Dimensioning to Midpoints
To dimension to a midpoint of a linear
edge in a drawing:
Click Smart
Dimension 
on the
Dimensions/Relations toolbar, or click Tools,
Dimensions, Smart.
Right-click the edge whose midpoint you want to
dimension, and click Select Midpoint.
Select the second entity to dimension, then click
to place the dimension.
You can also dimension to midpoints when you
are adding baseline
dimensions or ordinate
dimensions. Select the baseline or zero ordinate. Then follow step 2 to
dimension to midpoints as needed.
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