Hi, This is somewhat of a theoretical question stemming from some discussions with colleagues on the topic of implications with delineating basins with projected (e.g., Albers Equal Area) vs. unprojected (NAD 83) data derived from a 10m DEM that's in NAD 83.
Some have stated that it's not an issue as the values calculated from unprojected data simply get adjusted if you do decide to project.
I'm not sure this is the case though, as there are inherent differences between data in a geographic coordinate system and projected data. I tried one example going through the routine starting with unprojected DEM data, then tested the same site with projected DEM data. Steps taken for both were done (all work done in ArcGIS 9.3.1) using 10m DEM data.
One run was done using a DEM in NAD 83, and the second run was done by projecting the same DEM into USA_Contiguous_Albers_Equal_Area_Conic_USGS_version.
+ derive flow direction using the geoprocessing FlowDirection_sa tool + derive flow accumulation using the FlowDirection_sa tool + snap the pour point using a 50 meter distance + delineate the watershed using the Watershed_sa tool
In comparing the two I could notice a visual difference between the display of the Flow Direction grids. Images of the flow direction for approximately the same area are are also attached - one derived from the unprojected data named FlowDirection_Unprojected.png, and the other derived from the projected data named FlowDirection_Projected.png, and you can see differences between the two (I know visual inspection isn't the most scientific but can serve as a starting point).
Accordingly, there was a difference between the pour point with how it snapped for each run. And, there was a definite difference in the derived watersheds (see attached image named watershed.png). The watershed shown in green is the watershed derived using the projected DEM and subsequent projected-derived elevation derivative data. The watershed shown in the purple outline is the watershed derived using the unprojected DEM data.
I've come across these two other threads (links below) that discuss this issue in the old esri forums, but I'm still not clear as to how the Flow Direction tool works relative to projected vs. unprojected data (I understand the concept of hydrologic flow and flow direction though). If each cell still has the same elevation value in a projected DEM vs. an unprojected DEM (is this correct?), why is there a difference in a flow direction raster derived from projected data versus one derived from DEM data in NAD83???
Also, would any differences theoretically be less of an issue if doing delineations in a higher Latitude such as, Shenandoah National Park in Virginia versus doing delineations in the state of Texas?
I spoke with one person that thought that the east-west distortion you get as you move away from the equator could likely be an issue (like how in some maps Canada is extremely bloated and distorted), in that if you're more than 10 degrees of latitude away from the equator projected data is probably the way to go if you're concerned with accuracy.
One major unknown is the level of uncertainty with basins done using unprojected data that we're trying to get a handle on. There obviously is a difference, but what we're trying to get a handle on is the magnitude of potential errors (e.g., increasing exponentially with watershed size, etc.)
Thanks to anyone that can provide a straight-forward answer to this discussion, or just some helpful insight into this.
