Thursday, June 30, 2011

The Sensitivity of a Spirit Level

The accuracy with which a platform can be leveled is a limit on the precision of the determination of one's latitude and longitude. I included some adjusting screws on my platform to help level it with a spirit level. So one might ask how well can one level a platform?

To check the level of something one places the level on a surface and notes the position of the bubble between the two pairs of parallel rings. One then then rotates the level lengthwise 180° along the same line and again notes the position of the bubble. If the bubble has moved from its former position then this is an indication that the surface is not level and is in need of adjustment. The double rings on each side of center are spaced 2mm apart. They can be used to determine the sensitivity of the spirit level. Raising one end of my 2ft spirit level 5/32th of an inch will move the bubble 2mm from the inner ring to the outer ring. This corresponds to a change in slope of 6.5mm per meter which is one measure of the sensitivity of the level. Another measure is the angular change which moves the bubble the same amount or 22.4 minutes of arc per 2mm shift in this case.

The precision that one can level something in a particular direction is less than the 2mm separation since one can position the bubble more accurately. The platform needs to be leveled in two directions about 90° apart. So it may be possible to measure angles to within about 5 minutes of arc.

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