On the **Correct** Use of DEM VRTs in FORCE #371
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On the Correct Use of DEM VRTs in FORCE
Probably relatively few FORCE users despite using the GLO-30 DEM data delved into the Copernicus DEM Product Handbook. Consequently, as a community, we have been mostly unaware of the fact that the GLO30 dataset is distributed in six latitude bands (see Figure 5 in the handbook), each with a specific longitude reduction factor. In practice, this means that the x-resolution (pixel width) of the DEM data depends on latitude, with changes occurring above 50° N and S.
Figure: The x-resolution (in degrees) in relation to latitude. In black - longitude grid spacing in six latitude bands of GLO-30 data. In blue line represents the effective resolution (i.e. the average) of the European VRT mosaic. The red line represents the effective resolution of the global VRT. Image credit: @davidfrantz
❗This translates into several issues:
Figure: Pixel shift between two Level 2 outputs processed using DEM VRT of European and Global Extent
Figure: Hypothetical effective resolution (in meters) of European (blue) and Global (red) DEM VRT mosaics in relation to latitude. In black - resolution of GLO-30 data distributed in six latitude bands. Image credit: @davidfrantz
The solution
The best way to address these critical issues, is to process each Landsat and Sentinel-2 tile using only these DEM tiles that overlap with the footprint of the processed tile.
Starting from the development branch of FORCE v3.8.01 @davidfrantz has implemented a new parameter in the LEVEL2 parameter file called
USE_DEM_DATABASEthat allows for such tile-specific processing.USE_DEM_DATABASEis meant to be used in conjunction with theFILE_DEMparameter. WheneverUSE_DEM_DATABASE = TRUEtheFILE_DEMneeds to point not to a global, or regional VRT but to a folder with a collection of VRTs generated for each Landsat of Sentinel-2 tile.Pre-Built Global VRT Collections
To facilitate the use and adoption of this new way of using GLO-30 data, we provide a ready-to-use global collection of WRS-2- and MGRS-specific VRTs.
Under this LINK interested FORCE users will find
MGRT_VRT.tarandWRS_VRT.tararchives comprising, respectively, global collections of Sentinel-2 and Landsat specific VRTs. The list of GLO-30 tiles essential to process each WRS-2 and MGRT tile was identified based on openly available WRS-2 and MGRT footprints. Furthermore, we used 20 km buffer to account for potential deviation from the expected footprint's location.Use
VRTs are built with relative paths and must be placed in a directory located at the same level as the
copernicusdirectory containing the DEM tiles.Landsat and Sentinel-2 specific VRTs can be deposited in the same parent directory.
Special Case: Caucasus Region
Since the GLO-30 tiles from the Caucasus area are under embargo, under the aforementioned link, we provide SRTM tiles from this region (
SRTM_Caucasus.tar), and respective WRS-2 (WRS-2_VRT_Caucasus.tar) and MGRS (MGRS_VRT_Caucasus.tar) collections of VRTs.SRTM_Caucasus.tarneed to be placed in asrtm_asterdirectory located at the same level ascopernicusandDEM_VRTdirectories.WRS-2_VRT_Caucasus.tarandMGRS-2_VRT_Caucasus.tararchives.Documentation
Each
*.tararchive available under the abovementioned link comprises aREADME.MDfile with setup guidelines, as well as a*log*.csvfile listing all DEM tiles used to create a VRT for a specific WRS-2 or MGRS tile.Beta Was this translation helpful? Give feedback.
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