The length of the diagonal of a square can be calculated using 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 \( s \) is 700 feet. Plugging that into the formula gives:
\[ d = 700 \sqrt{2} \]
Using the approximation \( \sqrt{2} \approx 1.4 \):
\[ d \approx 700 \times 1.4 \]
Now, calculating this:
\[ d \approx 700 \times 1.4 = 980 \text{ feet} \]
Thus, the approximate length from one corner to the opposite corner of the square park is 980 feet.