For the second list/array (which you will use for finding the circumference), one of the major problems is that you could "bump" into the side of the picture when scanning for your next move. This could present a signficant challenge in figuring out the logic for the exceptions! However, here is a nifty trick: Put a "Frame" around the entire array of one extra position, all filled with blanks. Therefore, as you scan you CANNOT compute an illegal position! Thus, in your original array, you might use rows 1 through 24, and simply not use row 0. And, you might use columns 1 - 79 and not use column 0.
So, the original TITAN could be simply 25 x 80 and your new list / array would be 26 x 81 -- where you would fill row 0, column 0, row 25 and column 80 with blanks.