Plot Scale
Every drawing has a base scale, or plot scale. Before any text or cells can be placed in a design file the user must determine the plot scale. The plot scale should be the predominant drawing scale. The plot scale should always be one of the scales used on the drawing, and it should be displayed in the file in large letters above the drawing border.
Whenever possible a CAD file should be limited to just one scale. Multiple drawing scales in the same CAD file are more complex and difficult to work with. It is preferable to limit the majority of drawings to just one scale, and to consolidate multiple scale details on separate detail sheets.
All Drawings Are "Full Size"
All CAD drawings are created "full size". That means that a 50 foot long structure measures fifty feet in the design file. At the same time, all drawings are plotted at the finished paper size of 22"x34". In order to accomplish this, the border of the drawing (a cell) is placed in the CAD file at a scale factor relative to the desired plot scale.
For example: a building plan drawing will be 1/8"=1'-0" after it is plotted. It's "plot scale" is 1/8"=1'-0". The building is 100' long and 40' wide. In the CAD file, the building measures it's full size of 100'x40'. On the plot however, the building measures 12.5"x5". That is because the scale of 1/8"=1'-0" is used to represent the size of the building. The 22"x34" border that represents the edges of the paper plot, is placed in the file at a scale factor that makes the border 272 feet long and 176 feet wide.
In summary: All physical items (walls, pipes, pumps, lights, doors, etc.) are drawn full size in every file. CAD drawings are annotated with non-physical items. These include text, line terminators, and various cells (north arrow, section cut arrows, drawing border, etc.). The non-physical items are always placed in the file at a size or scale factor that relates to the finished drawing scale.
Scale Factor And Working Units
The size of an object in the design file is directly related to the working units setting of the design file. The default working units setting is 1':12":8000pu (96000 resolution). All scale factors and cells are based on these units. See the section on Working Units for additional information.
Drawing Accuracy
The use of CAD provides the means to draw extremely accurate drawings. At times changes in the design call for dimension changes on the drawings that are so small that they would not be visible on the hard copy plots. The District requires all drawings to be precisely to scale regardless of how small the difference might be. Even differences that are not visible to the eye are important to the CAD file.
Scale Factor Table
The scale factor is the active scale used when placing non-physical cells and terminators in the design file. It also applies to text size. A scale factor of 1 (AS=1) would create a symbol that is actual size. If a north arrow is 2 inches long, then the scale factor of 1 would place it in the CAD file as 2 inches long.
However, if the plot scale of the file is 1/4"=1'-0", then the north arrow must be placed at a different scale factor so that it will appear to be 2 inches long on the final plot. The correct scale factor for placing the north arrow in the 1/4" scale drawing is AS=48.
A complete list of scale factors for each common plot scale is show below.
|
PLOT SCALE |
SCALE FACTOR |
|
FULL SIZE |
AS=1 |
|
3"=1'-0" |
AS=4 |
|
1 1/2"=1'-0" |
AS=8 |
|
1"=1'-0" |
AS=12 |
|
3/4"=1'-0" |
AS=16 |
|
1/2"=1'-0" |
AS=24 |
|
3/8"=1'-0" |
AS=32 |
|
1/4"=1'-0" |
AS=48 |
|
3/16"=1'-0" |
AS=64 |
|
1/8"=1'-0" |
AS=96 |
|
3/32"=1'-0" |
AS=128 |
|
1"=5' |
AS=60 |
|
1"=10' |
AS=120 |
|
1"=20' |
AS=240 |
|
1"=30' |
AS=360 |
|
1"=40' |
AS=480 |
|
1"=50' |
AS=600 |
|
1"=100' |
AS=1200 |
|
1"=500' |
AS=6000 |
|
1"=1000' |
AS=12000 |
Multiple Scales On One Drawing
Multiple scales can be used on the same drawing by manipulating the working units. By changing the positional units details can be drawn at a scale other than the plot scale. The positional units setting can be changed temporarily, while a detail is drawn, so that it can easily be drawn full size. If a third different scale is required on the same drawing, the positional units can once again be changed for drawing that detail. By manipulating the positional units, there is no need to calculate a conversion for multiple scales on a drawing.
Changing the positional units setting in a file is always temporary. The units should be set back to the default of 96000 before the drawing is exited. All text and cells should be placed at the plot scale, using the default resolution of 96000
See the table, "Positional Units Table for Multiple Scales on Drawings", in PDF format. It shows the correct positional unit setting for scales other than the plot scale. The Master Units and Sub Units are never changed. The values on this table represent positional units per sub unit (inch). In MicroStation, you would multiply that number by 12 to get the number of positional units per master unit (foot). That number would then be entered into the resolution setting in the working units.
A tool is provided with the EBMUD custom application that will automatically set working units to the correct value for drawing details.