Hello everyone,
please tell me how to measuring the dimensions in the parallel projection , when I use the tape measure tool it measure the real dimensions not in the parallel view. 
Could you post an example of what you are trying to measure?
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It’s hard to tell from your description exactly what you’re trying to do and exactly what the issue is. Perpendicular (magenta) inferencing allows you to use any edge as a reference for either parallel or perpendicular measurements.
If the following depicts what you want and you still can’t achieve it, please expand upon your problem.
You know, insofar as perpendicular inferencing is concerned, it really makes no difference whether you are using parallel projection or perspective: it works the same way in either event. However, I’m puzzled by your preference for parallel projection; it is inherently a distorted, unnatural way of looking at things.
-Gully
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@darkprince: You need to use the Dimension tool (not the TapeMeasure tool,) to make a diagonal dimension.
Here is your file, with two scenes, the original “Front” scene, and a new “Top” scene. I also added a “Front” and “Top” layer. The “Front” layer is off in the “Top” scene, and visa versa. Each of these scene specific layers is assigned to it’s own dimension object.
test_diag_dim.skp (537.9 KB)
And here’s the file with a “Perspective” scene (and layer) added:
test_diag_dim_2.skp (561.7 KB)
Thank you very much for your attention Gully,
actually my problem is how to measure the dimensions in parallel projection , as you know there is a different in dimensions between the parallel projection and perspective especially if there is a slope in design .
So I want to know the method of measuring the dimensions in parallel projection.
for example I want to know the size of the line in the following picture.
the picture is parallel projection from the front.
You are mistaking the visual length on the projected image for the actual length in the model. SketchUp measures and displays the actual length, regardless of the projection and regardless of foreshortening in the view. Below, two scenes of exactly the same model, just switching from parallel to perspective and orbiting up a bit.
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Actually SketchUp aligns dimensions with the current active axes. (See example SKPs I posted above.)
Your seem to be thinking in 2D instead of 3D. Set up a point to dimension to that is on a line parallel to the camera and use that.
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My point was that in the upper view the edge is foreshortened, but this does not change the dimension length that SketchUp shows. A different question than what direction it measured.
That’s what I’m asking about thank you very much. @DanRathbun @slbaumgartner @DaveR
That is incorrect. The model is the model; its geometry remains the same however you view it. There may be an apparent difference in the appearance (of dimensions) on screen, but that is an artifact of the view state and the differences between the types of view. The geometry does not change.
I suspect there may be a language issue here. The concept itself of the geometry existing independently of the view state is rather straightforward.
-Gully
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@Gully_Foyle thank you for your attention , there is misunderstanding happened I didn’t mean that the real dimensions change , I meant the dimensions that appear on parallel project , the following pictures may describe my point.
The original post is about a dimension along an axis, which can also be accomplished by clicking the starting point, moving along the axis that you are interested in, hold shift key, then clicking the finishing point.
All of this seems to tie into a thread that I created awhile back. Basically, I think everyone works differently and believe that a dimension tool would be better if it acted like other software giving 4 dimension (along each axis and true length).
I read it as, it was specifically how to get a dimension aligned to a parallel projection view. The OP was attempting using the TapeMeasure tool, which was the wrong tool to use (or was mentioned in error.)
The real trick (when the object is rotated off axes like the OP’s example) is to make sure the axes are aligned to the view, because SketchUp draws dimensions (with the Dimension tool,) aligned to one of the axis. [Notice he did mark my answer as the solution.]
Dan,
My post was not to disagree with you.
Mostly, I was pointing out that the tape measure tool works with the inference engine in SU by holding the shift key to measure along an axis, which is what I do while modeling since dimensions are typically done for drawings after modeling is complete.
Hey it’s OK to disagree. Example, I have no use for the TapeMeasure tool. Hate it. I use other custom tools for guides and cpoints.