Snapping

CAD programs provide numerous ways to help improve the accuracy of drawing. When drawing on paper the end of one line and the start of another line is purely a visual fix. A rule can be used to give the position of the end point of a line and then the draughter, placing the start of another line at the same point on the paper, uses visual positioning to fix the next point. Whilst this is acceptable in manual draughting practice, it is un-acceptable in CAD draughting.

CAD draughting is capable of extremely high precision and is interrogated by software that gets its information from the drawing database. A program tells if one line ends and another one starts at the same point when the co-ordinates have exactly the same values. It is impossible to achieve this (apart from in some unusual screen/drawing-database scaling coincidences). CAD programs therefore provide the SNAPPING facility.
This piece of program code monitors the movement of the on-screen cursor position and constantly compares it to the database of vector lines making up the current drawing.
When a drawing contains hundreds or thousands of line, arcs or other shapes it is sometimes possible to see a slowdown in the movement of the cursor movements when the object snap is activated.

Object Snapping examples
Whilst trying to fix a line to the end or middle of another line I captured some shots to show the actions of the object snap code.
The first one shows the object snap cursor with one of its arms crossing the line I wanted to attatch to, but there was no indication that the snap code had found an object close enough to make a fixing to it.


With this example, if you look carefully you will see that the cross hair of object snap cursor is slightly closer to the object line that I want to fix to, but it is closer to the centre of the line than to the end of the line. The result is that the object snap code assumes I want the point that I am closest to. (a fair assumption) But this is wrong I want the end point.


Moving the cursor only slightly closer to the end point causes it to "SNAP" to the end point node, because it is closer to the end point than to the mid-point


After fixing my line at that point and fixing the other end by eye (guesswork), I want to fix another line to the end of the new line.
You can notice in the last two clips that the cursor is almost in the same position between the two lines, but in each case they are closer to the line showing the snap marker.



The point of this is that you need to consider which line, and which node on it, and move the cursor to give the snap code a reasonable chance of selecting to one that you know you want.
Objects drawn on a CAD system are represented by simply drawn lines. In the case of the objects below, black, deafault width lines.
Each object has a set number of control points called nodes that allow the CAD draughter to hook onto an object to move it or alter is shape. These are similar in concept to the object snap function.

The following clip shows the nodes, or hooks, used for interactive alterations.

The menu clip below shows the object snap options available to the draughter.
You can select the option that allows you to fix to a particular part of an object. BE CAREFUL. Don't select too many options and in particular be careful about the ("nearest") option only switch it on when you need it then switch it off. You canno rely on an end point snap when a nearest is on at the same time.
The ("aperture size") determines the effective box that is used by the snap code to determine whan an object is deemed to be the next one to work on by the snap code. the smaler the aperture box the closer you have to get to the object in question.



The Snap Toolbar shown below indicates the type of icon sused for each object snap function.


An important snap function is the "SNAP FROM" function. This is the SECOND icon from the left and is indicated graphically by showing an origin point, a vertical and hrizontal distance and then a snap circle.
This option allows you to delay the fixing of the first point of an object until you have moved the cursor to the correct position.
This is probably one of the most important snap functions available to the CAD draugther.
Select this before you select the actual object snap you want to use.
To draw a line starting at the endpoint of another line you would use LINE ENDPOINT

To draw a line starting a known distance from the end of an existing line you would use LINE FROM ENDPOINT