DWCON

Description

Performs four conditioning procedures for drainage and watershed programs. This program must be executed before any drainage and watershed programs.

Parameters

Name	Type	Length	Value range
InputDBEC: Input elevation channel*	Raster port	1 - 1	1 - 1024
OutputFDEP: Output fill depression channel*	Raster port	1 - 1	1 - 1024
OutputFDIRECT: Output flow direction channel*	Raster port	1 - 1	1 - 1024
OutputFACCUM: Output flow accumulation channel*	Raster port	1 - 1	1 - 1024
OutputFDELTA: Output flow delta value channel*	Raster port	1 - 1	1 - 1024

* Required parameter

Parameter descriptions

InputDBEC: Input elevation channel

Specifies the input channel that contains the elevation data. The input channel can be 8-bit, 16-bit or 32-bit.

OutputFDEP: Output fill depression channel

Specifies the output channel to receive the filled depression result. The output channel must be either 16-bit signed integer or 32-bit real.

OutputFDIRECT: Output flow direction channel

Specifies the output channel to receive the flow direction result. The output channel must be either 16-bit signed integer or 32-bit real.

OutputFACCUM: Output flow accumulation channel

Specifies the output channel to receive the flow accumulation result. The output channel must be either 16-bit signed integer or 32-bit real.

OutputFDELTA: Output flow delta value channel

Specifies the output channel to receive the flow delta value result. The output channel must be either 16-bit signed integer or 32-bit real.

Details

DWCON performs four conditioning procedures for drainage and watershed functions. DWCON must be executed before any drainage and watershed functions.

All functions are based on the article cited in References.

To use the drainage analysis functions, you must first run DWCON.

DWCON can handle up to 20K x 20K images on the workstation. If the limit is exceeded, you can execute the function repeatedly using different image window sizes.

Four output channels are required to store the four data set results. Due to the use of negative values in the results, the four output channels MUST be either 16-bit signed integer or 32-bit real channels. Because the maximum allowable value for a 16-bit signed channel is 32768, for a large DEM it is recommended that you use a 32-bit real channel for the flow accumulation output channel to avoid computer integer overflow problems. You can add channels to the PCIDSK file using the PCIMOD function.

Because all the calculations are in integers, to use real numbers with decimal places, you must multiply all values inside a DEM by a factor of 10. This can be done by using the ARI or MODEL functions.

Filling depressions

Digital elevation data always contains depressions that hinder flow routing. The objective of the first step in the conditioning phase is to create an adjusted 'depressionless' digital elevation image in which the cells contained in depressions are raised to the lowest elevation value on the rim of the depression. Each cell in the depressionless digital elevation data set will then be part of at least one monotonically decreasing path of cells leading to an edge of the data set. A path is composed of cells adjacent horizontally, vertically, or diagonally in the raster (eight-way connectedness) and that steadily decrease in value. If there are negative values in the elevation data, DWCON will look for the minimum negative value in the channel and add the absolute of the value to the entire channel so that all elevation data is positive.

Flow direction

This procedure builds the flow direction data set. The flow direction for a cell is the direction in which 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 surrounding conditions correspond to unique values when the powers of two are summed.

Flow accumulation

This procedure makes use of the flow direction data set to create the flow accumulation data set, where each cell is assigned a value equal to the number of cells that flow to it. Cells having a flow accumulation value of zero (to which no other cells flow) generally correspond to the pattern of ridges. Because all cells in a depressionless digital elevation data set have a path to the data set edge, the pattern formed by highlighting cells with values higher than some threshold value delineates a fully connected drainage network. As the threshold value is increased, the density of the drainage network decreases. To avoid computer integer overflow, it is recommended that you use 32-bit real channels for large database DEMs.

Flow delta value

This procedure produces a delta value data set. Delta value is the amount of increase in flow accumulation value in the flow direction. This data set is useful for automatic seed generation and pour point table calculations.

After DWCON

References

Jenson, S.K. and Domingue, J.O. "Extracting Topographic Structure from Digital Elevation Data for Geographic Information System Analysis". Photogrammetric Engineering and Remote Sensing, Vol. 54, No. 11, November 1988, pp. 1593-1600.