2 - Determining channel length and elevations in QGIS

okay in the last video we delineated a watershed area with multiple tools within the saga terrain analysis toolbox and essentially we're doing this to determined Q values of a stream at our bridge here but this is only part of the regional regression equation we also need to calculate stream length and elevation points within our stream and so this video will cover essentially how to do that in QGIS so the first thing we need to do is draw in our stream and the first thing we're going to do is turn back on our de m and then potentially we can use the channels as reference but I rather just follow the DM here so we want to create a new layer new shapefile layer and this is going to be we'll just call this channel and we want it to be line and one thing to always watch out for when you're creating a new shapefile or saving shapefile is this little line right here and you always want to set it here we're using Oklahoma state plain North feet projection but you always want to make sure you keep the same projection for all of your stuff so you know it's correct units okay hit OK and so now we just have this new shapefile and there's nothing in it real quick I'm just going to change change it to let's see just a - blue line so we know this is our channel and it shows up a little bit better and basically what I'm going to do is just trace the channel and you can see it coming here and for the regional regression equation you want to basically use the longest channel within your watershed so you can see how it branches off here and we're just going to stick with this main channel all the way through and for the most part I like to just use the DM you can see it pretty well kind of where the the creek channel is so we have our channel shapefile but we need to start editing it so you come up here again to this toggle editing the little pencil hit it assuming you've already clicked on the file you want and now it popped you can see how these popped up and so we want to add a line feature click that then it just give it gives us this little target and we'll just start in here and you just start clicking okay and you can get as precise as you want here you want to for the most part keep it within the center line of the channel because once you get your elevations if you're just slightly off and out here it can be a lot higher than it should be I'm gonna just quickly do this just for example okay so don't do this at the same resolution I am but you can get the idea really easy so you can see that obviously a lot of these creeks have quite a few bends in them and it takes you know a fair amount of time to really get a full handle on I'm getting these in okay so again you want to be a lot more zoomed in than what I'm doing but this is just for reference okay and then you can see how the creek channel is fading quite a bit within RDM as the definition of the channel is a lot less and you can see here it's almost faded and one good thing to do at this point is to click out of the DM and actually just go to the satellite and really determine whether this is truly a channel anymore or is it's just a drainage and so you can see kind of we're basically at the end right here so let's just end it and we'll right-click and this box should pop up we can just hit OK and you can see our channel now we want to go back to toggle editing and that way we can save our feature okay so now within this watershed we have our channel feature so the two parameters that we need for the regional regression equation from our channel are the elevations at 10% and 85% of the channel length as well as the distance between the 10% and 85% locations so the first thing we're gonna do is just figure out what the total length of the and this is similar to how we calculated the watershed area we want to right-click our channel geometry open the attribute table and go to the field calculator and here we want links in feet again it's a decimal number one thing I forgot to mention in the last video about the watershed is the reason we have to do this in feet-first is because the geometry layers only calculate within the units of your coordinate system and so that's feet which is fine but for the regional regression equation it likes things in miles or square miles for the area so that's why we need to convert it so first we're going to do it in feet so here similar to before we're going to come down and actually use length again not the dollar sign we're just going to use the regular length and then come back up to the dollar sign geometry close the brackets hit okay so here's the total channel length in feet and then now we want to convert that to miles so go back to the field calculator have a length miles category or column and again we want the field value of length and now we want to divide that by 5280 the number of feet a mile and we can hit okay so now we have the length in miles okay we'll toggle the editing to save that so we'll just remember this number for now but that'll give us the information we need to calculate the values in a minute so now we actually need to create and figure out where the 10% and the 85% values are and there's kind of a neat way to do that if we come to our processing tool box and type in points a long line and this is actually a G tall tool and it's under the vector geoprocessing so double click that so we want an input layer of our channel and we can either do this one at a time or I actually prefer to do it run as a batch process because we need the 10% and the 85 percent markers so if we run as a batch process down here essentially we can run both of these at the same time the downside is that it puts it into two separate files but that's okay for now so I want to hit the plus button so we have two two lines here so the input layer will select from open layers and this is just going to be our channel we'll select that for both and again this is different from the channels that we created for the watershed okay so this is distance from line start represented as fraction of line length so the first one we want 10% and the next one we want 85 so this goes from zero to one okay and then here points long lines this place is just asking for the file we want to save it to so let's go just to the desktop here and again we'll just save it as a shapefile and we'll call this the 10% that's okay we don't want to autofill and then again shapefile and this will be 85% okay so now we can run this okay all right and I actually forgot to add these to the drawing so let's just do that real quick okay so now we have two little shape files that are each just one point apiece we have our 10% the point location and these are all right on the line so that's what the tool did is points along the line so it restricts it to within the line geometry and then here we have the 85% okay so now we need to calculate the elevation at these points so you can either do it one at a time or merge these two together and and then that way we have one file that's what I like to do just so it's a little bit clearer so let's see I've merged and I want to merge vector layers so you can either do this with saga or with just the basic UGS tools which is what I'll use so I've got my 10% 85% make sure and set this destination CRS to our Oklahoma state plane and then save the file here that's just I'll just call it channel point okay not so it showed up is merged here I'm gonna remove these two just to avoid any confusion so again now you see these two points so one thing I like to do just so I know exactly what they are is to essentially label them you can see they're coming from a certain layer and that generally makes things easier but I like to toggle the editing and we'll select this one you see it shows up that's our 85% so I'm just gonna call that 85 and down here call it 10 and then that way we know exactly which ones which ok so what we want is the elevation of each of these two points based on our DM so we want to go back to our processing tool box we want to add raster values to points so here it's a saga tool and it's kind of between vectors and raster's so we want to add raster values two points so we'll double click that essentially what that's gonna do is gonna say at that exact point what is the raster value and so our points file is going to be our merged and we want to use the original da I'm not necessarily the field just to make sure it's the actual point value okay and we want to use here we have a small enough or not too small of a pixel value that we want to use nearest neighbor interpolation so that's just the exact value of that exact point some of these other ones will kind of take the mean of the closest pixel but here we have 2 meter raster so we want the exact location okay and so we'll save this to a file and we'll call it you know points elevation so again we're kind of creating a few extra files which we can clean up later but that's alright okay run that okay there we go so now here it pops up this result that's one thing you have to watch in key GS it gives kind of a temporary name to some of these but you can just hold the cursor on it for a second and see the actual file path or if you want the actual file you can just remove this and bring an actual file which this is just a different name but so let's open the attribute table and we can look and now you can see we have the elevation values that we need so the 10% and the 85% so that essentially gives us all the information at least we need from QGIS to determine our rural regression equations so we have the 85% and the 10% elevations and we have the total length of the channel but again we can divide that out and figure out the link between these two points fairly easily and then we have the area of the watershed and so the lengths and elevations will give us a slope we have the area all we need is basically the precipitation for this specific area and then that will complete all of our necessary parameters to calculate the flow rates using the USGS region or regression equation so that's it for this video thanks

Channel: John Edwards

Views: 1,965

Keywords: QGIS

Id: eJAcV8pBpIA

Length: 16min 59sec (1019 seconds)

Published: Mon Jul 15 2019