Flow Distance function

Available with Spatial Analyst license.

Overview

Computes, for each cell, the horizontal or vertical component of downslope distance, following the flow paths, to cells on a stream into which they flow. In case of multiple flow paths, minimum, weighted mean, or maximum flow distance can be computed. If an optional flow direction raster is provided, the down slope direction(s) will be limited to those defined by the input flow direction raster.

This is a global raster function.

Notes

To limit downslope directions along which flow distance is measured, provide an optional input flow direction raster that can be derived using the Flow Direction function. Choose from D8, D-Infinity (DINF) and multi-flow direction (MFD) flow models while generating an optional input flow direction raster.

If an optional input flow raster is provided, it is recommended that this raster be created using the Flow Direction function using the same input surface raster that you will provide to the Flow Distance function.

• If this input surface raster is void of sinks, the distance measurements would be to stream pixels represented by the input stream raster.
• If this input surface raster has sinks present, some flow paths can get terminated prematurely by flowing into sinks before reaching streams. In such cases, flow distance measurements for these pixels are calculated only up to the sinks into which they flow.

When the function is run without providing an optional flow direction raster, flow distance is assessed considering all possible downslope flow paths from each pixel to pixels on a stream into which they flow. When the function is run with a D8 flow direction raster, there is only one possible downslope flow path from every pixel to a pixel on the stream, and flow distance is measured along this single flow path.

It is recommended to create the input stream raster using the same input stream raster that is provided as an input to the function. You can create stream rasters from an input surface raster using the Flow Direction, Flow Accumulation, and Con functions respectively. If you wish to use streams from other data sources, for best results, convert them to the input surface raster first then run the Flow Distance function on the result.

When multiple flow paths exist from each cell to cells on the stream into which they flow, use the Statistics type to compute minimum, weighted mean, or maximum flow distance.

If there is only a single flow path from each cell to a cell on the stream, all statistics types produce the same result.

Parameters

Parameter	Description
Stream Raster	An input stream raster that represents a linear stream network.
Surface Raster	The input raster representing a continuous elevation surface.
Flow Direction Raster	The input raster that shows the direction of flow out of each pixel. When a flow direction raster is provided, the down slope direction(s) will be limited to those defined by the input flow directions. The flow direction raster can be created using the Flow Direction function.
Distance Type	Determines if the vertical or horizontal component of flow distance is calculated. Vertical—The flow distance calculations represent the vertical component of flow distance from each pixel in the domain to the pixel(s) on the stream into which they flow. This is the default. Horizontal—The flow distance calculations represent the horizontal component of flow distance from each pixel in the domain to the pixel(s) on the stream into which they flow.
Flow Direction Type	Defines the type of the input flow direction raster. D8—The input flow direction raster is of type D8. This is the default. MFD—The input flow direction raster is of type Multi Flow Direction (MFD). DINF—The input flow direction raster is of type D-Infinity (DINF).
Statistics Type	Determines the statistics type used to compute flow distance over multiple flow paths. If there is only a single flow path from each cell to a cell on the stream, all statistics types produce the same result. Minimum—Where multiple flow paths exist, minimum flow distance in computed. This is the default. Weighted Mean—Where multiple flow paths exist, a weighted mean of flow distance is computed. Flow proportion from a cell to its downstream neighboring cells are used as weights for computing weighted mean. Maximum—When multiple flow paths exist, maximum flow distance is computed.