

TransDEM News, Support, Hints and Resources
It is currently 28 Mar 2024 15:35

All times are UTC + 1 hour




Post new topic Reply to topic  [ 3 posts ] 
Author Message
 Post subject: SRTM News
PostPosted: 11 Oct 2014 14:13 
Offline

Joined: 05 Jan 2011 16:45
Posts: 1463
For SRTM 3 arc sec we have a new version around, which is called 3.0 or SRTM Plus or Void Filled, and has - yes, indeed - the voids filled by complementing the original data with other sources such as ASTER GDEM.
Documentation: NASA_SRTM_V3.pdf

Download from (new addresses, updated 2018-04-09):
http://e4ftl01.cr.usgs.gov/MEASURES/SRTMGL3.003/ Default 3-arc-second data for the world obtained by averaging the 1-arc-second raw data.
http://e4ftl01.cr.usgs.gov/MEASURES/SRTMGL3S.003/ Sampled 3-arc-second data for the whole world obtained by getting the middle 1-arc-second raw data sample out of a 3×3 matrix.


For SRTM 1 arc sec the USGS is in the process of releasing this higher resolution world wide. Once available the data can be obtained via Earth Explorer.


Top
 Profile  
 
 Post subject: Re: SRTM News
PostPosted: 30 Nov 2014 15:15 
Offline

Joined: 05 Jan 2011 16:45
Posts: 1463
Reading hint: 1 second stands for 1 arc second and equals about 30 m on the north/south axis and 20 m on the east/west axis for moderate latitudes. 3 arc sec are around 90 x 60 m.

Meanwhile the 1 arc sec have been made available. They too can be downloaded directly, without taking the detour via Earth Explorer.

Download addresses (new addresses, updated 2018-04-09):
http://e4ftl01.cr.usgs.gov/MEASURES/SRTMGL3.003/ Default 3 arc second data for the world obtained by averaging the 1-arc-second raw data.
http://e4ftl01.cr.usgs.gov/MEASURES/SRTMGL3S.003/ Sampled 3 arc second data for the whole world obtained by getting the middle 1-arc-second raw data sample out of a 3×3 matrix.
http://e4ftl01.cr.usgs.gov/MEASURES/SRTMGL1.003 1 arc second data for the world.

Both 3 arc sec and 1 arc sec data on this server are in the well known .hgt format. Earth Explorer offers the same data but in different file formats. For TransDEM, select GeoTIFF. However, SRTM data on Earth Explorer has a reduced resolution in latitudinal direction, i.e. 2 x 1 arc sec for 1 arc data and 6 x 3 arc sec for 3 arc sec data. I suppose that doesn't really mean less detail because the original source probably isn't any better anyway.

I have made a few comparisons and picked the upper Ruhr valley in Westphalia, Germany (SRTM file N51 E007) for them.

Starting with the original SRTM 3 sec data of the well known version 2:
Image
My edition is probably version 2.0 which still had holes for water surfaces.

Now new version 3 void filled data, 3 arc sec:
Image
No more holes for water. Notable differences will probably require proper mountain areas where version 2.1 often has voids.

Now the new SRTM 1 arc sec data:
Image
Even at this rather low scale the surface looks less smooth which means more details.

Finally ASTER GDEM2, 1 arc second as well:
Image
ASTER data always appears very uneven, implicated by the process involved.

Comparing at a larger scale is the more interesting story. For this I chose the valley of the river Lenne between Altena and Werdohl, southeast of the previous area.
Image

Original SRTM 3 arc sec data (Version 2):
Image

The new void-filled SRTM 3 arc sec data (Version 3):
Image
There are no obvious differences and none were expected.

Compared to that the new SRTM 1 arc sec data
Image
Contours appear somewhat crisper. However, one can also recognize a the sampling raster as a slightly disturbing pattern. It's a question, though, whether that will matter in practice.

Once again ASTER GDEM2 1 sec data. Nominally same resolution as SRTM 1 sec:
Image
The familiar crater-laden terrain surface as expected from ASTER. This seldom reflects reality, and is often described as artefacts.

ASTER data, now smoothed, applying the built-in TransDEM filter, using default filter values:
Image
The smoothed ASTER data still lack behind the more even surface of the new SRTM 1 sec data.

Conclusion: The new SRTM 1 sec data have a noticeable higher resolution than the existing SRTM 3 sec data. Compared to 1 sec ASTER GDEM, SRTM 1 arc sec still looks better, since it does not have the ASTER artefacts. Nevertheless, SRTM 1 sec will show the same errors as 3 sec. Forests, built-up areas and large human structures will not have been filtered out and will distort the data. However, when satellite DEMs are the only reasonable source for a route project, SRTM 1 arc sec is a valuable new option. (For the UK, free O/S Terrain 50 should still be preferred, While nominally at a lower resolution, it has the advantage of being terrestrial, "hand-crafted", not orbital.)


Top
 Profile  
 
 Post subject: NASADEM
PostPosted: 09 Apr 2018 15:58 
Offline

Joined: 05 Jan 2011 16:45
Posts: 1463
NASADEM: That's the name of the next version of SRTM. NASA says, it's a combination of SRTM, ASTER and other sources plus a lot of post-processing.
Resolution is 1 arc sec, as with SRTM V3 of 2014.

Data can be downloaded here (preliminary Version):
https://e4ftl01.cr.usgs.gov/provisional/MEaSUREs/NASADEM/
(Registration required, same as for SRTM V3)
Sub-drectory: hgt_merge
Void-filled DEM merge (*.hgt) and corresponding NUM files (*.num)

For areas without or without free terrestrial elevation data, let alone LIDAR, SRTM, now NASADEM, remains an important source. I haven't really looked into the data yet, so I cannot tell whether foliage or built-up areas have been addressed.


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 3 posts ] 

All times are UTC + 1 hour


Who is online

Users browsing this forum: No registered users and 5 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum

Search for:
Jump to:  
cron

Imprint & Privacy

Powered by phpBB® Forum Software © phpBB Group