When you’re creating landscape areas, Vectorworks tends to draw these as if they were projected down onto a flat plane. In reality, the site may be sloping, so the area of the site is actually greater than the plan area shown on the drawings. The difference between the plan area and the actual area can be calculated using basic trigonometry.
You need to know the plan area and you need to know the average slope, in degrees. The basic trigonometry for this is given by this formula. I’ve had a sketch with this formula on the back of my calculator since I was in high school. It really helps me to remember my basic trigonometry.