I was checking the values of a flow direction tiff created with Grass GIS and the values were: -32768, -4, -3, -2, -1, 1, 2, 3, 4, 5, 6, 7, 8.
This answer explains clearly the meaning of the values between -8 and 8.
I don't understand though why I have those -32768? It seems -32768 is the lowest value in the integer range in C... Can I assume that since I do not have any 0 (sinks in Grass), the -32768 are in fact the sinks?
The bottom of a value type (int16 - a signed 16bit integer in this case) is often the result when a value cannot be or has not yet been calculated (a Null value).
I would not assume these values are sinks without first inspecting their occurrence. Null values in the input will result in Null values in the output, unless they are properly dealt with.
To deal with both null and sink issues in the input data, one would generally fill null value cells by interpolation (r.fillnulls or r.fill.stats) and fill sinks (r.fill.dir) prior to calculating the flow direction.
