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In this session, we are going to take a closer look at Model Space. Our approach is going to be a little bit different though because we are going to look at Model Space from the computer's point of view. Taking this behind the scene's look can be very helpful in teaching us how AutoCAD maintains our line work. Model Space is essentially and infinitely large virtual grid much like a sheet of graph paper. We construct our geometry on this grid and AutoCAD uses the grid to maintain the accuracy of our drawing. Since, this grid is infinite in the size AutoCAD needed a way to reference specific locations and space, it does this through the use of baselines.
One of the baselines runs east and west, this guy is called the X Axis. The X axis is also a number line, everything to the right of 0 is considered positive X, everything to the left of the 0 is considered negative X. The other baseline runs north and south this baseline's called the Y axis. The Y axis is also a number line. Everything above the X axis is considered positive Y, everything below the X axis is considered negative Y. AutoCAD uses these baselines as a way of identifying any location on the grid.
Locations are identified using coordinates and AutoCAD references coordinates using the format X, Y. AutoCAD uses these coordinates to keep track of where we are drawing on the grid. There is a formal name for this grid and X and Y axis system it's called the Cartesian Coordinate System. Let's take a look at how it works. Take a look at the intersection of my X and Y axis. This location has a coordinate value of 0, 0. This location also has a name it's called the origin. I am going to pick another point on the grid.
What coordinate value would this point have? This guy would have a coordinate of 6, 2 because it's 6 units in the positive X direction and 2 units in the positive Y direction, all coordinates are measured from the origin. Let's try another point, what coordinate would this point have? This point has a coordinate of -7, 4 because it's 7 units in the negative X direction and 4 units in the positive Y. Remember, that AutoCAD references coordinates using the format X, Y. Let's try one more.
How about this point? This guy has a coordinate value of 9, -2 because using 9 units in the positive X direction and 2 units in the negative Y direction. This means when I draw a line on my screen I am picking two points and AutoCAD creates a line between them. From a computer's perspective though, AutoCAD is seeing a line that was drawn from a coordinate of -8, -3 to a coordinate of 8, 10 and AutoCAD can use these coordinates to calculate the length of the line as well as the angle.
AutoCAD uses coordinates to maintain the accuracy of all of our geometry. I've just launched my AutoCAD 2011 and I am sitting in a blank drawing. As I move my cursor around on screen take a look at the lower left corner, down here in Status bar. These numbers represent coordinates and they are showing me the current location of my cursor on the virtual grid. Take a look at the icon in the lower left corner, this is called the UCS icon and these lines represent the direction of positive X and positive Y. I am going to backup a little bit now pan my drawing over.
You can see that AutoCAD is actually showing us the baselines in Model Space. This red line represents the positive portion of the X axis and this green line represents the positive portion of the Y axis. Let's create some geometry using coordinates. I am going to launch my Circle command and before I choose my circle point I am going to come down and turn off my Dynamic Input. Dynamic Input tends to take some liberties with my coordinates and I don't want to get into that right now. For the center point of my circle I am going to type the coordinate 25, 25 and I'll hit Enter.
Then, I'll give this circle a radius of 5 and I'll hit Enter. Let's pan this down a little bit and I'd like to create another circle I'll do that by hitting my Spacebar to reenter the Circle command and I will enter a coordinate of 75, 25 for the center of this circle and then I am going to hit Enter to accept the default radius of 5. Now, let's say I'd like to draw a line from the center of this circle to the center of this one. That's actually quiet easy because I know the coordinates for the centers of these circles.
I am going to move up and launch the Line command and the start point of my line will be a coordinate 25, 25 and I'll hit Enter. I'll like to draw this line to the coordinate 75, 25 and then I'll hit Escape to cancel out of the command. Now, most of the time we won't be entering coordinates as we draft. However, it is important for you to understand the fundamental AutoCAD processes our line work in the background. Think of it this way, everything we draft is mapped out on an underline X-Y coordinate system and AutoCAD uses this system to maintain the accuracy of our drawings.
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