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The following HTML text is provided to enhance online readability. Many aspects of typography translate only awkwardly to HTML. Please use the page image as the authoritative form to ensure accuracy. 
FIGURE 6 Schematic views of monuments discussed in text.
MONUMENT DESIGN
It has been known for many years that increased surface noise suppression is obtained by increasing the measurement length of a strainmeter or tiltmeter. Instruments with dimensions of the order of 1 km out-perform instruments <1m long largely because the signal is increased by a factor of 1000 relative to the random-walk noise-level of an attachment point. The same principle suggests that a 10 m wide attachment point will obtain a factor of 1000 improvement in position stability relative to a 1 cm diameter rod driven into the surface. Although this assumption is reasonable, it may be inappropriate if noise signals with wavelengths less than 10 m are absent. Flawless crystalline rock can rarely be surpassed for control point stability (Figure 6) and a marker embedded in rock is typically subjected only to thermoelastic motions. In general, for soil installations, it is essential to increase the dimensions of a control-point to several meters at least. If a large slab of concrete is used as a monument in unconsolidated sediments its stability increases with area as long as it is rigid. However, a tower constructed on a concrete pad will amplify motions of the pad if the foundation is short compared to the height of the tower. Because thermoelastic and moisture-induced tilt variations in short platforms have been observed to exceed 100 microradians daily, diurnal movements at the top of a 10 m high tower would exceed 1 mm. To avoid amplifying surface noise, a rule-of-thumb is that the foundation must be rigid, and must be as wide as the tower is high. If a large horizontal foundation is impractical, and deep unconsolidated materials are present, the foundation should extend below ground as far as the tower extends above ground. Three approaches to anchoring a control point at depth are shown in Figure 6 A summary of other methods can be found in Bell and Bryant (1992). In a popular method pioneered Wyatt (Langbein et al. 1995) four inclined rods are cemented into drilled holes radiating from a central rod to which they are welded approximately 2 m above ground. The monument is suitable for installation in rock or unconsolidated materials and is preceded by drilling open holes into the base of which cement is injected, and the steel rods inserted. Collars isolate near-surface layers from the completed monument. Another promising method suitable for clay soils employs helical piers. Helical piers have been used for many decades to support bridges across rivers, and to
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