{"id":3353,"date":"2021-10-13T13:34:36","date_gmt":"2021-10-13T00:34:36","guid":{"rendered":"http:\/\/blogs.lincoln.ac.nz\/gis\/?p=3353"},"modified":"2023-05-07T02:52:11","modified_gmt":"2023-05-07T02:52:11","slug":"care-and-feeding-of-polygons","status":"publish","type":"post","link":"https:\/\/blogs.lincoln.ac.nz\/gis\/care-and-feeding-of-polygons\/","title":{"rendered":"Care and Feeding of Polygons"},"content":{"rendered":"<p><em>We look at different editing tools for working with vector polygons in this post.<\/em><\/p>\n<p>Digitising (creating and editing vector features) is a key GIS skill.\u00a0 Most find that it&#8217;s also one that can be more than a bit frustrating.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-3392\" src=\"https:\/\/d-blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/hwnwufpkqsx41.gif\" alt=\"\" width=\"346\" height=\"200\" \/><\/p>\n<p>Yes, we&#8217;ve all been there&#8230;<\/p>\n<p>At the risk of damage to your machine (and psyche), we&#8217;ll look at a couple of the editing options for some polygon data in this post.\u00a0 Specifically, we&#8217;ll look at the following editing tools:<\/p>\n<ul>\n<li>Create Features &gt; Polygon<\/li>\n<li>Edit Vertices<\/li>\n<li>Autocomplete Polygon<\/li>\n<li>Update<\/li>\n<li>Split<\/li>\n<li>Merge<\/li>\n<\/ul>\n<p>In a previous post, we looked at <a href=\"http:\/\/blogs.lincoln.ac.nz\/gis\/on-the-edge\/\" target=\"_blank\" rel=\"noopener noreferrer\">creating a catchment boundary<\/a> at Mt Grand and we saw how to create a line feature (and wasn&#8217;t that fun&#8230;).\u00a0 The three catchments of interest are Lagoon Creek, an unnamed tributary to the south, and further north, Cameron Gully.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-3356\" src=\"https:\/\/d-blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/Catchments.jpg\" alt=\"\" width=\"584\" height=\"769\" srcset=\"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/Catchments.jpg 584w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/Catchments-228x300.jpg 228w\" sizes=\"auto, (max-width: 584px) 100vw, 584px\" \/><\/p>\n<p>That was the precursor to the main event of creating a land cover layer based on some drone imagery.\u00a0 We&#8217;ve been trying some image classification approaches which are taking a bit longer than hoped, so in the interests of time, we opted to create our own layer based on visual interpretation of the imagery, balanced by some on-the-ground knowledge.\u00a0 The best land cover data we had going into this was, of course, the<a href=\"https:\/\/lris.scinfo.org.nz\/layer\/104400-lcdb-v50-land-cover-database-version-50-mainland-new-zealand\/\" target=\"_blank\" rel=\"noopener noreferrer\"> Landcover Database (v5)<\/a>.\u00a0 As you can see in the image below, the resolution is pretty coarse, being mapped at a scale of 1:50,000.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-3358\" src=\"https:\/\/d-blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/LCDB.jpg\" alt=\"\" width=\"537\" height=\"753\" srcset=\"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/LCDB.jpg 537w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/LCDB-214x300.jpg 214w\" sizes=\"auto, (max-width: 537px) 100vw, 537px\" \/><\/p>\n<p>We aimed to do something a bit better, along the lines of 1:5,000.\u00a0 How did we decide on this scale?\u00a0 Well, it meant a lot of back and forth and thinking about what&#8217;s the smallest feature that needs to be mapped for the research.\u00a0 Are we wanting individual plants?\u00a0 Or collections of plants?\u00a0 Or rough groupings of plants?\u00a0 What it gets down to is what level of detail is needed in the output?\u00a0 This is a critical question as it sets the scope for digitising and 1:5,000 seemed to fit all the criteria.\u00a0 With that set, we also needed to think about how we could keep it consistent across the catchments of interest.<\/p>\n<p>For this post, I&#8217;ll focus on Cameron Creek as it&#8217;s the simplest of the three.\u00a0 Now I&#8217;m no expert on land cover at Mt Grand.\u00a0 Heck, I&#8217;ve never even been there!\u00a0 What do I know?\u00a0 For this to work, I needed input from those closest to the source: Shaym Provost, PhD candidate, and\u00a0<a href=\"https:\/\/researchers.lincoln.ac.nz\/tom.maxwell\">Tom Maxwell<\/a>, el supremo of grassland management.