Until this morning I was unaware that Pi rulers existed, and yet at the mention of one it was obvious, and that makes me wonder if there are other similar obvious things that could be done with that idea. A Pi ruler, or Pi tape measure, is one where the indexed numbers, such as inches, are already divided by Pi (3.14159). An example of a Pi tape’s usefulness is measuring the diameter of a tree trunk. That is a difficult thing to do accurately because you can’t stretch a tape through the center of a tree. With a Pi tape you can measure around a tree trunk and the number indicated at the point of overlap would be the diameter of the tree. Having a Pi tape is useful if your loggers are told to only cut down trees between 24 and 36 inches in diameter, and leave the smaller and larger ones standing.
A reverse Pi tape, on the back of the original Pi tape, could do the opposite operations. Find the circumference of a log by stretching the reverse Pi tape across the diameter of the sawn face and the number at the edge would be the circumference.
A spherical ball could have its volume measured by having Sphere-tape that when wrapped around the circumference would indicate the volume at the point of overlap. Each of these measurements could be done with a standard tape measure, but then you would need to do a mathematical operation, which would need some paper and a pencil or a computer. With this Pi tape measure the answer is instantaneous, and that could be helpful if you are doing the operation many times.
You can make specialized tape measures for basic geometric shapes and volumes. A plain piece of non-stretchable tape can be indexed with any appropriate set of numbers for your needs, and then quickly applied to your specialized use.
Even an obvious idea once seen can lead to not so obvious ideas.