OT: Laying out engineered wood floor

Need advice from DIY types. I have an upcoming project which involves helping to install a floating engineered wood floor (the snap-together
type) in three rooms and a hallway. The homeowner has expressed a desire to have the entire floor installed continuously. That is, no transitions at the doorways.
For a single room, one uses the longest wall for alignment. Usually an outside wall is chosen, these being the straightest. But this would mean reaching the doorways of each room to the common hallway out of alignment, requiring a transition piece to hide the joint.
If I start along the longest wall of the hallway, I can work my way into each room from the hallway continuously. But I'd like to check the alignment of the hallway wall w.r.t. the back wall of each room (around corners, through doorways, etc) to establish proper alignment once I reach the back wall of each room.
Any tips or tricks to establishing parallel layout lines around corners and through doorways? Giant carpenters squares? Online web pages, videos, etc?
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Paul Hovnanian snipped-for-privacy@hovnanian.com
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I suppose you could use a surveyor's transit
I do not have a transit, but being an optical engineer this is the approach I used for a similar tile layout problem. It takes advantage of the geometry theorem that says when a line crosses two other parallel lines, the opposite angles are equal.
Take two laser pointers and mount them rigidly on a plate or holder at approximately 90 degrees (they could be off by many degrees). If the line A is oriented north/south, align one laser along the line in the North direction and the 2nd laser points East (lets say) through the doorway into the next room. Make a chalk line along the 2nd beam, this is line B. Go into the next room and align one laser along line B pointing West with the other laser pointing SOUTH (it is important this is opposite the direction used when aligning to A). Make a chalk line parallel to south pointing laser, this is line C. A and C are exactly parallel.
You can also use a laser and a penta prism. The penta prism always deviates the beam exactly 90 degrees relative to the input beam. They cost about $25 to $30 at online surplus optics stores such as http://www.surplusshed.com/
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anorton wrote:

I like this one. No large precision square required and I can extend line B quite a ways. Easy to make with a long straight edge and a laser stuck on it at approx. right angles.
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Paul Hovnanian snipped-for-privacy@hovnanian.com
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