{"id":3152,"date":"2021-08-12T12:16:29","date_gmt":"2021-08-12T00:16:29","guid":{"rendered":"http:\/\/blogs.lincoln.ac.nz\/gis\/?p=3152"},"modified":"2023-05-07T02:57:10","modified_gmt":"2023-05-07T02:57:10","slug":"on-the-edge","status":"publish","type":"post","link":"https:\/\/blogs.lincoln.ac.nz\/gis\/on-the-edge\/","title":{"rendered":"On the Edge"},"content":{"rendered":"

We look at creating a catchment boundary layer using existing data and nice visualisations.<\/em><\/p>\n

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https:\/\/www.nasa.gov\/image-feature\/dividing-line\u00a0\u00a0Image Credit: NASA\/Joe MacGregor<\/em><\/span><\/figcaption><\/figure>\n

So imagine you’re walking along a mountain ridge (like the one shown above from Antarctica – hope you’ve got your thermals on).\u00a0 When it starts to rain, water flows to one side or the other, depending on where it hits the ground.\u00a0 The line drawn along the ridge that separates these two flow directions defines the boundary between two river catchments and, along with some other key ideas, helps us define catchment boundaries.\u00a0 In this post we’ll look at how we can manually create these boundaries making use of all available data and some nice visualisations.<\/p>\n

Many may not be aware of Lincoln’s high country sheep farm in Otago, Mt Grand Station<\/a>.\u00a0 Near Lake Hawea,\u00a0 the station has been the focus of some recent efforts to “redesign<\/a>” its future, in light of climate concerns and sustainability.\u00a0 It’s also been the focus of some water quality modelling by Shyam Provost, who’s been looking into phosphorus and nitrogen levels related to land use.\u00a0 As part of his research, he’s needing to know the breakdown of different land uses within his catchments and to help out with that we’ve been working up a landcover layer that suits his purposes.\u00a0 Part of that has meant defining the catchment boundaries, so we’ll have to start there.<\/p>\n

Happily, a lot of high-resolution data have been collected at Mt Grand, including elevation at a 1 m resolution and imagery at 0.125 m.\u00a0 I’d venture to say that no high country station has data of such quality.\u00a0 It’s a veritable gold mine.<\/p>\n

There are three catchments that Shyam is interested in, Cameron Gully, and Lagoon Creek\u00a0with\u00a0one of its unnamed tributaries.<\/p>\n

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the topo map helps gives us a sense of the catchment extents but we need to be a lot more exact about this.\u00a0 As a starting point, I had a look at the River Environment Classification<\/a> data from the Ministry for the Environment.\u00a0 The main focus of this database is to classify rivers and streams into a hierarchy of different factors such as climate and geology (among six others).\u00a0 There’s a copy of the data in J:\\Data\\River_Environment_Classification.\u00a0 The\u00a0Catchments folder contains shapefiles of catchments of different sizes, or river orders.\u00a0 Higher order streams are larger and have more tributaries than do lower order streams – you could think of low order streams as the headwaters; the order increases as tributaries contribute more water to a river system.\u00a0 I wasn’t sure what order these streams would be but figured they would be pretty low, first or second order.\u00a0 Adding the first order catchments to a map shows that they’re too small:<\/p>\n

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But the second order ones capture these catchments pretty well (here I’m just showing the three of interest with black outlines):<\/p>\n

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Looking more closely, the borders seem pretty jagged, almost raster-like:<\/p>\n

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This isn’t too surprising if you know where these polygons came from<\/a>.\u00a0 The catchment boundaries were created using some DEM based tools.\u00a0 In this case, it was a 25 m DEM with the output\u00a0converted to vector polygons, which explains the jagged nature.\u00a0 (In another post we’ll look at how these tools work.)\u00a0<\/em> The key thing with these boundaries is they have to capture the areas that contribute runoff to a particular stream.\u00a0 If we were to do this manually, we’d start at a point on the stream (in this case, where the water monitoring equipment is) and then move up the land slope, perpendicular to the elevation contours until we reach the highest points of the catchment.\u00a0 Doing this on both sides of the stream helps define the whole catchment area.\u00a0 We ended up doing this\u00a0manually, making use of as much existing data as I could.<\/p>\n

As noted above, our 1 m DEM could offer us some critical aide in this effort.\u00a0 But in it’s raw form, it doesn’t tell us that much.\u00a0 We’d get a lot more out of this using a hillshade <\/a>layer:<\/p>\n

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The data don’t fully cover the catchments but we can work with this.\u00a0 See how the hillshade helps define changes in slope which are tied to the topography?\u00a0 Another arrow in our quiver is the high-res imagery which will be useful as a reference:<\/p>\n

