Robin Bodley conducted an investigation into the relationship between observer elevation and zenith angle to the sea horizon.  90° was subtracted from each angle to get the dip angle.  Data is plotted on a graph to visualize it.  The red curve represents the expected dip angle on a spherical earth with no atmosphere while the green curve has a correction of 0.13 for atmospheric refraction.

Each angle is the average of four sets of observations in both face left and face right.

The theodolite used is a Wild Heerbrugg Switzerland T2-55209, made in 1956.

The details of the survey marks can be found here:  http://apps.linz.govt.nz/gdb/

• CV3R 29.0m 0.16575°
• B9CG 41.2m 0.1895°
• A2MD 101.4m 0.2844°
• 8102 121.5m 0.33024°
• A27E 153.2m 0.36413°
• 8118 179.6m 0.4005°
• A2LL 181.9m 0.40413°
• 8106 196.4m 0.4167°
• BDJ9 226.6m 0.44167°
• B0P5 242.6m 0.45542°
• A2FP 275.9m 0.4863°
• A2LP 300.3m 0.504°
• A3Y3 353.7m 0.55514°
• 1212 446.6m 0.61281°
• A2M8 496.7m 0.6558°