Create Simple surfaces in AutoCAD and measure volume within these surfaces

I have successfully created a surface in autoCAD from contours lines (top of model). I want to create a second surfaced composed of a single rectangular layer (already created by a polyline).  

My question is

1) how do I convert this simple pline into a surface
2) How do I measure the volume found between these two surfaces?

I am a beginner so step by step is much appreciated.
Who is Participating?
silverkornConnect With a Mentor Commented:
when you say you used AutoCAD to create the surface are you using Land Desktop or Civil 3D software?
saunaGAuthor Commented:
The Pline is a closed polygon by the way
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