TIDAL DATUM PLANES 25 
Fig. 10.—Standard tidal bench mark, United States Coast and Geodetic Survey. 
stand frost action. A satisfactory mixture for the concrete consists of one part cement, 
three parts sand and five parts gravel or broken stone. 
If standard disk bench marks are not available, a small cross cut on a rock, build- 
ing, or other structure may serve the purpose of a bench mark. A copper bolt set 
into rock or into a cement block makes a satisfactory bench mark. Water hydrants, 
curbstones, and nails in growing trees, while frequently suitable for temporary use in 
leveling, do not make satisfactory permanent bench marks. 
The bench marks and zero of the tide staff should be connected by a double line 
of levels, the lines being run in opposite directions. The instructions of the Coast and 
Geodetic Survey prescribe that when the forward and backward differences in the ele- 
vations of two tidal bench marks differ by more than 0.035-/K feet (K being distance 
in statute miles leveled between the two bench marks), both the forward and backward 
leveling between these two bench marks are to be repeated until the difference falls 
within the required limit. It is important that the leveling record and the descriptions 
of the bench marks be made in such form that no ambiguity will arise in the use of such 
records at future times by other persons. 
For convenience the following table of maximum closing error allowed in leveling 
between bench marks is given. It is based on the formula given above. 
TaBie 1.—Mazimum allowable errors in leveling between bench marks 
Distance Maximum 
between error 
. Ms. allowed 
(feet) (feet) 
EEA OX )OCay EY ENSTS em ce ay mE et RE ST SO eae ee OE 0. O11 
TODO see Bs) ea So eT gar Nees Geom US, Re dope ries 2 a O15 
DN DO) secs as Ee Oe, RI A A Ae NR NS Oi eg ee . 021 
BODO) _ SoS Seance gS gat cece ts ans DOR oR eM 2512s A aR dg . 027 
ARVO) marca ea yee teagan Le! SOP WS BU arehays ate Oe ae i SgENG Yh 3 Lyi! . 030 
EB (OCD en ss wh aig a is CEN a i pe ae ee . 034 
949995—51——_3 
