How Create TIN (3D Analyst) works
Create TIN instantiates a new triangulated irregular network (TIN) and saves it to disk. Its extent, also known as a domain, and coordinate system are based on a specified spatial reference. The spatial reference can be taken from an existing geodataset. Specify a geodataset with an extent that's large enough to contain all the data you intend to add to the TIN. It should not be significantly larger than what's actually needed. You can add feature data to a TIN upon its creation using the Create TIN tool. Otherwise, features can subsequently be added to the empty TIN using the Edit TIN tool.
When at a command prompt, you provide the name of an existing geodataset for the spatial reference argument.
When using the Create TIN dialog box, you can browse for a spatial reference using the button next to the Spatial Reference input textbox. Choose Import on the Spatial Reference dialog box to browse for an existing geodataset to use as a source. It's not advised that you use the Select button to choose one of the predefined coordinate systems because their domains might not be appropriate.
TINs used for surface modeling should be constructed using projected coordinate systems. Geographic coordinate systems are not recommended. Functions such as slope, volume, and line of sight can produce misleading or incorrect results when x,y units are angular (for example, decimal degrees).