3-4. Slope Measurements. The slope of a tape length in a baseline measurement should not exceed a
10-percent grade. Where this limit is approached or exceeded, use special care in determining the
difference in the elevation of the tape ends or stands and performing horizontal taping. Measure the
differences in elevation between the fiducial marks of the tapes using spirit levels. Obtain rod readings
at all tape end supports and broken-grade intermediate supports. Read both the front (meter) and back
(foot) sides of the rod at each point. If the available rods are graduated in only one unit, the spirit levels
should run forward and backward over the line. Make a comparison of the observed differences of
elevation for each tape length. Be careful when adjusting the level and when balancing the backsights
and foresights for the observations. A maximum discrepancy between differences of elevation of ...3
millimeters should produce the desired 1/500,000 accuracy in the determination of the slope correction
for slopes up to 10 percent.
3-5. Wind Effect. The error caused by the wind blowing the tape horizontally, such as errors in
alignment, inclination, and friction of the tape on the supports, tends to make the measured length too
long. No first-order or second-order, Class I baseline measurements should be made when the wind is
strong enough to bend the tape more than 2 centimeters out of line. The wind would introduce an error
of 1 part in 500,000.
3-6. Instruments and Appliances. It is possible to make satisfactory baseline measurements over
somewhat rough and uneven ground if provisions are made to properly support and stretch the tape.
a. The following instruments and appliances are useful in running the baseline:
Two marking scribes.
Two pairs of dividers.
One level, with rod.
Two plumb bobs.
Two 1/10-meter scales, boxwood, with a reading in millimeters.
One stretch apparatus for tape, complete and consisting of two staves with loops and tape
attaching the clip, two balances, and an apparatus for testing the balances.
Copper strips for stake tops, of the same thickness as tape, 20 per kilometer.
One 30-meter steel tape (standardized or tested in the field) for measuring setups and
One 50-meter steel or Invar tape (unstandardized) for marking out the baseline.