Open the function editor, which should be empty. Add a Lenght node to the Position Input and connect it to the altitude output. Activate the Function Node Preview from the top tool bar.

It will display what the Length Node actually does. It measures the distance and returns a value (In this case an altitude value, which corresponds to the distance, length or whatever you call it)

At the point of origin the value will be 0 and gradually increases with increasing distance to the origin.

You can see the result directly on the terrain. You should see a reversed cone.

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