Reverse Engineering
The draughtsman's job is primarily to create an object, or objects that do not exist, produce technical drawings, and issue them for manufacture, however it is often required to measure parts that already exist, and turn them into 2D drawings or 3D models. This is referred to as reverse engineering.
 Reverse engineering may be required for many reasons, such as an assembly that uses 'off the shelf' components, as the balustrade example below illustrates.
In this instance, I measured an actual glass adapter from Q-railing and made it into a 3D part so that they could then be inserted into the staircase assembly. 
I model parts like this on a regular basis and store them on my computer for future use. I have a large, continuously growing catalogue of reverse engineered 'off the shelf' parts, which may mean that the modelling of your next project is already partly done.
There are many reasons to reverse engineer items. To replicate things that are no longer made, to amend 'As Built' drawings if things were modified on site or in the workshop, to catalogue components, to apply for retrospective planning permission, to reproduce hand made or bespoke items etc etc.
I own a selection of precision measuring instruments that allow me to measure and virtually replicate any item ranging from miniature items at a fraction of a millimeter, to full scale buildings and structures.
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