When you enter the Great Pyramid, you are not coming the way it was designed. You are coming in through a tunnel made later; Al-Ma'mun's tunnel. In this photo, the staircase is modern. It leads up to the Ascending Passage which was not accessible after the Great Pyramid was originally finished. The Plugging Blocks can be seen up to the left. There are three and you are looking at a partial third block as it abuts against the second one. The first one is not visible and that is where the problem begins. Without knowing where exactly the A.P. begins, there is no way to related the Inside of the Great Pyramid which all begins right here, to the outside which can only be connected through this later tunnel or the Entry Passage below which can be seen through a hole in the floor, also a later change. That hole is now gated.
This is where Stephen Brabin asked a simple question: How exactly did Piazzi Smyth connect the inside to the outside? Stephen noticed a problem and so he did his own measurement connecting the joint between the plugging blocks, the lower end of it on the floor of the A.P. and the floor of Al-Ma'mun's tunnel, the artificial corridor created through which visitors enter the Great Pyramid nowadays and where these photos are from.
And what did Stephen find? Piazzi Smyth, and thus Petrie, were off by >0.60 meters horizontally and 0.29 meters vertically. Why? Because Piazzi Smyth did not stand exactly due west of the plumb bob line. In Stephen's reconstruction, he stood south of due west. Consequently, his calculations overestimate the distance from the north base both of this joint and the spot on the Entry Passage below the head of the plumb bob hanging through the gated hole.
What Stephen has taken this to mean is that everything inside the Great Pyramid must be positioned circa 0.64 meters more north than previously assumed. In other words, all interior distances relative to the north base of the Great Pyramid are incorrectly too long by that much and also too high by 0.29 meters. This is not a small distance: 1 cubit ¼ sideways north and ½ cubit down. In inch, 25.2/11.4.
If the error is real and his data are accurate, Stephen's way of looking at the error correction is one way to interpret this. But the other way is that everything to the north of the Plugging Block joint is simply overestimated and too long, which means the correction which must be made is to shorten all distances from the cased north base to the vertical line of the plumb bob hanging from the floor of the A.P. where the second and third plugging blocks meet.
Since it is unlikely that Petrie made a mistake measuring the distance from the spot on the floor of the E.P. under the head of the plumb bob to the current mouth of the Entry Passage in the core masonry, he must have over-estimated the distance to the casing which no longer exists. How? By assuming a pyramid angle which is steeper than was actually built at the level of the Entry Passage.
How much steeper? The "Smyth error" at 0.64 m/0.29 m resembles a Two/One angle and so this may be the source of the problem Jean-Paul and I encountered. In other words, while our design model predicts a theoretical E.P. height of 32 and actual of 31.86 r.c., Petrie has 32.4 and Petrie's reconstructed corridor to the below-pavement fissure (1569 inch E; 1555 inch W) compared to ours to the pavement (1,475.45 inch) could be off by just about that amount, i.e. ~26 inch.