| Year | 1965 | 1970 | 1975 | 1980 | 1985 | 1990 | 1995 | 2000 | 2005 | 2010 |
| Number of years since \(1960 ( t )\) | 5 | 10 | 15 | 20 | 25 | 30 | 35 | 40 | 45 | 50 |
| Reduction in thickness since \(1960 ( h \mathrm {~m} )\) | 0.7 | 1.0 | 1.7 | 2.3 | 3.6 | 4.7 | 6.0 | 8.2 | 12 | 15.9 |
| Upper surface | Lower surface | ||
| \(x\) | \(y\) | \(x\) | \(y\) |
| 0 | 0 | 0 | 0 |
| 4 | 1.45 | 4 | -0.85 |
| 8 | 1.56 | 8 | -0.76 |
| 12 | 1.27 | 12 | -0.55 |
| 16 | 1.04 | 16 | -0.30 |
| 20 | 0 | 20 | 0 |