Need solution for this error

Error using fscanf

Invalid file identifier. Use fopen to generate a valid file identifier.

Error in egm3dinvert (line 42)

bou=fscanf(fid,'%f',[numcolumns numrows]); %open the input gravity file, read it and stores it in variable 'bou'

Answer 1

[Torsten]

First get a fid to open your file:

fid = fopen( filename )