CLARITY ON WCS AND UCS

[ESchomberg]

Hi fellow cad friends, a subject came up in my "inserting blocks" post that led to this subject.

Can anyone explain the benefits or necessity for one over the other and how it works?

This is a very confusing subject for me. I am sure it is not as confusing as it seems.

1) There are 3 screen views.

a) Model space(where you draw).

b) Model space that contains the paper space template.

c) Paper space.

I noticed that when I click on the lower left corner of my paper space it is about 0,0,0... I'm assuming that is where it should be? 

In Model/paper space there is the x,y,z symbol. I clicked on it and realigned it to 0,0,0...

It was brought to my attention that  should probably stick to WCS, so I clicked on the same icon and clicked "world".

I am probably making an issue of something that should not be an issue, but it is in my genes to dig and probe until I understand how something works. If there is no real answer to this, I am ok with it, I will just draw and let it go. But if there is a simple explanation I am curious to understand it.

[rkmcswain]

Not sure what the question is really.
Generally, I would say you should draw in WCS. But there are as many reasons for not being in WCS as there are things to draw. So without knowing the situation, it's hard to say.

The WCS is the basis for every other coordinate system if that helps. It can't be changed, it's a constant.

[Pointdump]

Hey Erik,

The UCS and WCS concepts are something I'd like to learn more about, so I've been nosing around the Innertubes.

Here's a pretty good short-n-sweet video:
https://www.youtube.com/watch?v=r-Gj55jMedk

Help is fairly useless:
https://knowledge.autodesk.com/support/autocad/learn-explore/caas/CloudHelp/cloudhelp/2016/ENU/AutoC...

Dave

[jeff_rivers · Accepted solution]

Good morning Eric.  

Think way back to learning trigonometry and geometry in school- you learned to graph functions on graph paper.  Custom was to designate a place as the origin, or the point where the X-coordinate and Y-coordinate were each zero.  Like a number line, positive X-coordinates are to the right, negative X-coordinates to the left.  Same for the Y-axis with positive being up and negative being down.  

AutoCad uses the same concept- your model space area has a World Coordinate System, with a fixed zero point (or origin) where the X-coordinate, Y-coordinate, and Z-coordinate are all zero.  It never moves, any line or other entity drawn at zero-zero will always be at zero-zero.  In Civil we customarily draw to some surveyed coordinate system, so all our lines, points, and other things are quite a distance up from the origin. 

The WCS origin never moves- it is not capable nor is it allowed to move.  It's a constant within your drawing that you can always use knowing it's at the same place.  Likewise every other point in the WCS is constant with respect to the origin.  The 'graph paper' that you are 'drawing' on in model space never changes.  With respect to the WCS, a point at (or a line through) the point coordinate 100,100 will always be at (or through) that point at 100,100. 

Sometimes you need to use different drawing coordinates and axes.  For example, if I am drawing features along a road, and that road is straight but instead of running perfectly east-and-west it is at some angle, say it runs Northwest-to-Southeast.  I want to draw things parallel to the road, but that road is not parallel to my WCS X-axis.  

I will create a UCS (a User Coordinate System).  My new origin is now not at the WORLD zero-zero.  It's at some convenient point on my road, say where my major road alignment intersects a side road.  And the X-axis is now  aligned with the northwest-southeast sloping centerline of my road.  

So now I can turn on ORTHO, and every line I draw will be parallel to my road centerline, even though that line is running at an angle on my screen / in modelspace.  That's just one example of using a User Coordinate System, in a way that makes drawing a little easier or more efficient. 

There are a lot of tools for controlling the UCS- you can save them, give them a name, and recall them later for re-user.  I do that with the road centerlines a lot, as I explained above, because I often need to draw things on channelization plans that are parallel to my centerline, and very few roads are actually perfectly North-South or East-West.  

You can grip-edit the UCS icon and drag it around with the mouse.  

You can rotate the axes around each other - very handy for drawing things in three dimensions.  I detail out traffic signal poles in 3D and check them with known utility objects up in the air above my road.  

Any time you're working in a custom UCS you can return to the World Coordinate System by typing UCS > W.  

If your drawing gets a weird rotation to it due to some funky UCS, go back to WCS, then type PLAN <enter> to reset to an orthogonal plan view relative to WCS.  

