Second Order Differential Leveling
Leveling standards are being addressed at the office. We use Topcon DL-502 levels. I found a September 2006 CALTRANS publication (Chapter 8 - Differential Leveling Survey Specifcations). We do Second Order work, not sure if Class I or II. We compute misclosure using a k factor of 0.035'. I am mostly interested in maximum sight lengths and minimum ground clearance of sight lines. I was told to keep the maximum sight lengths at 250', which I think the boss meant 260', because he likes the idea of 10 BS and FS readings (5 turns) per mile (approximately). This distance exceeds the 230' in table 8-1. Is this also a federal standard? The boss is less particular about a minimum ground clearance. The 1.6' in the table seems a bit much to me, but I think the microclimate surrounding a level rod is being considered. Any thoughts on this subject would be greatly appreciated!
linked below are the FGCS digital leveling specifications:
We do a lot of second order leveling, mostly class II but some class I. First of all, you MUST use an invar rod. And the level must be capable of second order. We limit sight lengths to 70 meters for class II and 60 meters for class I, as per the spec. If you want to submit the leveling to NGS, there are two other differences between the two classes: for class I the rod must be calibrated, and temperature profiles are required. The latter means that you must have two thermistors on the tripod, high and low, and record the values for each sight. To me this requirement only makes sense for a long linear line, where the error can accumulate if going uphill or downhill. But we have to do it even for small area leveling if it is to be bluebooked, and we have a monitoring job we do every year over a 2000 foot deep salt mine where we have to bluebook it every fifth year.
And yes, a line of sight skimming the ground IS definitely a problem, the heat coming off of the ground will bend the line of sight upwards a bit.
You can be fooled with loop closures if you close back to the POB, errors can accumulate going up hill and then de-accumulate coming back down. This is greatly minimized when using a one piece invar rod.
The NGS offers a class on geodetic leveling to FGCS standards. Lots of good presentations are linked from the class web page here:
Screen capture of class page with details about class:
There is a class scheduled for December 2020 at the Instrument and Methodologies Branch near Corbin, VA. Beautiful area. Everyone I know that has taken the class spoke highly of it.
had a 14 m difference in elevation in about 100 m
Was that error perhaps 14 mm?