To determine the diagonal length of a square park with a side length of 700 feet, we can use the property that the diagonal \( d \) of a square is given by the formula:
\[ d = s \sqrt{2} \]
where \( s \) is the side length of the square. Given that \( s = 700 \) feet, we can substitute this value into the formula.
Given \( \sqrt{2} \approx 1.4 \), we can calculate:
\[ d \approx 700 \times 1.4 \]
Calculating the product:
\[ d \approx 700 \times 1.4 = 980 \text{ feet} \]
Therefore, the approximate length from one corner to the opposite corner of the square park is \( \boxed{980} \) feet.