Watershed Prepare
Fills sinks in terrain elevation data based on specified criteria, a necessary first step before many Watershed operations.
Choice of Fill height and Fill flow control how sinks are filled as follows:
- If both Fill height and Fill flow are negative or zero, the image is left unchanged.
- If Fill height is positive and Fill flow is negative or zero, sinks are filled based solely on height, that is, the depth of the sink.
- If Fill height is negative or zero and Fill flow is positive, sinks are filled based solely on flow, equivalent to the area of the sink.
- If both Fill height and Fill flow are positive, sinks are filled based on both height and flow.
Fill height and Fill flow allow us to specify whether we want to fill sinks on the basis of their vertical depth or on the basis of their areal size or on a combination of both characteristics.
Fill height is a measure of the vertical depth of the sink. Sinks that are deeper than the Fill height specified will not be filled and will be left unchanged. Sinks that can be filled with the Fill height specified will be filled.
Consider a lake formed by a dam where the lip of the dam is 50 meters above the height of the lake. The lake is a sink until the level of the lake rises up above the lip of the dam and can spill over past the dam downstream. The lake is a sink with a vertical depth of 50. The Fill height required to fill it is 50. At any Fill height values of 50 or greater, the lake as a sink is filled. At any Fill height less than 50 the lake is unfilled and remains as a sink.
If we use a very large value for Fill height, such as 20000 (a very big height difference whether we are measuring in feet or meters) that will be enough to fill in any sink.
Fill flow is a measure of the areal size of the sink. A sink is a closed drainage basin, the total flow of which is found by assuming one unit of water falls on every pixel within the basin. To fill the entire basin we must have at least that much flow available to fill the basin. If a sink is 1000 pixels in areal size we must have at least 1000 in Fill flow available to fill it.
If we choose 1000 as the value for Fill flow that will be enough to fill all sinks that are 1000 pixels in size or less. Sinks that are larger than 1000 pixels in size will be too big to fill with the Fill flow we have specified. A Fill flow specification of 500 would be enough to fill a sink that is 400 pixels in size, but it would not be enough to fill a sink that is 1000 pixels in size and thus requires a minimum of 1000 in flow to be filled.
Launch the template by choosing a Tile field and then double-clicking the Watershed Prepare template. When the template launches we can specify options.
See the Watershed Prepare: Filling Sinks topic for an example.