stabilize hill slopes. The helix consists of one or two 20-cm-diameter propellers welded to the base of a thick steel bar which is driven clockwise into the ground by a hydraulic torque converter attached to a backhoe. Additional 5-cm-square, 3-m-long steel bars are added as the pier pulls itself into the subsurface until the assembly refuses to go further, or until the required depth has been reached. The piers can be driven at any angle and can be withdrawn anticlockwise and repositioned if they encounter large boulders. The ends of three or more steel inclined piers are welded together to form a stable subsurface tripod. We have used this method to anchor creepmeters on each side of the Hayward fault in California using the geometry shown in Figure 5 but terminated 1 m below the surface. A symmetrical quadrapod or tetrapod is desirable for above ground attachment to avoid thermoelastically induced lateral displacement of the control-point.

The monolith shown in Figure 6 consists of a reinforced steel concrete monument poured in place within a 15-30 cm diameter borehole. The center of the monolith hosts a 7 cm diameter plastic inclinometer lining. Two orthogonal grooves permit biaxial tilts of the monolith to be monitored throughout the column (Bilham 1993). As discussed above, this form of monument permits the long term stability of the monument to be monitored to sub-mm precision, and also permits estimates of the effectiveness of noise suppression as a function of depth to be evaluated.

The stability of all of the monuments illustrated is no better than the stability of the deepest part of the monument, and in general the stability of the surface attachment point relative to the base of the monument is assumed to be adequate (sub mm). In principle several methods may be borrowed from strainmeter and tiltmeter technology for monitoring surface motions relative to the subsurface.

For example, the inclinometer monolith automatically provides a measure of lateral offset and can be included in both of the buried multiple anchors. An example, from the Hayward fault, is shown in Figure 7. At Point Pinole the central helical pier (7 m deep) was constructed from 8 cm diameter steel tube instead of 5 cm solid steel bar. An inclinometer tube was cemented within this hollow central pier, and two inclined solid helical piers provided short term lateral stability of the attachment point as shown in Figure 6. Despite operating in a thick clay deposit of variable moisture content, the biaxial inclinometer data reveal that the tops of the piers have moved less than 0.5 mm during their first 18 months of operation.

Towers and buildings are sometimes utilized as GPS monuments because no alternative sites are available with a suitable combination of security and sky coverage, or because telemetry or recording facilities are more easily available from these locations.

The inclinometer method can be extended to control points that are currently installed on the tops of existing buildings. Concrete, or brick structures, even those that after centuries show no signs of collapse, undergo cyclic thermal and hydraulically induced deformation whose amplitudes are often difficult to assess. An inclinometer tube attached to the building can monitor seasonal and secular tilt of the building more accurately than most geodetic alternatives, and in some circumstances it may be possible to insert an inclinometer tube from the control point to levels tens of meters below the basement of the building (Figure 8). Sub mm monitoring accuracies can be anticipated from buildings in which high frequency tilt noise is absent. The method is

Front Matter (R1-R6)

Contents (R7-R10)

I Workshop Summary (1-2)

Executive Summary (3-5)

1 Introduction (6-9)

2 Networks and Data Sources (10-13)

3 Remote Sensing of the Atmosphere (14-17)

4 Dynamic Positioning and Navigation (18-20)

5 Static Positioning (21-22)

II Workshop Proceedings (23-24)

6 Civilian GPS Planning and Policy Making in the Federal Government (25-39)

7 Networks and Data Sources (40-109)

8 Remote Sensing of the Atmosphere (110-163)

9 Dynamic Positioning and Navigation (164-200)

10 Static Positioning (201-226)

Appendices (227-227)

A Statement of Task (228-228)

B Biographical Sketches of Steering Committee Members (229-230)

C Workshop Attendees (231-233)

D Working Group Participants (234-235)

E Workshop Agenda (236-239)