> For the complete documentation index, see [llms.txt](https://docs.openbrim.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.openbrim.org/templates/custom-girder-bridge-workflow/geometric-criteria-cg/clearance-zone-cg.md).

# Clearance Zone \[CG]

A Clearance Zone is the **region** a clearance requirement is measured over: the roadway, track, channel or path the structure crosses. It holds the region itself, the surface the clearance is measured up from, and which objects are in scope. It does not hold the clearance you require — that belongs to the **Vertical Clearance Check**, which can measure more than one requirement over the same zone.

The region is a polygon in plan. Only its X and Y are used: the floor comes from the reference surface below, so the elevation of a picked point is discarded. Region points are stored in project coordinates, because the crossed feature is a physical place on the ground and must not move when the bridge alignment is edited.

**Show Region \[YES/NO]:** Draws the region outline on the ground, so you can see what is being measured over.

**Region:** The polygon the clearance is measured over. The cell is read-only as text — a point list cannot sensibly be typed — and is edited from its cell action, which lets you pick the corner points directly in the 3D view.

**Floor Objects:** What is below, as a list of object names — the objects whose **top** surface the clearance is measured from. This is a list rather than a single choice because the ground under one span can genuinely be a survey surface *and* a modelled roadway *and* an adjacent bridge deck; where they overlap, the highest one wins. Naming the imported Digital Terrain Model here is the usual case.

**Constant Floor Elevation:** The level the clearance is measured up from when nothing below is modelled — a water surface, or a top of rail. It is not used once a Floor Object is named.

**Check Objects:** The objects that must clear this region, as a list of object names — the girders, for example. Left empty, everything above the region is checked. Naming the objects explicitly is also the fix when a coarse default scope reports a minimum caused by something you did not intend to measure.

**Exclude Objects:** Objects removed from the scope, as a list of object names. Use this to drop something that legitimately intrudes into the region.


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