Understand the World and User Coordinate Systems There are two coordinate systems: a fixed system called the world coordinate system (WCS) and a movable system called the user coordinate system (UCS). Objective: Explain the relationship of the WCS icon (World Coordinate System) to the AutoCAD coordinate system. Explain the difference between the WCS and the UCS (User Coordinate System). The World Coordinate System is based on the Cartesian Coordinate System. Having an understanding of this coordinate system (which many people have, despite of probably never heard of the name) is essential to understanding how the.
But when we present this on paper, we want the plot drawing to look like this:
In order to get from figure 1 to figure 2, we somehow have to twist the drawing. There are essentially three ways that we can do this:
- Softdesk North Rotation - Rotate the roadway entities in the master drawing. After doing this, we can also set the Softdesk North Rotation. The purpose of setting the North Rotation is to essentially fake Softdesk into thinking that north is no longer up in the Y direction. It is now in a direction corresponding to the rotation of the roadway. Even though the roadway is rotated, north has been rotated as well to maintain accuracy.
- Dview Twist - Rotate the world coordinate system. In this case the roadway drawing entities are not rotated. The entire world is rotated.
- UCS - UCS stands for user coordinate system. By default, when you start a drawing, the user coordinate system is the same as the world coordinate system, which never changes. However, you can change the user coordinate system. Using this method, the UCS can be rotated in accordance with the road.
Also realize that twist is very important in relation to annotation. Before annotating the roadway in figure 1, the user had to know how to orient the text based on the desired twist. Choosing a method of rotation should therefore take into account the ease with which text can be oriented.
So what method should be used? Unfortunately, they are all fairly complicated. Users should have a high degree of proficiency in AutoCAD before using any of these methods. Each method has its advantages and disadvantages and they are discussed next.
Softdesk North Rotation
a) For proper Softdesk Cogo labeling orientation and accuracy, this is the best method.
a) Setting a north rotation other than the Y direction can be very dangerous if you forget that it’s set or another user does not know it’s set. All Softdesk defined alignments, dtm’s, pipes, etc. are based on the changed north rotation. Inconsistent data can therefore be created between users with different north rotations.
b) Equating initial survey information to post-design stakeout can sometimes be difficult.
c) If the user needs different orientations in different sheets or viewports, north rotation cannot help. It is only useful for one orientation.
d) The same North Rotation has to be set individually for every plan drawing in the project.
a) The accuracy and consistency of Softdesk data will not be effected. The entire world is being rotated, so the relationship between the drawing information and the coordinate system does not change.
b) Different dview twists can be set in different viewports and drawings.
a) It can be difficult to correctly orient Softdesk cogo labels and AutoCAD text and dimensions.
b) It can be difficult to set the exact desired orientation.
a) It is easy to correctly orient AutoCAD text and dimensions.
a) When the UCS is set to something other than the world coodinate system (WCS), creating and defining Softdesk alignments, dtm’s, pipes, etc. will result in inaccurate and bogus Softdesk data.
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b) When the UCS is set to something other than the WCS, drawings that are externally referenced will be inaccurately located.
c) When the UCS is set to something other than the WCS, lines and curves cannot be labeled accurately with Softdesk label commands.
d) One drawing can have only one UCS. It’s not possible to have different twists in different viewports using UCS.
CMI believes that Dview Twist is the best method to use. The disadvantages of using the Softdesk north rotation or UCS rotation far outweigh any advantages of UCS / north rotation and disadvantages of dview twist. Accuracy of data is most important and both UCS and north rotation can have a negative impact on accuracy.
Expert users will sometimes use a combination of dview twist and UCS. This may be okay as long as the user understands the ramifications of using UCS and always sets the UCS back to the WCS when working in Softdesk, externally referencing a drawing, or finishing a drawing. But if a user doesn’t remember to set the UCS to the WCS to do these tasks (very possible), drawing accuracy can be affected.
Extends objects to meet the edges of other objects.
To extend objects, first select the boundaries. Then press Enter and select the objects that you want to extend. To use all objects as boundaries, press Enter at the first Select Objects prompt.
The following prompts are displayed.
Current settings: Projection = current, Edge = current
Select boundary edges..
Select objects or <select all>: Select one or more objects and press Enter, or press Enter to select all displayed objects
Select object to extend or shift-select to trim or [Fence/Crossing/Project/Edge/Undo]: Select objects to extend, or hold down SHIFT and select an object to trim, or enter an option
Boundary Object Selection
Uses the selected objects to define the boundary edges to which you want to extend an object.
Object to Extend
Specifies the objects to extend. Press Enter to end the command.
Shift-Select to Trim
Trims the selected objects to the nearest boundary rather than extending them. This is an easy method to switch between trimming and extending.
Selects all objects that cross the selection fence. The selection fence is a series of temporary line segments that you specify with two or more fence points. The selection fence does not form a closed loop.
Selects objects within and crossing a rectangular area defined by two points.
Specifies the projection method used when extending objects.
Specifies no projection. Only objects that intersect with the boundary edge in 3D space are extended.
Specifies projection onto the XY plane of the current user coordinate system (UCS). Objects that do not intersect with the boundary objects in 3D space are extended.
Specifies projection along the current view direction.
Extends the object to another object's implied edge, or only to an object that actually intersects it in 3D space.
Extends the boundary object along its natural path to intersect another object or its implied edge in 3D space.
Specifies that the object is to extend only to a boundary object that actually intersects it in 3D space.
Reverses the most recent changes made by EXTEND.