# How Surface Volume (3D Analyst) works

Surface Volume calculates the projected area, surface area, and volume of a surface relative to a given base height, or reference plane. The surface can be a raster, TIN, or terrain dataset. The results are written to a comma-delimited text file.

If the Input Surface is a TIN or terrain dataset, each triangle is examined to determine its contribution to the area and volume. The sum of these parts is used for the output. If the Input Surface is a raster, its cell centers are connected into triangles. These are then processed in the same fashion as the TIN triangles.

The Output Text File is a comma-delimited ASCII text file to which the results are written. If the file already exists it will be appended to. The first line in the file contains the field headers. These are "Dataset, Plane_Height, Reference, Z_Factor, Area_2D, Area_3D, Volume". Subsequent lines contain the actual values.

The Plane Height or {base_z} in python represents the height of the horizontal reference plane from which calculations are derived. This value should be given in factored units. It will not be multiplied by the Z Factor. The default Plane Height is determined by the option used for Reference Plane. If the Reference Plane is set to ABOVE then the Plane Height is set to the minimum height of the surface. If it is set to BELOW, the default Plane Height is set to the maximum height.

The area and volume are calculated between the reference plane and the surface. The Reference Plane argument determines whether these calculations are performed above or below the plane. Use ABOVE or BELOW keywords to specify which option to use. The default Reference Plane is ABOVE.

When setting the Reference Plane to ABOVE, the projected area and surface area are calculated for the portion of the surface above the given Plane Height. The volume represents the cubic area between the plane and the underside of the surface.

The following two examples depict a Reference Plane set to ABOVE and the Plane Height value that sets the plane below and intersecting the surface.

When setting a Reference Plane to BELOW, the projected area and surface area are calculated for the portion of the surface below the given Plane Height. Volume is the cubic area between the plane and the top of the surface. The following examples depict a reference plane set to BELOW.

The following two examples depict aReference Plane set to BELOW and the Plane Height value that sets the plane above and intersecting the surface.

The Z Factor is used to convert z values to another unit of measure (e.g., feet to meters}. The surface heights are multiplied by this value. For the results of this tool to be correct, xy and z units have to be the same. The tool will attempt to determine a proper Z Factor. It can do so if there is a spatial reference, the coordinate system is in projected units (i.e., not geographic), and xy and z units are all defined.