As you discovered, each Paperspace (layout) has an origin point (a zero,zero) in the WCS.  It's the same WCS and has an origin at zero-zero, but you're drawing on the "sheet of paper" that holds your border, titleblock, and your viewport into modelspace.  

Be very careful where you are defining blocks, and what WCS or UCS you may be working in.  Do not create blocks in paperspace if you will be using them in modelspace.  I'd suggest just always making blocks in modelspace, always using the WCS.

Do not make a block in some random place in your drawing- if, like me, you draw up at project plane (or state plane), your drawing coordinates are up around 600000,1200000 (give or take- I'm in Washington State).  If you draw the thing you want to make into a block up there, but accidentally use 0,0 as the INSERT POINT for your block, that block will always be six hundred thousand feet away from where you want it to be (or worse).  

That's why the choice "pick insert point" is so important when you define a block.  Suppose you want your new block to be a manhole cover, for example.

And you just drew it up at your project coordinates where you need to show a manhole.  If you pick the center of the circle as your Insert Point, all is good, and that block will come in centered around whatever point you pick.  

However, if you forgot, and left the checkbox checked to use WCS zero-zero as the insert point, your block insert point is 600000 by 1200000 feet away, because that's where your project data all lives in the WCS.  When you next insert the block and pick the point where you want it, the block will be drawn six hundred thousand feet away or more.

I am not sure what your middle bullet point means: "Model space containing your paper space template".  By 'template' do you mean your titleblock?  

The convention where I work is that all drawing happens in modelspace, all titleblocks are in paperspace.

Each plottable sheet, containing the titleblock and viewports, is on a separate layout tab.  Each layout tab has a titleblock, and one or more viewports that "look through" the page into the model, at the appropriate zoom factor.  

[ESchomberg]

Dude, I really appreciate that.... That was excellent.

[jeff_rivers]

Thanks.  It's good for me to go back and re-remember all the things I learned and try to write them out. This got me to revisit paperspace/WCS stuff.

A couple more important points about using coordinate systems.  

These are of course, just the native AutoCAD modelspace coordinates, and we are not talking about survey coordinate systems (e.g. WGS-84 and all that stuff.  Fair warning, I am not a surveyor and so survey coordinate systems are still a foreign language to me).

If you ever attach and work with XREF's, you can get burned if you aren't paying attention to your WCS/UCS.  When attaching an xref of another drawing, you must have your coordinate system set to WCS.  If you have set some custom UCS for a specific task, then later forgot, your XREF will not attach at the correct origin.  

Many other C3D commands need to know that you are in WCS, and not in some custom UCS.  I think it's better now, (and someone else please chime in here) but it used to be that inserting points, creating surfaces, making alignments and profiles and corridors, and many other civil commands were very unhappy and unpredictable if you attempted to use them while working with a custom UCS.  

The drafting crosshairs change colors now to indicate you are not working in WCS, which is a great visual reminder.  Used to be they were just always white, and so you could easily forget you had a custom UCS set.  Now when the crosshairs turn red + green you know someone has set a custom UCS or DVIEW and you need to be careful.  

There's another powerful command called DVIEW, and one of its subcommands is TWist.  You can rotate all the entities in your model (everything in modelspace) about some point, in order to more easily view and work with it.  

It works both in modelspace, and through a paperspace viewport.  I use it to rotate my model so my roads appear to be truly aligned North-South or East-West.  Purely to make working in the model easier, as I can see more of the road and it eliminates having to work at some odd angle.  This command rotates the WCS too, so it stays true to your model.  

Imagine again you are drafting on a sheet of graph paper, and you rotated the entire sheet of paper 45 degrees clockwise--  the grid lines on the graph paper haven't changed: The paper and everything drawn on it is now just rotated from your point of view. 

Same with Cad: doing a DVIEW-TWist rotates the model AND the WCS together, so any new entities you draw are all still orthogonal to the other drawing entities.  

If you have a long complex road project, for example, and the road meanders, you can set up a different TWist in each paperspace viewport so that the road runs mostly left-to-right along your plotted sheets.  This lets you most efficiently fill each sheet from edge to edge with minimal wasted white space.