how to calculate r-tree's height?

i have a deep doubt about r'tree's height.

if leaf node : N, fanout : m

then r'tree's max heig开发者_如何学Pythonht : log m (N) - 1 ?

why this differs from b'tree?

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Leaf pages have approximately the double capacity of directory nodes when storing point data: for a rectangle you need to save max and min for each dimension.