Level One - CAD Environment - Cartesian Coordinates

HNC CAD: 2D CAD Concepts -

Relative Coordinates

In the diagram below, say point 1 was know to be 9 units along the X-axis and 5 units along the Y-axis. This, as we know from the previous section, is an absolute measurement because it is taken from the origin of the coordinate system.
Now take point 2. If the information you have is the distance from point 1 then that distance is known as a relative distance, measured from point 1. To specify the distance we have to use a temporary coordinate system centred at that point. The green box signifies the temporary coordinate system. Now, in relation to this coordinate system, point 2 is -4 units in the X-axis and 1 unit in the Y-axis, relative to point 1.

point 2

point 3

-13 units

X-axis

-2 units

Y-axis

relative

point 2

Exercise 2

Continue with this exercise by entering all of the coordinates for each point relative to the previous point. Don't forget that point 1 is measured from absolute zero so it is an absolute coordinate. Enter your values in the columns headed X and Y. Comments will be placed in the Marked comments column when you click the Click here for marking button.

Although you have entered the coordinates using a relative method (which measures a distance from some point of reference other than the origin) , they are converted to absolute coordinates (which measures distances from zero, zero) once the computer program has received them. So, relative methods are only a facility provided by the CAD program in cases where the information you have at hand is more conveniently from some other point. There are many other ways in which positional information can be entered into the computer, but they are all converted to absolute coordinates by the CAD program.