All about scales in drawing, viewing and plotting:
A look at the relationships between and the use of the AutoCAD commands: SCALE, ZOOM, ZOOM-XP, LTSCALE, PSLTSCALE, DIMSCALE, and the related concepts of TEXTSIZE and text height and scales for plotting.
The biggest jump for those just learning CAD is getting used to the idea that, unlike drawing on paper, it is commonly agreed that we should “draw” “full size” or at a scale of 1 to 1. But what does this mean?
We can can ceate pre-scaled drawings (a crazy idea introduced to match the features of other programs) or work in drawing units such as millimeters or inches so that our models seem to be full-size.
As an Australian I would model the Statue of Liberty in millimetres whereas an American might model it in inches. We both say that we model at one-to-one even though we are using different units of measurement and thus our models represent different sizes. Well are they? Neither of our statues of Liberty has any real size. They are just made up of entries in a data-base with attached dimensions that are read by the drawing editor in order to display an image on the computer screen at a size convenient for viewing and then sent to a plotter to print at a convenient scale. Our two models are different only because the Australian model has dimensional data attached that is roughly twenty-five times larger than the American (approx. 25mm to the inch) – at least the numbers in the data-base are. Thus unless you are using the pre-scale concept the drawing-file has no drawing scale unless you think of it as a scale of 1 to 1.
Each of us could use the other’s model by importing it into our own environment using AutoCAD’s SCALE command to enlarge or reduce the objects to the local measurement units.
ZOOM is purely a viewing aid that that changes the image size on the screen but leaves the drawing database unaffected. Options of the ZOOM command involving a “scale” are: Zoom: nX or nXP thus zooming by a user set scale “n” relative to the current image or relative to a VIEWPORT (MVIEW) in PAPERSPACE.
ZOOM-XP is typically used to set a desired “plot-scale” scale within a VIEWPORT that has been created in a Paperspace image of a drawing sheet at a scale of 1 to 1 ready for plotting. i.e. one unit on the paper represents "XP" units on the model.
Note that the “scale” option in the PLOT command dialogue-box (typically set to 1:1) determines the size of the printed output relative to the nominated size of the sheet. For example an A1 size sheet can be plotted at A3 size by setting the plot scale to 0.5. The different views displayed in each of the Viewports on a sheet plot at the scales set by the ZOOM-XP command for the individual VIEWPORT unless further modified by the overall plot scale of the sheet.
When using different line types (hidden, dashed etc.) the LTSCALE must be set to a suitable value to enable them to be seen on the screen and appear in plots. Visibility of line types will also be affected by the current degree of ZOOM. The LTSCALE is the length of the line-type pattern-repeat in drawing units (millimetres inches etc.) required to make line type display correctly. Hence if working in mm an LTSCALE of 500 produces repetitions of the dash pattern at 500mm intervals. Typically this is suitable for working with drawings (designed in mm) that are to be plotted at 1:100. If the same drawing is to be plotted at 1:20 (five times larger) then the LTSCALE would need to be set to 100 (one fifth of the 500). LTSCALE can be an arbitrary value that suits the way you work with your drawings except when you come to plot – then it needs to be set to an agreed value that meets the office standard with regard to appearance of plotted drawings.
PSLTSCALE is a system variable used to determine the way line types are displayed in VIEWPORTS in Layouts (Paperspace). PSLTSCALE is either: 1 (ON) or 0 (OFF). When PSLTSCALE is ON all occurrences of line types regardless of any differences in ZOOM XP scale in VIEWPORTs will appear at the current LTSCALE relative to Paperspace. In a mm environment this would typically be 5 thus rendering dashed line pattern repeats every 5mm. When PSLTSCALE is OFF, line types appear according to the ZOOM XP scale of each viewport and appear at the current LTSCALE in Paperspace.
DIMSCALE is a system variable that determines the overall scale factor of dimensions. There are many dimension variables such as arrow-size and text-size that determine the appearance of dimension styles whereas “Dimscale” is a multiplying factor applied uniformly to all the individual dimension variables to produce dimensions at a size to suit the plotted scale of the drawing; a bit like setting a particular text height.
Dimscale is only read-only when DIMANNO is set to 1. By creating and setting current a non-annotative style it is then possible to (setvar “Dimscale” n) where n is any positive real value. AutoCAD 2014 defaults to annotative dimensions when first installed.
TEXTSIZE is a system variable that stores the current default text height. The TEXT command prompts for a Text height and displays the current value that you can accept or change. This is the size of the text created by that particular application of the TEXT command. However be careful: when creating or changing a text STYLE you will be asked to nominate a text Height. This is zero by default allowing the TEXT command to determine the text height for each use of the TEXT command but if non-zero this will cause the TEXT command not to prompt for a height but assign a fixed size to all text created using that style. You cannot then resize that text without redefining its style. A common recommendation is that: except in unusual circumstances do not change text Height from zero in the STYLE command.
Choosing Text Height and Dimscale relative to Drawing Scale:
New CAD operators often have difficulty deciding on an appropriate text height or Dimscale when annotating drawings. Office standards normally set out the required text and dimension sizes that should appear on the plotted drawings including consideration of different sizes for differing priorities of notes, labels, room names etc. The difficulty for the CAD operator is setting the size in drawing units relative to the finished plotted scale of drawings. This is further complicated by differing plot scales within viewports and text or dimensions in Paperspace on the drawing sheet itself.
The best metaphor that I found (for architects) was to visualise the text painted on the floor of the building and think of the size that it would need to be painted in order for it to show up on the plotted plan. In this way I can see that 300mm high painted text will appear 3mm high in a drawing plotted at 1:100. Similarly if I were to plot at 1:50 (twice as large) I would need to paint the text half as high (150mm).
Thus there is a simple relationship: plot-factor x plotted text height i.e. 100 x 3 = 300 and 50 x 3 = 150 respectively.