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(Resolved) Why high-resolution terrain data does not match actual altitude？

K

Kang Huang

New member
Hi,
I have now made a three-layer nested domains simulation with a ratio of 1:3:3, the grid accuracy of the finest nested area is 1km, and then the longitude and latitude of my target site is very close to one of the grid points in the 3rd nested domain. The distance is only about ten meters. But the output altitude (HGT) of this grid point in the output file after geogrid.exe is more than 100 meters different from the actual one (actual about 900m, simulated about 760m).
Regardless of whether I use the default 30s terrain data that comes with WRF, or the high-resolution terrain data of SRTM 3s and SRTM 1s, or change the interp_option option in GEOGRID.TBL.ARW (set it four_pt or nearest), this problem will still occur.
I want to know what causes this and is there any solution? Because altitude is also a vital parameter when performing WRF simulations.
Ps:
  1. I used gdal & convert_geotiff to implement merge and binary conversion of srtm.tif files.
    Code: 
    gdal_merge.py -o MyTarget.tif -a_nodata -32768 --optfile tif_list.txt
    Code: 
    convert_geotiff -w 4 -t 1500 -u "meters MSL" -d "3s topography" -b 0 -m -32768 MyTarget.tif
    Because the ocean part is not included, no additional processing is done. At the same time, the coordinates, resolution and other information of the index file are also set according to .hdr&.tfw. You can see it in the attachment.
  2. I check the origin srtm.tif file using arcGIS, by positioning the accurate longitude and latitude coordinates, we can see that the altitude is still close to the actual value. After domain nesting setting in namelist.wps and geogrid.exe, we got a grid point in D03, located in (40°23′45.38″N，117°34′28.28″E)。
  3. I read the Chapter 3 in WRF User Guide: Ideally, the map projection and its accompanying parameters should be chosen to minimize the maximum distortion within the area covered by the model grids, since a high amount of distortion, evidenced by map scale factors significantly different from unity, can restrict the model time step more than necessary. Does this mean that because of the limitations of projection, I cannot locate a certain point on the static terrain very accurately?
I hope I made my question clear and expect for reply.
Best regards！
 

Attachments

  • 20240305152922.png
    20240305152922.png
    1.2 MB · Views: 7
  • namelist.wps
    1.6 KB · Views: 4
Here are some attachments that may help you understand the problem，thanks for your time！🧐
 

Attachments

  • index&.tfw&GEOGRID.TBL.zip
    647.2 KB · Views: 3
Last edited:
haiqingsong said:
Do your original DEM values at that point match up with reality?
Click to expand...
Yes，the actual altitude value matches the DEM values as showed in the arcGIS because this place is a site we know well. But when I apply SRTM to WRF static terrain data, the same latitude and longitude coordinates do not match the actual ones.
 
Kang Huang said:
Yes，the actual altitude value matches the DEM values as showed in the arcGIS because this place is a site we know well. But when I apply SRTM to WRF static terrain data, the same latitude and longitude coordinates do not match the actual ones.
Click to expand...
Please modify the interpolation algorithm in the geogrid section of your WPS.
 
haiqingsong said:
Please modify the interpolation algorithm in the geogrid section of your WPS.
Click to expand...
Hello，sorry for replying so late. As I mentioned before, I have tried to modify the interpolation algorithm in GEOGRID.TBL.ARW (set it four_pt or nearest). However, the variation range of the results is between a few meters, which is still far from the actual altitude.
 
Kang Huang said:
Hello，sorry for replying so late. As I mentioned before, I have tried to modify the interpolation algorithm in GEOGRID.TBL.ARW (set it four_pt or nearest). However, the variation range of the results is between a few meters, which is still far from the actual altitude.
Click to expand...
Make a 300-meter resolution nested area and compare it with your measurements.
 
Thanks for your suggestion！Sorry, I was so busy with chores that I get the chance to reply to you now.
I have tried to make a nested area d04 in 333-m resolution, and I found that the altitude nearest grid point (which is nearly the same position we want) is 852m, that's close to the actual altitude.This shows that your suggestion is effective and the elevation of the static terrain data is reliable.
In addition, I also want to ask two questions:
  1. Three and four nested areas were set respectively, and approximate grid points with almost the same target point were obtained. Do they represent the same point?
  2. What is the main cause of this error? Is it the difference in nesting methods or the influence of interpolation methods?
 
Kang Huang said:
Thanks for your suggestion！Sorry, I was so busy with chores that I get the chance to reply to you now.
I have tried to make a nested area d04 in 333-m resolution, and I found that the altitude nearest grid point (which is nearly the same position we want) is 852m, that's close to the actual altitude.This shows that your suggestion is effective and the elevation of the static terrain data is reliable.
In addition, I also want to ask two questions:
  1. Three and four nested areas were set respectively, and approximate grid points with almost the same target point were obtained. Do they represent the same point?
  2. What is the main cause of this error? Is it the difference in nesting methods or the influence of interpolation methods?
Click to expand...
My guess is that the coarse resolution causes the DEM of your site's location to be interpolated and smoothed.
 
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