if i am correct, the right way to measure a bike is to measure from center of BB to the center or top of the top tube, correct? does this follow a vertical line directly above the bottom bracket, or the length of the seat tube? how does it change things up when you start measuring bikes with "compact frames"? do you still measure the frame the same way? seems like this could cause some confusion, you might end up purchasing the wrong size frame if you only went by knowing your frame size. say you rode a 52cm, and you bought a 52cm frame, but it ended up being a compact frame, with a longer top tube, essentially being a 54 or 56cm frame.
i hope this question makes sense and i phrased things correctly, any help would be great.
follow allong the seat tube. on a frame with a horizontal top tube you stop where the ST and TT intersect. on a compact frame you need to stretch a string from where the TT and the HT intersect, to the seat post. the string should be level. mark that point on the post. starting at the center of the BB, measure up the ST to that point. boo ya!