Tree and building height from the tilt angle

It depends almost entirely on the angle to the base, and the page shows the number rather than making you guess. With the phone at 1.5 m and a steady hand of about half a degree, sighting a base 45° below the horizon gives a distance good to roughly 2%; at 10° it is 5%; at 2° it is 25%. Height inherits that and adds the top sighting. In practice this means a tree at fifteen meters measured from ten meters away comes out within a few per cent, and the same tree measured from eighty meters away comes out within a quarter — which is why the page keeps telling you to walk closer.

Because the triangle starts at the phone, not at the top of your head. If you hold the phone at your eyes, that is a few centimeters below your height; if you hold it at chest level it is more. The whole measurement scales with this number, so an error in it is an error of the same percentage in every result: three centimeters out of 1.5 m is 2%, and ten centimeters is 7%. It is worth being right about, and it is worth measuring once with a tape rather than estimating each time.

Yes, and that is the real limit of the method. The distance comes entirely from the assumption that the base is at the same height as your feet; a tree at the bottom of a slope will read further away than it is, and one up a bank will read nearer. On sloping ground the height figure goes wrong with it. If you cannot get onto the same level, the honest options are to measure from somewhere you can, or to accept that you are measuring something else.

That the crosshair was pointing slightly upwards when you sighted the base, which cannot happen for something standing on the ground you are standing on — your eye is above the ground, so the ground is always below the horizon. Usually it means you sighted the wrong thing: the bottom of a wall hidden behind a hedge, a dark doorway rather than the pavement, or the object is actually on higher ground. The tool refuses rather than returning a negative distance.

Because that measures an angle too, only worse. Turning pixels into an angle needs the camera’s focal length, which browsers do not reliably report, and it needs an assumption about lens distortion that phone cameras violate near the edges of the frame. The accelerometer, by contrast, is measuring gravity, which is the same everywhere and needs no calibration. Pixels are the right tool when there is a reference object of known size in the shot — that is a different measurement and a different page.

No. The base sighting is the whole method, and it needs a base — a point where the object meets ground at your own level. A boat on a lake, a cloud, the far bank of a river: all of them have a visible bottom edge that is not on your ground plane, and a reading taken there would be a confident number with nothing behind it. If you know the object’s height already, the two-angle method can be run backwards to get the distance; that is a calculation, not a measurement, and this page does not pretend otherwise.

Some phone cameras expose the distance they are focused at, and where they do it is a genuine short-range rangefinder — useful from a few centimeters to a few meters, which is exactly the range where the tilt method is useless. Two warnings. It only reports while the camera is actually focusing on something, so it often returns nothing. And the specification does not say whether the number is meters or dioptres, and implementations differ, so the page states which one it read rather than converting silently in the wrong direction.