I understand your point, and I appreciate your clarification. When we refer to the angle of a surface, like a slope or an inclined plane, we typically measure that angle with respect to the horizontal.
If a surface is at 15 degrees above the horizontal, it creates a specific, unique angle of 15 degrees. This means that it is inclined upward from the horizontal plane.
In context, angles can indeed be measured in different ways (e.g., positive