Flow Direction Flow Accumulation

the following tools our tutorial will be an introduction to tools you can use to model the flow of water across a surface so these tools need an elevation to begin but they're not in the surfer status that they're aptly named up in the hydrology data set we're going to focus really today on flow direction and accumulation but a couple others that could be interesting Basin uh be able to delineate drainage basins I don't love this as a tool I would probably use watershed if I were you a watershed gives you a little bit more control because you can actually determine the pour points you know you can say look I'm interested in finding the drainage basins for these major creeks or for all of these streams or so on and so forth rather than you know just look at my elevation liner determine every drainage basin because it will find you some really really tiny ones that aren't too of interest to you you can also use some of the outputs that we have here to define the order and the linkage systems of streams it's kind of great but really we're going to focus on here is as these great so flow directions the most important tool in this data set but it doesn't really have any value on its own right the whole purpose of flow direction is to create something that will be an input to everything else all right it's almost in a weird way it's own version of the network database from ArcGIS vector right of Network analyst you're taking a road layer and you're converting a new data type a network data set and that network data set feeds other analyses but even though you may have all of the information that you need on the elevation layer or the the road layer in that previous example you needed to be configured in such a way that it can be easy as an input of analysis for some other tools that's what flow direction is going to do right it's going to take the input elevation and it's going to output something that tells you the direction in which your data tends to flow right flow all cells outward I'm just saying at the end of my analysis window just flow everything out and so it'll take its moment it's going to produce something that initially seems confusing but let me just run through it notice that there's eight values essentially what this means is one I flow east to I flow southeast four I flow south eight I flow southwest 16 I flow west 32 I flow Northwest 64 I flow north and 128 I flow northeast right you don't have to take my word for it of course you can go check and we'll go to our GIS and we'll try to find floats our chests flow direction if you ever look at these values and find yourself really annoyed and stressed out this is what is being calculated yet and it's eventually looking at the downward slope direction and it's categorizing something based on all right my value is going to become one of these based upon which cell I would flow into given right a surface of elevation and that's essentially what this is calculating itself as right here cool so you've got your flow direction so the primary we're going to focus on is flow accumulation so the purpose of flow accumulation is given a direction raster which essentially tells each cell what they should flow into accumulation is going to imagine well what if we did flow like what if one raindrop for example or as many raindrops as you want actually fell everywhere where were they all theoretically wind up right so this is what's going to allow you to do a first step into delineating stream channels alright so we'll do that faithful fellow accumulation I'm going to make an integer just in case if we want to look at the the attribute table so input weight raster is very cool like right now it's just going to imagine that every single cell has a value of 1 and every cells going to take that value of 1 and they're going to try to flow that value of 1 as far as it can go and anytime the two values of 1 accumulate they become 2 and any a cell that has to meets a cell that has 12 it would become 14 and so on and so on until there's going to start to be some cells that have a value of thousands right because a thousand different droplets would meet there based on the way that things would have to drink a wait raster would allow you to throw something on top of it like maybe you interpolated annual rainfall you know based on some some access to some rain rain monitoring stations you have and then that would be different depending on where you were and then you could actually try to model what the average right drainage of rainfall would be right based on where areas get a lot of rain versus what areas get a limited amount of rain you can maybe even do the same thing with like imagining it was a torrential downpour right use a lot of rain and see what areas are really susceptible to flooding so we'll run it this is gonna be one of those institution which the first thing that ArcGIS returns to you is not going to be the most attractive and helpful so we're gonna have to play around with it so really highlight what we're looking at and so it kind of looks like a lightning bolt here but let's mess with the symbology a little bit right again every cell is now reporting the number of cells that have flown into it so we can start to delineate them let's say that if you are below 100 you don't show up and if you're above 10,000 then you're just this solid round now we start to zoom in and we can see what we've essentially created we've defined stream channels mini ones that maybe only emerge with a couple of little water water droplets flowing down into ones that are maybe dry screams but miniature streams during the wetlands area or during the wet season it flow into major stream channels that converge with other major stream channels so