Hello. I have datasets exported from map data as large grids
(1024x768). I want to transform them into 3 long (768000 rows)
columns, with X, Y, and the value from the grid. Example:

B A A
C D A
C C D

to be put into:

X Y value
1 1 B
2 1 A
3 1 A
1 2 C
2 2 D
3 2 A
1 3 C
2 3 C
3 3 D

Can anyone help?