[GIS] Merging DEMs of different grid size

demmergeqgissaga

I'm trying to merge two grids with different cell size. One has a cell size of 10m and the other 50m. The 10m one covers a subarea of the 50m one. I know that the most accurate one is the 10m one and hence I would like to keep its values in the overlapping areas, and not do any kind of interpolation. My problem is that the resulting merged grid (after resampling the 50m grid to 10m) has obvious step changes as seen in the picture

Merged grid

Is there a way I can feather out these discrepancies without editing the grids cell by cell? I'm using Qgis and Saga.

Thanks

Best Answer

Just in case anybody else would be interested on this the best way I could find was using saga gis software module Mosaicking. The module has two options 'blend boundary' and 'feathering'

enter image description here

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Typing saga_cmd io_grid 3 and cutting the header:

Usage: saga_cmd [-GRID <str>] [-FILE <str>] [-NODATA <str>] [-NODATA_VAL <str>]
  -GRID:<str>           Grid
        Data Object (optional output)
  -FILE:<str>           File
        File path
  -NODATA:<str>         No Data Value
        Choice
        Available Choices:
        [0] Surfer's No Data Value
        [1] User Defined
        Default: 0
  -NODATA_VAL:<str>     User Defined No Data Value
        Floating point
        Default: -99999.000000

you will notice the -GRID optional parameter. It allows to specify the name of the SAGA grid to store on your disk and not in memory. The concept is quite the same for saga_cmd io_gdal 0.

So you need to adapt your commands, storing on your disk every output grid in SDAT (SAGA GIS Binary Grid File Format) format using the .sgrd extension.

saga_cmd io_grid 3 -GRID=G:\path_to_file\foo.sgrd -FILE=G:\path_to_file\foo.grd -NODATA=0 -NODATA_VAL=-99999.000000
saga_cmd io_gdal 0 -GRIDS=G:\path_to_file\bar.sgrd -FILES=G:\path_to_file\bar.adf
saga_cmd grid_tools 3 -GRIDS=G:\path_to_file\foo.sgrd;G:\path_to_file\bar.sgrd -TYPE=7 -INTERPOL=0 -OVERLAP=1 -BLEND_DIST=10.000000 -TARGET=1 -USER_XMIN=0.000000 -USER_XMAX=0.000000 -USER_YMIN=0.000000 -USER_YMAX=0.000000 -USER_SIZE=1.000000 -USER_COLS=0 -USER_ROWS=0 -USER_GRID=mosaic -GRID_GRID=
saga_cmd io_grid 5 -GRID=G:\path_to_file\mosaic.sgrd -FILE=G:\path_to_file\mosaic.xyz

Finally, a useful remark from How to create a python script for saga:

Instead of running through the GUI or the command line you can also run modules from python. Compared to the saga cmd interface this has the advantage that you don't have to save all intermediate results, apart from the fact that you can use all the useful features of python to eg iterate over a number of files.

QGIS – How to Merge DEM Rasters in QGIS and ArcGIS

This was a tough problem to track down because I had thought that the effect was at the edges of tiles when it fact they are throughout the data. You're right that the phenomena isn't in the data before mosaicking the data. The problem results from the resampling process that is inherent in mosaicking. You need to use either the cubic convolution or bilinear resampling methods when you mosaic your DEM tiles instead of the default nearest-neighbour resampling method. The phenomenon is the result of duplicated rows and columns of values during the nearest-neighbour resampling. The following is an example of a plan curvature image derived from the mosaicked tiles you provided using nearest-neighbour:

enter image description here

It displays the same phenomenon that you describe in the question. Nearest-neighbour resampling is most suited to applications with categorical data while continuous data, such as elevation, is better resampled using bilinear or CC resampling methods. Here is the same data after mosaicking the data using cubic convolution:

enter image description here

Beware, however, that bilinear resampling and cubic convolution will result in a slight smoothing of the topography (bilinear slightly less than CC).

Best Answer

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