So i am a bit confused by the column space of a matrix. I seem to read various answers in different places as to what actually spans the column space of a matrix.
1) Lets say i have a matrix, most places seems to suggest that one should find the reduced row echelon form of the matrix and pick the columns with leading 1's from the original matrix.
2) Other places i've seen seem to suggest that the columns with leading 1's from the RREF matrix also makes up the column space.
Are both of these ways correct? and if so why is 2) correct?