To find the length of the diagonal of a square, we can use the formula:
\[ \text{Diagonal} = \text{side} \times \sqrt{2} \]
Given that the side length of the square park is 700 feet and using the approximation \(\sqrt{2} \approx 1.4\), we can calculate the diagonal as follows:
\[ \text{Diagonal} = 700 \times 1.4 \]
Calculating this gives:
\[ \text{Diagonal} = 980 \text{ feet} \]
Therefore, the approximate length from one corner to the opposite corner of the square park is 980 feet.