Why is my dimension negative?

When you add a dimension from a Line to a Point, the dimension itself can be Negative or Positive.

Why is this?

The Line from which you dimension the point has its own X-Y Coordinate System.
The positive x–axis of the Line is from the start-Point to the end-Point' (the places where you mouse-button down and mouse-button up at each end of the Line)
Therefore, when looking along the positive x–axis of the Line, a
odimension to a Point that is to the positive X-axis of the Part, is negative with respect to the y-axis of the Line.
odimension to a Point that is to the negative X-axis of the Part, is positive with respect to the y-axis of the Line.

When you add a dimension from the X or Y–axis of a Part, the dimension itself can be Negative or Positive.

Why is this?

Each axis is treated as a Line. Thus, the axis itself has its own X-Y Coordinate System.
The less obvious case is the Y–axis:

The positive X–axis of the Y–axis(!) is from the Origin and in the positive Y–axis direction. Therefore, when looking along the positive X–axis of the Y–axis, a

odimension to a Point that is to left-side is negative.
Therefore, a dimension to a Point in the +X–axis direction of the Part is negative.
odimension to a Point that is to right-side is positive.
Therefore, a dimension to a Point that is in the -X–axis direction of the Part is positive.
tog_minusMore Details

Dimensions-Axes

Why is the Dimension along the X–axis negative?

Note: X-axis, Y-axis : coordinate-system of Part, x-axis, y-axis : coordinate-system of element, and a Line.

Dimension convention in MechDesigner:

All Lines and Axes have a Coordinate System (Origin and +x,+y) . This includes the Part-axes : X–axis and the Y–axis. Each axis has an origin and a +x–axis direction and a +y–axis  

When we dimension using the Y–axis, or a Line in the Y-axis direction, then its +x–axis direction is along the +Y–axis of the Part.

Dimensions-axes2

 

In the image, the area with the 'patch' is all –ve in the Coordinate System of the Y–axis, or Line in the Y-axis

Therefore, is this case, when we dimension the Point1s-red along the X–axis of the Part, it is along the negative direction of the y-axis of the Line.

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