4

There are two seemingly identical Spatial Analysis tools:

(1) Surface -> Aspect:

Derives aspect from a raster surface. The aspect identifies the downslope direction of the maximum rate of change in value from each cell to its neighbors.

Aspect can be thought of as the slope direction. The values of the output raster will be the compass direction of the aspect.

(2) Hydrology -> Flow direction:

Creates a raster of flow direction from each cell to its steepest downslope neighbor.

Manual comparison

To test this I created an aspect layer and a flow direction layer, converted the aspect layer to the format of flow direction using Raster Calculator:

64*(("aspect" >= 0)  &  ("aspect" < 22.5)) + 
128*(("aspect" >= 22.5) & ("aspect" <67.5)) + 
1*(("aspect" >=67.5) & ("aspect" <112.5)) + 
2*(("aspect" >=112.5) & ("aspect" <157.5)) + 
4*(("aspect" >=157.5) & ("aspect" <202.5)) + 
8*(("aspect" >=202.5) & ("aspect" <247.5)) + 
16*(("aspect" >=247.5) & ("aspect" <292.5)) + 
32*(("aspect" >=292.5) & ("aspect" <337.5)) + 
64*(("aspect" >=337.5) & ("aspect" <=360))

and then compared to the flow direction layer:

("aspect_conv" == "flowdir")

The vast majority of the resulting layer is 1 (true) and I suppose the few areas with 0 is because of flat aspects or nulls.

It therefore appears that the only difference is that "Flow Direction" returns unique values (powers of 2), while aspect returns a continuous range of floats. Is that all?

2

From what I see in your tests and data results, i would say the same as you :

"the only difference is that "Flow Direction" returns unique values (powers of 2) while aspect returns a continuous range of floats"

I would add Flow Direction is a sort of subtool (maybe somewhere based on the aspect tool and doing exactly what you did) used to precisely create the requisite coded output of 8 neighbourood cells flow direction needed in the D8 flow direction model (Réf. Jenson and Domingue, 1988 below) on which all esri tools are based for their hydrological analysis :

FLOW DIRECTIONS

The second procedure of the conditioning phase builds the flow direction data set (Table 3). The flow direction for a cell is the direction water will flow out of the cell. It is encoded to correspond to the orientation of one of the eight cells that surround the cell (x) as follows:

64 128 1
32 x 2
16 8 4

For example, if cell x flows to the left in the matrix, its flow direction will be encoded as a 32. Flow direction encoding is done in powers of two so that surround conditions correspond to unique values when the powers of two are summed for any unique set of neighbors.

Source

S. K. Jenson and J. O. Domingue, “Extracting Topographic Structure from Digital Elevation Data for Geographic Information System Analysis,” Photogrammetric Engineering and Remote Sensing, Vol. 54, 1988, pp. 1593-1600

Download here : http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.138.6487&rep=rep1&type=pdf

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