Understanding Overlay Options
The Overlay options define how the surface within a structure boundary is created.
When no Overlay options are set, a structure will be calculated from inputted altitudes, so contours and spot elevations are ignored.
When an Overlay option is selected, the surface of a given structure is produced from contours or spot elevations, and inputted altitudes for that structure is ignored.
Surface Option Checked
With only the surface option ticked the boundary points as input are used in conjunction with any contours or spots which cross or are within the structure boundary to define the structure surface. The user can use precise control for the boundary points specifying their altitude and use contours and spots to create a complex surface within the boundary. A good example of this use is; you have a car park or road with well-defined edges with known points defining the perimeter in addition to contours or spots defining an internal shape for drainage.
Points Option Checked
With only the boundary points option ticked the input boundary point altitudes are ignored and the altitude of the points projected onto the surface defined by the spots and contours is used instead. A linear surface is created within the boundary. This option is the least commonly used but has some uses when the boundary point altitudes are not know and it is felt the overlaying to obtain the altitudes using the contour and spot information would be accurate enough without seeking further detail.
Surface and Points Options Checked
With both surface and boundary points ticked the structure surface is created using the spots and contours. This is a bit like laying a cloth over the surface so it matches exactly. The most common use of this option is where there is landscaping and there are different areas of materials e.g. One area may be turfed and another may be flower beds.
Tip: Using overlay options will add complexity to your project and increase calculation time.