If Plumb is a line pointing directly to the center of the earth and Level is a line at right angles to this, then the line is a curve and plumb is only accurate at one point. However, for all normal purposes, a Plumbline with a heavy weight suspended in oil to prevent excessive swing is the most accurate, and a water level is most accurate over small distances. I allways use a "plumb rule" for upright. (a 6ft length of 4" x 1" yellow pine with a lead weight on a line hanging down the middle in a hole with the line following a gauge mark). The small vertical bubbles in a 6ft level can create quite large errors - half a mm on a 25mm bubble is 30mm at 6ft.
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My advice is check each vial for accuracy using the methods allready given. If you have one that is inaccurate blank it out so you don't mix them up.
There are also two types of surveying - plane surveying and geodetic surveying. Plane surveying uses a straight line of collimation and is fine for most work sites and jobs over hundreds of metres. Geodetic surveying uses the earths curvature and is normally only used for jobs that require high degres of accuracy such as bridge towers.
Of course it really all comes down to specified accuracy. Most job sites only need be 1:500 or even less. Large engineering jobs might be 1:25000 or more.
There are also two types of surveying - plane surveying and geodetic surveying. Plane surveying uses a straight line of collimation and is fine for most work sites and jobs over hundreds of metres. Geodetic surveying uses the earths curvature and is normally only used for jobs that require high degres of accuracy such as bridge towers.
Of course it really all comes down to specified accuracy. Most job sites only need be 1:500 or even less. Large engineering jobs might be 1:25000 or more.
My spirit level broke after I used it as a hammer 
H
holmslaw
Then adjust them, take off the covers and the phials can be adjusted.
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My spirit level broke after I used it as a hammer
Try using it as a crow bar until it's right again. Worked for me.
R
RedHerring2
I think this might show that even though two points may appear to be the same distance from the centre of the earth they may still not be in a level plane:
http://news.bbc.co.uk/2/hi/science/nature/8767763.stm
http://news.bbc.co.uk/2/hi/science/nature/8767763.stm
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