I can make the same measurement from the same reference point and wind up with two values that are 20+ inches different. If I make one measurement in parallel projection mode but with an arbitrary rotation, I get 178". If I make the same measurement from parallel project top view, I get 200".
How am I supposed to actually measure anything accurately if the rotation angle of the camera affects the measurement?
I have 3 weeks worth of work that I now have zero confidence is actually correct…
There may be some element in your model that is causing your top view measurement to read on an angle which would result in the greater dimension. To ensure accuracy, selectively zoom in on the points you want to define by distance. Please don’t view this as an insult, but bear in mind that when a problem exists in modeling, the problem usually lies with the user rather than with the software.
Feel free to post the offending model here and one or more of us will be able to evaluate and analyze your issue more definitively.
Sounds like you aren’t anchoring both ends of the dimension to the same points in each case. One way to ensure that you are dimensioning to the right points is to insert them in perspective mode. Create the 2D view after.
Be aware that dimensions are created aligned with the current editing context’s axes.
So if you have changed the model’s main axes, you change the alignment for any subsequently created dimensions, in the top level model’s entities context.
If you you have entered into the editing context of a group or component, any subsequently created dimensions are aligned with the local axes.
Misaligned dimensions are the most common cause of “funky” measurements.
I can assure you that SketchUp doesn’t make mistakes in the order of say 10% with dimensions like 200". Neither with the ‘Tape Measure’ tool, nor with the ‘Dimension’ tool.
It must be something quite simple. See how SketchUp measures geometry in 3D view and you might find out that you measured two points that aren’t even in one plane when viewed in 2D. Also take note of the current axes orientation as noted by @DanRathbun. Same applies to using the ‘Tape Measure’ tool. Best if you upload your example to clarify what is going on.
FYI: I retitled the topic to be less accusatory towards the software and moved it to the main SketchUp category (because it is an operator issue, not a technical issue.) Also added filter tags.
Related SketchUp User Guide articles:
Related Tutorials:
hey there… I’m getting radically different measurements in the form of many feet. what am I not figuring? I copied the two geometries from an earlier version of the design and then made them into groups for ease of mobility.
I found the issue! I assumed that because I was working in parallel projection and from top down that as I copied and pasted from the old version to the new that everything would be on the same plane. to my surprise pastes were posted at a higher elevation. MY SOLUTION: select all then used Enroth Flatten to Plane. Had to make a few adjustments connecting lines, and now all is back in working order. I hope this helps someone in the future 
You always have to bear in mind that SketchUp is a 3d modeler, even when you are viewing down an axis or in parallel projection. It is easy to accidentally draw things out of plane if you assume that the view controls the plane things will land on.
