drillers' depth

Here's a question: some people indicate that there are differences between the various sources of well depth information.  But for starters, how DO we actually measure driller's depth ?  What actual processes are used to arrive at whatever the bit depth is ?  And how sure are we of the value arrived at ?  Is this strapped pipe tally ?  Do we use laser pipe measurement ? Is this pipe tally with some sort of stretch correction ?  Do we use thermal expansion correction ? How do we account for tension and compression (if we account for this at all ?). What about buoyancy and pressure effects ? How do we reconcile LWD depths when running in and drilling ?  What sorts of uncertainties are accounted for ? I'd appreciate some different inputs to see what range of practices are actually in play.

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