I posted this before, but the search engine isn't pulling it up, so here it is again:
"Phase-shift laser rangefinders" measure distance along a line of sight, and this gives them much better range and accuracy than ultrasonic or infrared rangefinders and proximity detectors. You can find lots of them for less than $100 on Amazon, but I'll just mention the specs on one as an example of what you can get for around $130: the Bosch DLE50. It's tiny (175g or 6 ounces, including batteries), has a range of 5cm to 50m (2 inches to 55 yards), and is accurate to within 1.5mm (0.06 inches) at a range of 30m or less. Lasers are notorious battery drainers, but that's not a killer: you can use other sensors until you need the greater accuracy and range. An annoying caveat is, I haven't yet found a cheap one with a data port, and the only one I've found with Bluetooth is over $600. But I expect this flaw will be remedied soon.
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