A lake can be used as a large natural tiltmeter by measuring changes in water-surface elevation relative to nearby stable benchmarks at two or more sites around it. Differential gaging systems can be either permanently installed or portable. The precision of either type of system is directly proportional to the size of the lake; tilt resolution increases with lake size. Permanent systems can accurately measure water level to within 0.5-1 mm, the equivalent of 0.5 to 1.0 microradian on a small (1-km wide) lake. Although portable systems can attain a similar precision on larger lakes, their precision on a small lake is estimated to be 1-3 microradians, depending on environmental conditions at the time. Factors such as wind conditions, seiche, instrument precision, temperature and barometric effects, and bench-mark stability need to be taken into consideration when gathering lake-level data.
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