So you would like to measure things on your smartphone more accurately? The Arrim ONE has you covered. It attaches to your phone and lets you measure straight lines, circles, angles, and curves. Arrim ONE has an on/off button, and most of its features can be controlled on your smartphone.
The device is accurate to 1.5mm within 20 meters range. It weighs only 35 grams and has a 100mAh battery good enough for 100 measurements. Works with iOS and Android devices.