Database Elements for Underlays and Overlays

Figure 17:1. Draft Database Hierarchy - Overlay/Underlay Elements

Elements for use as backing and overlay sheets may be created in Draft and stored in the Draft database. The relevant part of the database hierarchy is shown in Figure 17:1 above.

The SHLB (Sheet Library) exists as a member of a LIBY and is used as an administrative element to group together BACK (Backing Sheet template) and OVER (Overlay Sheet template elements.

A BACK would probably be used as a frame sheet and would contain frame lines, authorization boxes These would exist as (member) NOTE elements created via Draft’s geometric 2D drafting facilities. Any hash codewords defined by TEXP elements will be expanded when the BACK is referenced. These codewords will typically extract data from the Draft database. See Accessing Data from the Draft Database.

OVER elements may also own NOTEs as well as VIEW elements, and would probably be used to overlay graphic details such as keyplans on drawing sheets. An OVER shares many of the attributes of a SHEE. Note that Point Construction (see Section 11) can be used to create BACK and SHEE elements.

Most Overlay Sheet attributes are held by the Overlay (OLAY) element.

BACKs and OVERs are referred to by the relevant drawing Sheet’s BSRF and the relevant Overlay’s OSRF attributes (respectively). (BSRF may also be set at Drawing level.) The BACK/OVER to be used may be specified by using commands such as:

The Overlay’s OSLV (‘overlay sheet view layers visible’) attribute defines those Layers of the overlay sheet (that means, the LAYE element(s) owned by the VIEW(s) owned by the OVER referenced by the drawing Sheet) that are to be visible.

The Overlay’s XYSCALE attribute allows independent scaling in the X and Y directions of the instanced overlay sheet or plotfile. For example:

XYSCALE 2 1

Either or both values may be negative, but zero is not allowed.

Note that when an Overlay Sheet is instanced (that means, attribute OSRF is set), text is only affected by the scale in the Y direction. It is not possible to generate backwards or distorted text. However, when a plotfile is used (that means, attribute OSFN is set), then there is no such safeguard and text could be distorted if differing X and Y scales are specified. See also Scaling and Mirroring Special Labels.

Control of complex overlays may be achieved by using multiple OLAY elements.

The Layers which are to be visible may be referred to either by Layer purpose (that means, the PURP attribute) or by their member list position under the VIEW(s) owned by the OVER. Up to 12 Layers may be made visible at once. The Layers that are to be visible are specified by a further option of the OSHEET command. For example:

The Design graphics may be made visible by including Layer DESI. If the overlay sheet has more than one VIEW then the Layer visibility statement will apply across all VIEWs. (It is not possible to specify, for example, a Layer with PURP DIMS to be visible for /VIEW1 but not for /VIEW2, assuming both VIEWs own such Layers.) The setting of a Layer’s LVIS attribute has no bearing on the visibility questions discussed in this section.

The above functionality is only available when an overlay sheet template reference is used. No distinction between Layers can be made when using overlay sheet graphics from a file.