How many 300-foot tape measures fit in your pocket and cost under $2?
I had a very nice, somewhat huge, open reel-type 300-foot surveyor's tape measure that worked well until one day it wound funny and I spent a couple of fruitless hours trying to un-jam my $100 measuring investment. I also own an electronic (ultrasonic) tape measure... but that needs a big hard surface to bounce off and only works up to about 30 feet. Actually, you can buy conventional tape measures up to about 25 feet that can be stuffed into a largish pocket... but I want something closer to the size of a 6-foot tape measure. This device is honestly a bit awkward, but it can get the job done and is smaller than an Altoids tin .
I'm sure some readers are thinking "why not just measure what you can with a short tape measure, mark where you stopped, shift the tape measure to there and repeat until the destination, adding the measures to get the total distance?" Glad you asked. Two problems. First, out in the real world, it is hard to be accurate about starting the next measure exactly where the previous one stopped. Second, it is difficult to keep a straight line from start to destination, biasing the result toward overestimation. In summary, accuracy would be much poorer for long distances. Making a calibrated "tape extension" line solves these problems.
Step 1: Materials
Conventional tape measures consist of a tape with calibrated markings spaced at the resolution of the device. For example, a tape measure that reads to a precision of 1/8 inch has the complete length of the tape marked in 1/8 inch increments. To keep accuracy close to precision, the tape is made of a non-stretchy material.
Our "tape" measure also needs to be made of a non-stretchy material, but we can use something much cheaper and more compact than metal, cloth, or fiberglass tape if we don't have to make readable marks on it at the resolution. The trick is to combine a resolution-marked short measuring tape with a less-frequently-marked longer tape or cord -- here we'll use fishing line.
The materials you'll need:
1. An actual tape measure that is way too short. The tiny less-than-$1 ones are usually between 1m and 2m length.
2. A spool of fishing line. To make the line show better for photos, I used 30-pound-test for the unit shown -- don't do that! Cheap 4-pound-test is plenty strong, not very stretchy, fits a much longer length, and is even easier to tie. A spool of 150 yards (450 feet) of 8-pound-test can cost as little as $1.
3. An end for the line. A golf tee or a screw is a good choice if you'll be using this to measure things on the ground; a bent paper clip is a better simulation of the end of a regular tape measure.
If the spool of fishing line you got is one of those flat ones that fits in your pocket, you might be able to bypass step 4 and stow your tape measure within that spool. Otherwise, you'll need to make a spool using either cardboard and scissors or wood and a scroll saw -- as step 4 describes.