Background

HEC-FDA 1.4.x did not have a concept of geospatial position, which made working with two-dimensional hydraulics difficult at best. FDA 2.0 uses geospatial data in the form of shapefiles, raster data, and HEC-RAS native output. This saves modellers from the tedious per-processing steps of the past. GIS data is much like any other data, except that it includes a set coordinates which relates it to a geospatial position on the globe. When that data is encoded, it has to define a coordinate system. We often refer to that coordinate system as the data's "projection". A Projection, more broadly, is the method used to portray a part of the spherical earth on a flat surface (like our computer screens), There are numerous resources available online from the USGS and countless non-federal experts on mapping if you wish to learn more. When working with FDA, the most important things are that we know the source projection of every piece of geospatial data, and we have chosen projection to use while were intersecting that data to pull water surface elevations, terrain elevations, or identify which structures are contained in a given impact area. 

Geospatial Operations

Geospatial data comes into HEC-FDA in the following places: Terrain, Hydraulics, Structure Inventory, Index Points, and Impact Areas. The latter three can all be added as shapefiles. The source projection for a shapefile exists in the .prj file, which is a necessary component file of the shapefile itself. Terrain and Hydraulics come in as either RAS Native files, or .tifs. Properly formatted .tif data will have the projection encoded in the file itself. RAS Native results do not carry with them a projection, it is assumed from the accompanying terrain. You have an option to set the destination projection directly in the study properties as well via .prj file. In all cases, when there is a terrain present, it's projection will be used as the study projection even if the user sets one explicitly in the study properties. The user selected projection is only used in the absence of a terrain.