by Dennis Collin
When working with plans or indeed any drawing involving sites or buildings, it is highly likely that the building is not orientated to ‘sheet north’. If positioned in the relation to the real world they are not only likely to be drawn at an angle but are situated a long distance away from AutoCAD’s world coordinate system origin (WCS)
Fig 1. In real world terms coordinated site plans are some distance from the ‘world’ origin. (Ref Fig 2.)
This can present the novice user with a dilemma when working with or printing these drawings as they don’t naturally follow the sheet. However, they would like them to! Some users then ask should they move or rotate the building, so that the drawing it follows the sheet. The answer to this question is a resounding NO!
Fig 2. Whilst the true north might be up the page a more convenient grid would follow the building axis.
AutoCAD’s default coordinate system is the previously mentioned WCS. This is the global coordinate system where North is up the page, and the grid’s Eastings follow a left to right direction parallel to the screen. The origin is down to the bottom left, usually marked with a Icon marker.
On a typical survey drawing or site plan the WCS is aligned with the Ordnance Survey UK grid and the site typically will be hundreds of miles away from the origin at typically quite an angle. As the position of the site is critical for coordination purposes, users should never rotate or move the building. However, what users can do is setup a local coordinate system to the building and get the view to relate to that. This will orientate the site to sheets yet maintain the locations correct position in real-world terms.
To do this we can setup a user defined coordinate system (UCS)
Decide on a suitable reference point, it could be a grid line, corner of building or another suitable reference.
Type in UCS and AutoCAD will naturally select a suitable reference point for the origin. The user will now need to indicate directions for the X and Y axes.
Fig 3. Local UCS defined but view still aligned to true north.
Once set the UCS origin marker is now aligned to the building or other feature as shown.
Run the UCSMAN command (UCS Manager) note the Unnamed UCS, Right click on the unnamed text and call it Local Building Grid or similar.
It will now be possible to use functions like ORTHO snap to draw features like grid that run parallel or perpendicular to the building. Furthermore, elements can be drawn using relative coordinates to the local grid origin if desired.
For presenting the view on the sheet annotations would read better if they followed a horizontal aspect. This can be done by rotating AutoCAD’s view to the current UCS.
This can be done by the PLAN command and choosing Current UCS. This will rotate the view to suite the screen/sheet.
Fig 4. View aligned to the current UCS, note direction of true north arrow symbol.
This approach can also be used on paper space viewports within sheets. By following this method, clear readability of drawings can be achieved without moving or rotating real elements to incorrect coordinates.
Fig 5. Viewport with rotated UCS aligned to sheet.
Creating and managing UCS along with templates and paper space is just some of the topics that we cover on the 2-day AutoCAD Intermediate Course. For more details visit here: Talk to one of the team where we can help and discuss your needs.
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