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I downloaded a .tif raster file from opentopography.org. It's an Alos (AW3D30 ) topography file showing mountainous terrain in Sweden. On the very right the file contains terrain, and on the left a lake surface. The lake surfaces have values of -9999 for each cell.
Is there some way to interpolate these -9999 values into values which would somehow correspond to the lake surface elevation? For example, the elevation of the nearby shore/coast is around 300 something meters. Is there a way to replace all the -9999 values with 300 values in order to get a somewhat acceptable transition from shore/coast to lake surface?
The mentioned file can be downloaded from here: https://www.dropbox.com/s/kai6202uewas47j/no_value.tif?dl=0