Similar in nature to the Drape and Merge tool, the single parameter envelope tool will create a seamless surface across multiple (generally, but not necessarily) overlapping results. However, this tool will not change any hydraulic parameters. Where the Drape and Merge tool overlays water surfaces onto a common DEM framework (thereby potentially changing/refining any parameter related to depth), this tool will only merge a single parameter onto a blank grid. This means that a parameter such as depth can be enveloped without any change to depth values. However, when merging depth surfaces, the coarsest grid will normally limit the output cell size. If a finer output size is desired, the drape and merge tool is likely to produce better results.
The surfaces can have different extents and cell sizes (or even be TINs). During processing, you can specify the desired cell size of the output file, or leave it as "-1" and have it calculated for you as the maximum or minimum of any grids included in the envelope.
