ChuckM said:
Every single tape we can buy out there is an accurate tape -- as long as you use that same tape throughout for the project. I never ever switch tapes, and use the very same tape to transfer measurements until the end. No calibration of any kind. Every cut has been accurate -- if any cut isn't, it's not the fault of the tape but something else.
Not so unfortunately. It is basically hit&miss with the better quality ones having a higher -chance- you do not get burned.
The real problem is that we do not used tapes to measure the same distance all the time. And the tapes inaccuracy may not be spread evenly along its length. It actually never really is. The problem is "how much" is it nonuniform.
Case in point, I have a tape at home which I was checking against a calibrated one and which looks like new as is not really used.
It accuracy looks this:
(real values are +-0,2, cannot measure more accurately by eye)
0-500 > real 501.0
- this is just horrible
0-1000 => real 1000.4 /so 500-1000 gives 499.4 (!)/
- seems not so bad /if not aware how came to be/
0-1500 => real 1500.4 /so 1000-1500 gives 500.0/
- darn good /if not aware how came to be/ EC precision class 1 (highest) demands real 2000.8 /so 1500-2000 gives 500.4/
- seems OK /if not aware how came to be/
Now you can see the problem ... you can get almost 2mm compound error bu two measurements one in 500 mm and other in 1000 mm territory and this from the same measure.
The calibrated Class 1 tape I got delivered lately looks like this (random calibration acc. +/-0,1 mm, excluding systemic errors)
0-500 > 500.0
0-1000 > 1000.0
0-1500 > 1500.1
0-2000 > 2000.1
0-2500 > 2500.2
0-3000 > 3000.2
0-3500 > 3500.3
0-4000 > 4000.3
0-4500 > 4500.3
0-5000 > 5000.3
Now, with such a gradual lengthening - like on this one, even if by more per meter - Yeah! You can build a well-fitting project as it will just be a bit scaled like the tape was.
But without having your cheapo tape measure calibrated - in some way - you will never know. And I am not sure how many woodworkers have the length-measuring tools to do so with any real accuracy.
Actually, I bought this tape lately precisely because it was the cheapest way to get a calibrated measuring instrument with known deviations.
Not to the actual point, that is of course moot.
What the PGs allow and tape + pencil does not is not accurate measuring but accurate -transfer- of measurement to the tool. As usual, errors are always compound.
A PG does not solve measurement accuracy issue. You need a calibrated/known-good tape or ruler for that. But it reduces the transfer error from >0,5 commonly to < 0,1 mm which is great in this size/cost band.