Techniques of Water-Resources Investigations (TWRI) Book 3, Chapter A19--Clarification of Leveling Procedure
 

In Reply Refer To:                                February 4, 1993
Mail Stop 415




OFFICE OF SURFACE WATER TECHNICAL MEMORANDUM NO. 93.12

Subject:  Techniques of Water-Resources Investigations (TWRI) 
          Book 3, Chapter A19--Clarification of Leveling Procedure

The purpose of this memorandum is to clarify procedures concerning 
the establishment of turning points as discussed in TWRI Book 3, 
Chapter A19, "Levels at Streamgaging Stations."

Instructions in the TWRI state on page 16, "Run the level circuit 
or series of circuits from the reference mark that appears most 
stable, turning on the other marks until the farthest one is 
reached.  Then, continue each circuit back to its starting point, 
turning on the same marks in reverse order."  These statements, 
and the examples in the manual, imply a requirement that the 
instrument be reset after each foresight, and a backsight taken to 
establish a new height of instrument (HI).

The instructions in the TWRI acknowledge, however, that side shots 
are acceptable, if the side shot is taken twice, and from 
different HI's.

The policy of WRD is that level procedure shall be considered 
acceptable, if a complete circuit is run such that at least one 
turning point is taken, and all marks are shot twice and from 
different HI's.  The two shots on each mark could be either two 
foresights from different HI's or a foresight followed by a 
backsight.  The circuit should be closed by a final shot on the 
base Reference mark to allow determination of error of closure.




                                 Charles W. Boning, Chief
                                 Office of Surface Water

WRD DISTRIBUTION:  A, B, FO, PO