To find the length of the diagonal of a square park with a side length of 700 feet, you can use the formula for the diagonal \( d \):
\[ d = \text{side} \times \sqrt{2} \]
Given the side length is 700 feet and using 1.4 as an approximation for \( \sqrt{2} \), you can calculate the diagonal as follows:
\[ d = 700 \times 1.4 = 980 \text{ feet} \]
Therefore, the approximate length from one corner to the opposite corner of the square park is 980 feet.