I have a DEM map of 8-meter resolution, where some groups of pixels are blank due to the clouds (I guess). Is there a way in ArcGIS or GRASS to fill into those null areas with data from 30-meter DEM map?
Here is one solution using ArcGIS:
1) Resample your 30-meter DEM using the Resample Tool. In the Output Cell Size you can write the path to your 8-meter resolution DEM. For the Resampling Technique choose either Bilinear or Cubic, as they are appropiate for continuous data. Also, before running the tool, click on the Environments... button and click on Processing Extent:
Here, write again the path to your 8-meter DEM in both the Extent and the Snap Raster boxes. This will ensure your output raster has the same dimensions as and aligns with your original 8-meter DEM.
2) Now, assuming you have a Spatial Analyst license you can use the Raster Calculator to fill the NoData values from your original 8-meter DEM with the (resampled) values from the 30-meter DEM. If your original 8-m DEM is called
input_dem and the resampled DEM (the output from the first step) is called
resampled_dem you can use the following expression in the calculator:
Con(IsNull("input_dem"), "resampled_dem", "input_dem")