Correction to Section 92:

Petrie unfortunately erred in his computation of the azimuth of the direct distance from the center of the second pyramid to the center of the third pyramid. In the following revised table, the corrected azimuth is highlighted:

This error was discovered by Stephen Brabin, who noticed that Petrie's azimuths and direct distances, which supposedly describe the sides of a triangle, do not actually form a triangle.

An analysis was then undertaken in which the figures in the above table were recreated from Petrie's original survey data as recorded in Chapter 5, "Co-ordinates." During this process, the nature of his error became apparent:

1. The survey data recorded in Chapter 5 are reckoned from an assumed north. The azimuth of true north on Petrie's system of coordinates is east of his assumed north by + 1º 12' 22" ± 6".

2. The figures in the section 92 table are relative to a coordinate system that is parallel to the average azimuth of the first and second pyramids, which lie approximately - 5' (or west of) true north.

3. A conversion from the coordinate system of the original survey to a coordinate system aligned with the pyramids therefore requires that both of the above azimuth corrections be employed—which Petrie did, quite correctly, but only on the first two rows of the table. On the third row, rather than adding the east correction and subtracting the west, he mistakenly reversed the signs, subtracting east and adding west, thus yielding the incorrect value of 34º 10' 11".

Quod erat demonstrandum.

I am indebted to Stephen Brabin for both discovering the error and producing the final detailed analysis, thus demonstrating not only Petrie's error but also the correctness of the remaining figures in the table. Stephen's spreadsheet analysis may be downloaded here.

— R.B.

© 2005 Ronald Birdsall. All rights reserved.