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atrushwo108 post(s)
#05-Apr-23 21:21

Hey Everyone,

If I specify both a fill height and a fill flow, how does the function fill sinks?

  1. Does it fill sinks when both conditions are met... aka AND?
  2. Does it fill sinks when either condition is met... aka OR?

atrushwo108 post(s)
#06-Apr-23 16:21

I figured this out with some testing. Turns out it is when either condition is met... aka OR. This removes a lot of potential functionality because running it with 2 parameters is no different than filling sinks in 2 different passes.

Allowing for an AND condition could allow users to identify anomalies in their surfaces (i.e. small sink areas with large depths). Not sure how difficult this would be to implement though.

Dimitri


7,433 post(s)
#07-Apr-23 06:31

The Watershed Prepare: Filling Sinks topic has some useful explanations, for example:

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.

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.

and

Fill height and Fill flow

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.

Since it usually is easier to note the height of a dam or other obstruction that it is to compute the areal surface of a drainage basin, in many cases it will be easier to simply use a Fill height value when filling in sinks created by dams. Fill flow may be more useful when filling in small sinks in undulating terrain where we are interested in larger effects and do not care about highly local sinks that are, in comparison with larger watershed areas, mere puddles.

adamw


10,447 post(s)
#19-May-23 08:53

I figured this out with some testing. Turns out it is when either condition is met... aka OR. This removes a lot of potential functionality because running it with 2 parameters is no different than filling sinks in 2 different passes.

One pass is faster than two, that's the benefit.

We might try to add an option to choose AND instead of OR though. Can you clarify as to why specifically do you want it? Do you want to leave unfilled sinks which are higher than H even if they have a small flow? Or sinks which have a flow bigger than F even if their height is small?

atrushwo108 post(s)
#19-May-23 14:23

Thanks for getting back to me.

One example is basement suite entrances in bare earth LiDAR. They are usually a staircase down the side of a house for direct, separate access into a basement suite. Typically, these are ~2m deep and have very small contributing areas (fill flow). If we were given the functionality to fill sinks based on both criteria (AND), we could only fill these small areas. We would also need functionality to either fill or not fill depressions which met the characteristics.

If these functionalities existed, we could then isolate these small areas from the LiDAR by subtracting the original raster from the fill sinks raster. This depth raster could then be traced to pull vector locations of basement suites.

By allowing users to fill particular depressions, it would also give more functionality to watershed analysis by consolidating catchment areas to more prominent depressions. It would also give more functionality for analysis of depressions by removing anomalies.

Thinking outside of the box a bit, with this functionality, one could also invert their raster and use the same logic to fill trees. This might not work out that great if there are trees on hills though...

Anyways, I hope my ideas inspire ideas of your own!

danb

2,064 post(s)
#19-May-23 22:15

Its funny you should say that atrushwo, your original post gave me exactly that thought.

Initially I inverted the DEM and harvested the tree crown "sinks" and then tried with the DEM not inverted using the stream flow and the sinks to delineate the edge of the crowns. I have no real purpose for this at the moment and haven't tested it further, but have squirreled the project away for a rainy day.

Attachments:
TREE CROWN.jpg


Landsystems Ltd ... Know your land | www.landsystems.co.nz

oeaulong

524 post(s)
#20-May-23 02:55

Oh Dan, that is clever. I could have used that 10 years ago. Was working with a long time colleague trying to figure out how to digitize Tree crowns for spectral classification, canopy height and such for prospecting a forest inventory. This was for the purposes of establishing Carbon sinks for cap & trade offsets. Now another thing to play with.

atrushwo108 post(s)
#23-May-23 14:35

Yeah, I hope this goes somewhere. One of the reasons I like Manifold so much is that you have the flexibility to change things to suit your purposes.

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