I am currently reading this book and I found the answer a bit confusing. Would be nice if someone could elaborate on the accuracy of the following sentence:
The values 64 and 16 have the highest frequency, which means the direction of the surface flow is north and west.
Also, if there is a better reading in regards to this topic.
The D8 flow direction is based of work by Jenson and Domingue (1988). Based on this model any one cell has 8 possible directions of flow out of the cell. The direction of flow is determined by the elevation raster and the direction coding just indicates which of the 8 cells it will move (64 and 16).
I believe that ArcGIS encoding is as follows:
The encoding in 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.
All the frequency field does is count the number of cells in the image that flow in the given direction. So one can assume that the largest count indicates the overall direction of flow for the image
A great read is this:
https://gisgeography.com/flow-direction/
And the original paper is this:
Jenson, S. K., and J. O. Domingue. 1988. "Extracting Topographic Structure from Digital Elevation Data for Geographic Information System Analysis." Photogrammetric Engineering and Remote Sensing 54 (11): 1593–1600.
