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Chuck Varney:

Thanks for those pages on calibration of a level like this. Interesting material! That "tape on trees" method sounds like it would aim for roughly one-inch placement accuracy at a range of 30-50 feet. Going with the higher end, that implies an angular ratio of 1/(50×12) or 1/600. Multiplying by 3438 (the "magic number" --really just 60×180/pi-- to convert ratios to minutes of arc) yields roughly 6 m.o.a. Sounds about right... Five to ten minutes of arc seems like a reasonable maximum accuracy for a basic bubble-level device like this.

One of these days I want to set up a really long fluid level: maybe a 75-foot garden hose connecting two open-top reservoirs, transparent if possible. Fill the hose with water adding enough to get the reservoirs about half full too, get rid of bubbles, and the two reservoirs will automatically reach the same level. Stick a marker (piece of tape?) on each reservoir at the fill level, and then drain the water. Now we can sight along it as a perfect line to the true horizon whenever needed. That should be accurate to about a quarter of an inch at 75 feet range. That's a ratio of 1/3600 so very close to one minute of arc angular accuracy. Sounds like a backyard project for next week. :)

Frank Reed