\u00a0 As a first cut, I asked Shyam and Tom to draw what they were thinking on a printed out image:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-3355\" src=\"https:\/\/d-blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/CameronsRaw.jpg\" alt=\"\" width=\"1266\" height=\"735\" srcset=\"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/CameronsRaw.jpg 1266w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/CameronsRaw-300x174.jpg 300w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/CameronsRaw-1024x595.jpg 1024w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/CameronsRaw-768x446.jpg 768w\" sizes=\"auto, (max-width: 1266px) 100vw, 1266px\" \/><\/p>\n<p>The accompanying legend told me that orange = Riparian, green = Mixed scrub, purple = Kanuka and everything else is Mixed Pasture.\u00a0 <em>(NB: there ended up being more classes than these, as we&#8217;ll see later.)<\/em><\/p>\n<p>This gave me a starting point.\u00a0 I could look at\u00a0the areas marked out on the printed image and match them against what I can see on the image itself and get a better sense of what the different land covers look like.\u00a0 Where to start then?\u00a0 The aim here is to end up with a polygon layer showing the different land covers.\u00a0 I started by <a href=\"http:\/\/blogs.lincoln.ac.nz\/gis\/how-do-i-create-a-new-feature-classshapefile\/\" target=\"_blank\" rel=\"noopener noreferrer\">creating a new polygon feature clas<\/a>s in the project geodatabase, using New Zealand Transverse Mercator as the layer&#8217;s coordinate system.\u00a0 This gives me an empty canvas to work with.\u00a0 I want to come up with a bit of a plan so I&#8217;ll start with looking at the whole catchment and decide where to start:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-3357\" src=\"https:\/\/d-blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/CameronBDY.jpg\" alt=\"\" width=\"1152\" height=\"767\" srcset=\"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/CameronBDY.jpg 1152w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/CameronBDY-300x200.jpg 300w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/CameronBDY-1024x682.jpg 1024w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/CameronBDY-768x511.jpg 768w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/CameronBDY-330x220.jpg 330w\" sizes=\"auto, (max-width: 1152px) 100vw, 1152px\" \/><\/p>\n<p>I&#8217;m thinking I&#8217;ll start at the\u00a0outlet\u00a0ofthe\u00a0catchment that looks like it&#8217;s in cropping.\u00a0 Not a class that Shyam&#8217;s provided but I&#8217;m adding it as I can see it&#8217;s there.\u00a0 With that new, empty polygon layer on the map, I can go to the <span style=\"color: #0000ff\">Edit<\/span> tab and pick <span style=\"color: #0000ff\">Create Features<\/span>.\u00a0 In the pane that opens to the right, all the editable layers are shown, plus some different digitising tools for each editable layer:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-3359\" src=\"https:\/\/d-blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/Create.jpg\" alt=\"\" width=\"1914\" height=\"858\" srcset=\"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/Create.jpg 1914w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/Create-300x134.jpg 300w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/Create-1024x459.jpg 1024w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/Create-768x344.jpg 768w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/Create-1536x689.jpg 1536w\" sizes=\"auto, (max-width: 1914px) 100vw, 1914px\" \/>The simplest tool to use is the <span style=\"color: #3366ff\">Polygon<\/span> tool, which does just what it says it does &#8211; creates polygons.\u00a0 With this tool selected, a polygon is started with a click, then every subsequent click draws a line, changing direction as needed.\u00a0 Finish off the polygon by double-clicking.\u00a0 My plan is to start rough, knowing that later on I can clip the land cover layer to the catchment boundary to keep everything inside:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-3360\" src=\"https:\/\/d-blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/sktech.jpg\" alt=\"\" width=\"753\" height=\"817\" srcset=\"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/sktech.jpg 753w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/sktech-276x300.jpg 276w\" sizes=\"auto, (max-width: 753px) 100vw, 753px\" \/><\/p>\n<p>This is referred to as a sketch &#8211; if I&#8217;m unhappy with it I can just click the\u00a0<img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-3361\" src=\"https:\/\/d-blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/X.jpg\" alt=\"\" width=\"38\" height=\"43\" \/>\u00a0and start over.