Expert: Bill DeShawn - 5/20/2010

Question
what is co-ordinate system.what is its use.explain different types of co-ordinate system

Answer
Farhana:
The Cartesian coordinate system that AutoCAD uses is explained here:
http://www.shodor.org/interactivate/lessons/CartesianCoordinate/
and here:
http://en.wikipedia.org/wiki/Cartesian_coordinate_system

In AutoCAD there are two kinds of coordinate systems that each are cartesian coordinate systems.  One is the World Coordinate System (WCS).  That system is present in every drawing that you create.  It's origin never changes and all points on the drawing are relative to that origin when the WORLDUCS system variable is turned on (set to 1).  The other coordinate system is a user coordinate system (UCS) that can be on the same elevation as the World Coordinate System or it can be on a totally different elevation/plane altogether.  It can have the same origin or a different origin.  It's plane can even be rotated on the X, Y, or Z, axis or a combination of rotated axes.  In a cartesian coordinate system (the way I see it), there are only two axes:  X and Y.  The systems can be at different elevations, but the cartesian coordinate system itself does not define any Z axis points or elevations.  Any Z axis points that are non-zero (relative to the plane of the current UCS or WCS) are considered to be 3D points.  When the SNAPBASE system variable is set to 1 (turned on), and you use the UCS command to set the UCS to a line, the origin of that new UCS is the starting point of the line, and the direction of the line is the angle which will now be zero degrees in the current UCS, and your snap origin will be the starting point of the line.  An angle in a cartesian coordinate sytem is relative to 0 degrees which is measured from the "EAST" (the right side when view is not twisted or rotated) and graduates upward from zero in a counter-clockwise direction.  

Please let me know if you have any more questions.

Bill DeShawn
http://my.sterling.net/~bdeshawn