points of the test section should be maintained while the baseline is measured. Use stakes for marking
the test section; they should be braced in all four directions.
b. The spring balance used when stretching the tape should be tested and set to read correctly using
the 15-kilogram test weight, as weighted at the NBS. Record this test before and after each day's work,
at midday if possible, and more often if the temperature range is greater or if it is suspected that the
position of the dial pointer has changed. There is an adjusting screw alongside the drawbar by which the
dial can be reset with a screwdriver.
c. The special tape thermometers are tested at the NBS before being sent to the field to ensure that
they are accurate to within 0.3, centigrade. Although no thermometer standardization correction is
required in the field computations, use care in reading the thermometers to avoid thermometer lag,
which is usually caused by rapid changes in temperature.
d. At least three tapes will be used in measuring each baseline. The portions measured with each
tape should be about the same length. To accomplish this, divide the total length into 1-kilometer
sections (one section may end up being longer or shorter). Group these sections in three divisions of
about equal lengths. Each division must begin and end on either a kilometer mark or terminal station.
Use a different pairing of the tapes on each of the three divisions in order to secure a complete
intercomparison of the lengths. Run each tape forward on one division and backward on another. Only
one forward and one backward measurement of each section should be made, unless the discrepancy
between the two exceeds 10 millimeters √K (where K is the length of the section in kilometers), in
which case the measurements are repeated, preferably with the tapes originally used, until the forward
and backward measurement of the section agrees within the limit.
3-3. Baseline Alignment. The alignment of the stakes (or tape ends) should be done with care so that
the distance introduced does not exceed 1 part in 500,000. This requirement can be met by adhering to
the following limitations:
No marking strip or point marking a 50-meter tape end should be more than 2.5 centimeters
out of line between the two adjacent marked points.
The 2.5-centimeter tolerance should be decreased proportionally for distances less than 50
No point between terminal stations should be more than 15 centimeters out of line.
A broken baseline (with more than one tangent) may be used where topographic conditions
demand, provided the terminal stations are intervisible and the angles at each break and end station are
measured to form a closed polygon. For all orders of baselines, no portion should deviate from the final
projected line between terminal stations by more than 20, and, for first order, should be held to 12, or
below when possible. The use of the broken baseline should be kept to a minimum, since it requires
making angle measurements and adds another possibility for error.