Measuring a forest plot on a hillside is tricky because the ground isn't flat. When viewed from above, you get the planimetric area—the horizontal projection that appears on a map. However, the actual surface area (the area of the sloping ground you'd walk on) is larger.

Imagine you have a 400 m² plot on a steep slope when looking from above. If you were to cover it with a quilt, you'd find that the quilt must be larger than 400 m² to fully cover the uneven ground. This extra area comes from the slope "stretching" the ground, much like a longer hypotenuse in a triangle.

Estimating the Slope

To adjust your area measurement, you first need to determine the slope angle using simple tools:

1. Mark Two Points: Place a stake uphill and one downhill.
2. Measure Horizontal Distance: Use a string with a level to create a horizontal line between the stakes. Suppose the distance is 10 m.
3. Measure Vertical Difference: Find the height difference between the two stakes. For example, a 1 m rise.
4. Calculate the Slope Angle:

Correcting the Area

To get the true surface area, adjust the planimetric area using the cosine of the slope angle:

For example, if your plot has a 30° slope:

• cos(30°) ≈ 0.866
• For a 10,000 m² planimetric area, the surface area would be approximately 11,547 m²

This means that although the map shows 1 hectare, the actual ground you cover is about 11,547 m².

Arboreal Forest measures the area perpendicular to gravity. So if you are using our application, you don't need to measure the slope—we take care of it for you.

Summary

• Planimetric area is the flat, horizontal area measured from above.
• Surface area is the actual area along the slope.
• Arboreal Forest measures the planimetric area, so you don't need to apply a correction to get the real number of trees per hectare, basal area, and volumes.

We have made two tools that let you try different slopes and different elevation profiles and see how it affects the area.