on until they flow outward into the Susquehanna River at various points cool little trick here right if this is my analytical layer this is actually my stream channel that I might use or other tools such as delineating wood or cher sorry for defining stream orders and so on and so forth I can actually make this a little bit easier to see on a map by using focal statistics right we we looked at Foucault's statistics previously as an analytical tool but it has a couple of visual cosmetic uses too so in this instance I'm taking right the reason we can't see this very well is a raster doesn't abide by the same rules as a vector when you zoom in or out with vector the points shrink or grow light dependent but with raster something is a hundred feet by 100 feet fell size it is always 100 feet by a hundred feet it doesn't you know try to change itself so you can see it as you zoom at so one of the things that might be helpful is maybe just increasing the cell size right so we'll say we'll say folk F a very simple I'm just going to increase in the most immediate neighborhood and I'm going to say everything take on your maximum value and so what I'm essentially doing here is all these little cells right on the edge here they're going to become what these values are right because they're much smaller they're going to take on their maximum happens everywhere I won't use it for any kind of analysis but I'll have it around in case I need to very quickly and visually and I'm just going to input the same value that I had previously demonstrate to somebody the output of my tool now it is much easier to see even when i zoom out now I can have something that I keep right at a larger extent to show to a client or to show to a class and I can very easy classify what these look like even if I'm not necessarily going to do this for analysis and so that's flow accumulation very very very cool tells you the number of cells that flow into a given cell based on surface of elevations so if we can do it for every set you can always do it for just a few and so where's my little starting point and so I've got these three points here no maybe give them each a value of 1 and their ID in order to be used as an input in this tool I need to convert them so I'll go up to convert go to to raster I'm going to do point to raster wait the value field is going to be there fi d xh1 them to value of one column rascal point cell size is all set there you go can't see them they're real tiny but if i zoom in you know and I looked under the starting point I'd be able to see a very tiny grid cell all right so now I have these three grid cells I'm going to return to the flow accumulation but now instead of like what I did here right every cell flowed their value so there's obviously going to be times right if I use my pixel inspector and went here right we do to do to get my flow accumulation hey there's going to be instances where there is 120,000 cells right that come in or I'll use the regular pro quick conversion so it's much easier to see maybe look at that I mean there's gonna be times of 120 thousand five hundred seventy nine cells meet here hey versus a smaller one here wait maybe they only have seven sixty one seventy sixty two and then suddenly who they're in an area where tons of cells are meeting and because the tons of cells are meeting right the value jumps up enormous late until it comes down and meets more cells and so on and so forth so now instead of every cell getting that value to flow just these three are going to get that value to flow so the maximum value I'm ever going to have anywhere is three and this allows you if the other one can see the generalized flow pads this allows you to very specifically determine if you just dropped something or a number of things what paths would they take to flow up and did I mess up there Oh fill it so I need my flow accumulation flow direction in my flow direction need my last point for my weight do that one and now I get a much cooler thing I'm gonna have to zoom in to really see it right maybe we'll do the thing we did before with the focal statistics so you can really see just in your immediate neighborhoods take on the maximum value totally cosmetic right just doing this so we can see this a little bit easier and now starting with these three points right let's turn them into these bright red so something really easy to see like blue circles starting here each one follows their natural flow and in this instance these two happen to meet here right so this would be a flow path taken by tool so very cool and something that I want to get you start or have your brain start thinking beyond just its applications here and if I can start looking at a pathway of how one thing flows imagine if my surface wasn't elevation but it was travel time right a weird version of travel time maybe where I allowed the user the opportunity to cross graphs per to cut through buildings or do any number of things and I would increase the amount of time it would take to cross over grass versus a road versus crossing through a building but I can actually see the best path someone might take maybe my surface is wealth or elevation or property value in a city and I can see the neighborhood's are where wealth would flow right within cities based upon that proximity thing the point is when you start to think of flow direction as flowing more than just water right you can flow it over surface of travel time or surface of wealth or surface of population density those are going to be cool applications that we'll explore to a small degree in a another lecture later on something called cost this

Channel: Shea O'Neill

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Length: 13min 11sec (791 seconds)

Published: Sun Apr 12 2015