Screening vs Dithering
The term "screening", as used in this manual, is used interchangeably with the term "dithering". Generally, screening is a dotted or pen up-down pattern. Dithering is generally produced as a gray scale value. Either method is acceptable for District drawings. The choice to use screening or dithering depends on the reproduction methods that will be used when the drawings are printed. Dithering tends to turn darker, almost to black, when photo copied. If drawing sets will be photo copied, screening is a better choice than dithering. Pen tables have been provided that both screen and dither various levels and reference files. The consultant may use whichever method produces the best result.
What is Screening (dithering) used for?
"Screening" is a process that plots background information with a close dot or dither, rather than a solid line. This allows the screened portion of the drawing to appear lighter, or less important, than the solid portions. Screening is generally used to indicate existing structures or topography. Screening can also be used to show background information from one discipline, on a drawing of a different discipline. For example, when drawing a Conduit and Power plan, it is preferable to screen the building outline and all mechanical equipment, so that the conduit shows clearly on the drawing.
Screening can also be used to indicate "Future" installations. Care should be taken not to mix screening to indicate existing elements, with screening to indicate future work. If a project requires both types of elements, then the future work should be shown with a standard line code, such as line code 6.
Are there other options?
Occasionally it is necessary to lighten the appearance of elements in a file, without actually screening the elements. This is not often required on drawings, but may be needed on complex site plans or on projects where staged construction is shown. The pen table provides two options for lightning elements.
Pen Table Summary
The pen table allows the user to selectively screen files, screen levels within files, or lighten files. The control lies in the logical name given to the reference files. This pen table works in the following manner:
Active File:
The Active file will always plot screened on levels 160 through 169 (G-SCRN-01, ...02, ... thru 10), and solid on other levels.
The level G-GRID-MINR, for the minor profile grid lines, always plots screened on all files.
Reference Files:
You can control your reference files by the logical names you give them.
To SCREEN ALL LEVELS in a reference file, name your reference file with any logical name that begins with the letters SC
example: SC302 or SCSITE
To Screen ONLY LEVELS 160 to 169 (G-SCRN-01 through G-SCRN-10) in a reference file, name your reference file with any logical name that begins with XSC
example: XSC301 or XSCPLN
To LIGHTEN the ELEMENTS on all levels in a reference file, without screening them, name your reference file with any logical name that begins with the letter L
example: LC301 or LXSITE
To plot all levels and all line weights as WEIGHT = 0, name your reference file with any logical name that begins with WT0.
example: wt0C301 or WT0XSITE
To have ALL LEVELS NOT SCREENED and NOT LIGHTENED in a reference file, you can use any logical name, except one that begins with SC, XSC, or L