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Use Set Start Point
and Set End Point 
on the Point Cloud toolbar to position measurement points over the ends of the pipe. An example is shown below.
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Switch to the referenced piping sketch in the Detail window, and select Dimension > Dimension Pipeline Command
.
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Define a start point by selecting the section of the pipe to dimension. You can only select discrete points on the pipeline, such as the ends of a pipe or the ends or the middle of a component. The points can span only one length of unknown pipe.
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Select the pipe again to define the end point.
The software displays markers to indicate the span of the dimension, and displays an empty Dimension box.
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Drag Start/End Points Distance
from the Point Cloud Measurements window and drop it in the Dimension box.
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Select File > Save
to save your changes.