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This doesn’t fully cover my area of interest either but I’ve got the built in imagery in Pro plus some other satellite imagery that I can call on.\u00a0 Time to get started.<\/p>\n

I first created a new polygon feature class<\/a> for my catchments.\u00a0 To demonstrate how this whole process worked I’ll focus on Lagoon Creek and it’s unnamed tributary.\u00a0 The symbology is set to hollow fill to make it easier the see the line as\u00a0they are digitised.\u00a0 From the Edit<\/strong> <\/span>tab, I can click on Create<\/strong> <\/span>to open up the digitising tools<\/a>:<\/p>\n

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Knowing where Shyam’s water sampling instruments were gives me a starting point.\u00a0 This will be an irregularly shaped area so the Polygon tool is the best one to use.\u00a0 To start, I clicked on the location of the sampler and then started drawing a line that went perpendicularly up the elevation contours (since water always flows downhill):<\/p>\n

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Moving around while digisting can be a challenge – to pan it’s easiest to hold down the mouse wheel to move around and left-click\/hold\/move mouse to zoom in and out – gradually the boundary line gets built up – I click once every time I want to change direction:<\/p>\n

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Here I’ve made it to a local high point but still a ways to go.\u00a0 When digitising I can always go back to a vertex and either delete it (right-click on it > Delete Vertex) or move it around.\u00a0 I’ll do a quick double check of my progress using the hillshade:<\/p>\n

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And the imagery:<\/p>\n

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Reasonably happy so far, though I’m a little uncertain about the area around 3\/4s of the way up – Might be nice to look at it in 3D just to make sure – easy to do by converting this map to a scene (View\u00a0<\/strong><\/span>> Convert<\/strong> <\/span>> T<\/strong>o Local Scene<\/strong><\/span>):<\/p>\n

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This is with the hillshade on but I can easily turn layers on and off and change basemaps to make the most of what the data tell me.\u00a0 I’m finding this 3D view helpful so I’m going to go one step further and put these two views side by side and maximise their real estate so I can see both at once:<\/p>\n

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I can edit in either window (thought it is much faster in the 2D map – I really need a decent GPU<\/a>!!!) and just need to be sure I hit the Finish Sketch button,\u00a0\"\"<\/a>, before moving from one to the other.\u00a0 To further enhance this, I can connect to the two views using Link Views<\/strong><\/span>:<\/p>\n

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Now when I shift the view in one, the other moves to match.\u00a0 This became very handy, especially when the exact boundary became unclear.\u00a0 The linked views were an efficient way to do a bit of quality control while I was working.\u00a0 To me, this is one of the big benefits of using Pro over ArcMap, multiple maps and views in one project.\u00a0 (I also had Google Earth open at the same time as yet another point of reference.)<\/p>\n

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My strategy here was do a relatively quick first draft and then go back and refine it where needed.\u00a0 With digitising, sometimes it’s easier to get that first draft and then do further edits using the suite of editing tools, particularly the Edit Vertices<\/strong><\/span> tool,\u00a0\"\"<\/a>\u00a0.\u00a0 This one lets you create new vertices, reshape or delete existing ones to refine things by right-clicking on a vertex.<\/p>\n

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In the end I’m pretty happy with the results:<\/p>\n

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For comparisons sake, here’s my Cameron Gully boundary compared to the REC catchment:<\/p>\n

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It’s fairly different in certain parts and I think mine is 1) higher resolution and 2) a better representation of the catchment that Shyam was sampling.\u00a0 Digitising is a standard GIS thing to do but hopefully you can see how better data and the ability to visualise things has made this an easier (though not a quick) task .\u00a0 \u00a0There’s more to say about this project so stay tuned.<\/p>\n

C<\/p>\n","protected":false},"excerpt":{"rendered":"

We look at creating a catchment boundary layer using existing data and nice visualisations. So imagine you’re walking along a mountain ridge (like the one shown above from Antarctica – hope you’ve got your thermals on).\u00a0 When it starts to rain, water flows to one side or the other, depending on where it hits the […]<\/p>\n","protected":false},"author":3,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[],"class_list":["post-3152","post","type-post","status-publish","format-standard","hentry","category-uncategorized"],"_links":{"self":[{"href":"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-json\/wp\/v2\/posts\/3152","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-json\/wp\/v2\/users\/3"}],"replies":[{"embeddable":true,"href":"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-json\/wp\/v2\/comments?post=3152"}],"version-history":[{"count":1,"href":"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-json\/wp\/v2\/posts\/3152\/revisions"}],"predecessor-version":[{"id":4075,"href":"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-json\/wp\/v2\/posts\/3152\/revisions\/4075"}],"wp:attachment":[{"href":"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-json\/wp\/v2\/media?parent=3152"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-json\/wp\/v2\/categories?post=3152"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-json\/wp\/v2\/tags?post=3152"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}