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In dialog box of volume surface calculation and also in ASCII file there are 2 out come with names 2D Area and 3D Area. What is difference between these two numbers? What is included in 3D that is not in 2D? For example :

 Plane_Height, Reference, Z_Factor, Area_2D, Area_3D, Volume
 45,00, ABOVE, 1,000000, 644929,78213426, 716708,47208592, 17433798,859008

Note: Decimal point is a comma.

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2D area is calculated from a flat surface in plan view (looking down). 3D area takes into account 3D characteristics of the surface. A 3D polygon with peaks and valleys will have a larger 3D surface area then a flattened "plan view representation" of the same polygon in 2D. Suppose you have a 3D polygon of a pyramid, the 2d area is the the pyramid's footprint (a square when viewed in plan view) whereas the 3D area a a sum of the areas of the 4 triangular sides.

As a side note; although the field name of the value does in fact say 3D_Area it is 2.5D area not 3D. This function calculates volume between a plane and a surface and ArcGIS only supports 2.5D continuous functional surfaces. ArcGIS treats all raster, tin and terrain as continuous functional (2.5D) surfaces. True 3D surface is a discontinuous surface (3D model) must be able to store more than one z-value for a given x,y location. To visualize the difference; continuous functional surfaces cannot render vertical plane (buildings fir example) whereas discontinuous surfaces do.

Not taking into account the recently introduced city engine technology which I know nothing about, ArcGIS can only render 3D surfaces and 3D models (fully enclosing a volume) as features in a multipatch feature class. You can use the Add Z Information tool to calculate the voume of a "fully enclosed" 3D multipatch feature.

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