Global Mapper v25.0

Combine ASTER GDEM and SRTM

naugustin
naugustin Global Mapper UserTrusted User
edited September 2014 in Elevation Data
Dear all,

I tried to understand the Cobine/Compare Terrain Layers options a bit more and wondered if I could create a combined grid out of good 90m SRTM Grid and 30m ASTER grids which are quite noisy and full with artifacts. I downloaded the needed data (USGS Earth Explorer) so I have the grids physically on my computer and prepared the area of interest.

However, I thought I could create some surfaces to roughly characterize the noise of the ASTER data (compared to the SRTM) and just subtract that from the ASTER Grid. But that was not very successful yet - I guess the main problem is that the filters do different than I think they would do things (so, I´m sure it a classic user error ;-)).

I tried to combine the ASTER grid with Maximum/Minimum Elevation grids that I created by comparing the ASTER with the SRTM data but all ended in quite weird results. The "best" result I got with a simple "Filter (Keep first if Second Valid)" by defining SRTM as first and ASTER as second grid. But I since both layers have no gaps I don´t really know what GM did here and think that there is still some room for improvement. I attached a picture for illustration. The result (however) appears to be a bit more crisp than the SRTM data and also some small craters in the coned are back but it still loos a lot of detail from the ASTER (even though most is noise anyway).

Do you have any ideas what I could/should try?

Best Regards,
Nico

Unbenannt-1.jpg

Comments

  • tjhb
    tjhb Global Mapper User Trusted User
    edited August 2014
    since both layers have no gaps I don´t really know what GM did here and think that there is still some room for improvement.

    I think what GM has probably done in these circumstances is simply to resample the SRTM surface to the same resolution of the ASTER surface (using--from the look of it--bicubic interpolation). Values from ASTER would not have been used at all.
    Do you have any ideas what I could/should try?
    One simple thing you could try is to set the transparency of the upper surface (last listed) to 50%. For a more complicated version, try exporting a version of the ASTER surface using a median filter, at reduced resolution, then opening the result, adding it to the stack with (say) 33% transparency, and exporting again at original ASTER resolution with no resampling.
  • naugustin
    naugustin Global Mapper User Trusted User
    edited September 2014
    Thanks for the suggestion, but thats not what I was looking for. I hoped that I could smooth the Aster grid file by comparing Aster with SRTM data. However, it was just an idea but I miss the time to try on that further - Thanks anyway!