Actually -I am a few thousand years  late  on defining it </p>

 datum (singular )  and  data  (plural)</p>

 Data springs from a datum-point of origin  etc..</p>

 On my kits  from a few years back - I always  made a hotizontal line  such as the canopy floor , which all other  measurements  were tied to.</p>

 Using the  wing as adatum line creates all sorts of  problems </p>

 Being exceedingly lazy -I always tried for  the  most  simple approach.</p>

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