To find the length of the diagonal of a square, you can use the formula:
\[ d = s \sqrt{2} \]
where \(d\) is the diagonal and \(s\) is the side length of the square.
In this case, the side length of the square park is 700 feet, and you are using \( \sqrt{2} \approx 1.4 \).
Now, substitute \(s = 700\) feet into the formula:
\[ d = 700 \times 1.4 \]
Now, calculate it:
\[ d = 980 \text{ feet} \]
Thus, the approximate length from one corner to the opposite corner of the square park is 980 feet.