Hi David
It's difficult for any survey to pinpoint the correct base, the correct height, hence the correct slope of a pyramid if the outer casing has gone walkabouts. You could also argue that regardless of the debris around the base Petrie sighted correctly.
The missing casing always leaves a sour taste in the mouth for there is no way of telling where it came down to meet the pavement. Petrie argued for the four sockets but we don't know where the casing sat upon those four depressions or if any adjustments were made before the casing took shape. Petrie admitted that there was no way to know the thickness of the casing to the upper parts of the pyramid so all in all it's really a casing problem.
I may be wrong but I prefer to stick with one survey and one alone. However tempting it might be I cannot bring myself to pick and choose measures from various surveys to support this proposition or that proposition. If I cannot rely on Petrie to solve my problem then it won't be solved. It's as simple as that.
Petrie wrong?
According to who / what? He made some blunders with figures but that's really a transferal problem, notes to notes to print. All up he's probably 99.9% perfect.
I like the idea that pyramids continue to crumble and so too the measures. Modern measures are therefore not as good as those of a hundred years ago, at least not when we consider that Petrie had some astonishing new tools at his disposal including the theodolite.
If we are looking for evidence of slope then I ask: How can we possible pass up the casing stones still "in situ" Have they moved? No. Have they deteriorated due to sun, wind, rain and sand? Possibly but we have a margin of error already in place and it's difficult to see it being extended one way or the other.
Graham