1. **State the problem:** We need to find the area of a triangle given the base and height measurements. 2. **Formula:** The area $A$ of a triangle is given by the formula: $$A = \frac{1}{2} \times \text{base} \times \text{height}$$ 3. **Identify base and height:** From the problem, the base is $0.7$ miles and the height (altitude) is $0.6$ miles. 4. **Calculate the area:** $$A = \frac{1}{2} \times 0.7 \times 0.6$$ 5. **Simplify the multiplication:** $$A = \frac{1}{2} \times 0.42$$ 6. **Calculate the final area:** $$A = 0.21$$ 7. **Interpretation:** The area of the triangle is $0.21$ square miles.