Hi.

I've been reading in ISO 5459:2011 about datums and datum systems. It's very informative but somehow leaves me with more questions than I had before reading it...

I'll just post one of those for now.

So the standard explains establishing a datum system from a cylinder and a perpendicular plane, where the

Next,

the standard explains establishing a common datum from a plane and a perpendicular cylinder, where the

Why? Why does one use common datums in some cases? What would the difference be if the second example was also a datum system?

As far as I can see the end result is a straight line and a point in both examples. So what is the difference?

Anyone?

I've been reading in ISO 5459:2011 about datums and datum systems. It's very informative but somehow leaves me with more questions than I had before reading it...

I'll just post one of those for now.

So the standard explains establishing a datum system from a cylinder and a perpendicular plane, where the

**cylinder**is the primary datum**(A)**, perpendicular to the**plane**(secondary datum**B**).Next,

the standard explains establishing a common datum from a plane and a perpendicular cylinder, where the

**plane**is the primary datum**(A)**, perpendicular to the**cylinder (B)**. Only in this example it's a common datum (A-B).Why? Why does one use common datums in some cases? What would the difference be if the second example was also a datum system?

As far as I can see the end result is a straight line and a point in both examples. So what is the difference?

Anyone?

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