Is there a standard distance, or a standard percentage of a distance, that BS and FS distances should be balanced within on a third order level run?
Is there a standard distance, or a standard percentage of a distance, that BS and FS distances should be balanced within on a third order level run?
Any good Surveying textbook has the info you are looking for.
Standard distance depends on your eyes, magnification factor of the lens, and density of the air. In my area (PNW), 300 ft is a good distance, any longer than that and you are struggling to decipher what you are reading. Alaskans could go longer, Texans would go less. (Interestingly enough, digital levels seem to use about the same distances for reading)
The balanced leg method was originally published as a solution to poorly calibrated levels (failed peg test), so if you perform a peg test prior to level runs, balanced legs are less of a worry.
Variation of legs (per previous posters link) is 10 meters, which is in the realm of 10% of what I consider a typical max length of leg. You can do the math and determine level of possible error and length of leg vs variation of leg length and see what is acceptable.
Be aware that You will definitely encounter situations where you cannot meet stated methods (leveling across river for instance), and you should be able to note and revise your methods to adapt while still meeting acceptable level of accuracy.
Besides possibly out of cal instruments, a major reason for balanced sights is that refraction is hard to pin down precisely.?ÿ You need to calc refraction and earth curvature for any serious leveling but equal sights cancel those out.
The balanced leg method was originally published as a solution to poorly calibrated levels (failed peg test), so if you perform a peg test prior to level runs, balanced legs are less of a worry.
Less of a worry, but significant nonetheless for anything but casual leveling. Although I don't have hard data to back this up, I suspect that one of the reasons I get such tight closures with my digital level is that it makes balancing sight distances within a few centimeters so fast and easy.
Besides possibly out of cal instruments, a major reason for balanced sights is that refraction is hard to pin down precisely. You need to calc refraction and earth curvature for any serious leveling but equal sights cancel those out.
I somehow wiped out the 'like' button, so here you go; Like
Interesting graphic, I should have bookmarked, was a display of the magnitude of atmospheric refraction through a typical day.
Bill93 don't forget about going up and down hills. If going uphill your backsight will be high on the rod but your foresight will be no lower
than 0.5 meters?ÿ and therefore your refraction will be different, even if you balance the distance between the rods.
?ÿ
JOHN NOLTON
True, but your sights are likely to be short because of the grade, so refraction differences will be immaterial except for the most precise of work.
On the matter of correcting for refraction and temperature gradients, see:?ÿ http://adsbit.harvard.edu//full/1979IAUS...89..305H/0000312.000.html
Another good source on the issue of refraction see:?ÿ https://www.ngs.noaa.gov/PUBS_LIB/ResultsOfLevelingRefractionTests_by_NGS_TR_NOS92_NGS22.pdf