\u00a0 When I&#8217;m satisfied, clicking\u00a0<img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-3362\" src=\"https:\/\/d-blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/Save.jpg\" alt=\"\" width=\"41\" height=\"39\" \/>finishes the sketch, followed by a\u00a0<span style=\"color: #0000ff\">Save<\/span> from the <span style=\"color: #0000ff\">Edit<\/span> tab:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-3363\" src=\"https:\/\/d-blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/EditSave.jpg\" alt=\"\" width=\"562\" height=\"152\" srcset=\"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/EditSave.jpg 562w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/EditSave-300x81.jpg 300w\" sizes=\"auto, (max-width: 562px) 100vw, 562px\" \/><\/p>\n<p>This makes it permanent.\u00a0 Note that we&#8217;ve now got\u00a0two different kinds of saves available &#8211; this one saves any editing changes while the Save Project,\u00a0<img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-3364\" src=\"https:\/\/d-blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/SaveProject.jpg\" alt=\"\" width=\"29\" height=\"27\" \/>, saves changes to the whole project &#8211; but <span style=\"text-decoration: underline\">not necessarily<\/span> the edits.\u00a0 Okay, so I&#8217;ve got a rough start, but on close inspection, I&#8217;m not 100% happy so I&#8217;d like to reshape it a bit around the stream.\u00a0 Clicking <span style=\"color: #3366ff\">Edit<\/span> tab &gt;\u00a0<span style=\"color: #0000ff\">Modify<\/span> brings up a whole suite of editing tools:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-3365\" src=\"https:\/\/d-blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/ModifyFeatures.jpg\" alt=\"\" width=\"296\" height=\"858\" srcset=\"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/ModifyFeatures.jpg 296w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/ModifyFeatures-103x300.jpg 103w\" sizes=\"auto, (max-width: 296px) 100vw, 296px\" \/><\/p>\n<p>For this job I&#8217;ll use <span style=\"color: #3366ff\">Reshape<\/span> &gt;\u00a0\u00a0<span style=\"color: #0000ff\">Edit Vertices<\/span>.\u00a0 Each time I clicked, I created a vertex and the line that joins the new vertex to the previous one is an edge (remember that for later).\u00a0 This tool allows me to rearrange, or add or delete vertices once the feature is selected &#8211; they are visible below as green boxes with one red one, that being the final vertex of the polygon.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-3366\" src=\"https:\/\/d-blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/EditVertices2.jpg\" alt=\"\" width=\"706\" height=\"599\" srcset=\"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/EditVertices2.jpg 706w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/EditVertices2-300x255.jpg 300w\" sizes=\"auto, (max-width: 706px) 100vw, 706px\" \/><\/p>\n<p>Now I can grab a vertex and reposition it to tidy up my first attempt &#8211; I&#8217;m just going by what I can see on the image and interpreting as I go.\u00a0 I can just see a fence line running west and then angling to the southeast:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-3367\" src=\"https:\/\/d-blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/EditsToPoly.jpg\" alt=\"\" width=\"358\" height=\"355\" srcset=\"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/EditsToPoly.jpg 358w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/EditsToPoly-300x297.jpg 300w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/EditsToPoly-150x150.jpg 150w\" sizes=\"auto, (max-width: 358px) 100vw, 358px\" \/><\/p>\n<p>Once I finish the sketch and save the edits, it&#8217;s saved to the layer:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-3368\" src=\"https:\/\/d-blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/NewPoly.jpg\" alt=\"\" width=\"669\" height=\"608\" srcset=\"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/NewPoly.jpg 669w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/NewPoly-300x273.jpg 300w\" sizes=\"auto, (max-width: 669px) 100vw, 669px\" \/><\/p>\n<p>If I right-click on a vertex I can delete it or, by clicking on an edge I can add additional vertices.<\/p>\n<p>Polygons on the map is one thing, but labelling each with the correct landcover is another.\u00a0 By adding a text field to the layer, called LandCover, I can use this to classify each polygon.\u00a0 But as you can imagine, there&#8217;s extra effort involved in always typing things exactly and consistently.\u00a0 As far as Pro is concerned, &#8220;Mixed Scrub&#8221; is different from &#8220;mixed scrub&#8221;, let alone &#8220;Mixed scrub&#8221;\u00a0or &#8220;Nixed Shrub&#8221;.\u00a0 I&#8217;m going to do something a little tricky and set up a drop down list of pre-baked options to choose from for Land Cover so I take my potential mistyping out of the equation.\u00a0 (I can already see the shape of the next blog post&#8230;).\u00a0 Here&#8217;s how that looks:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-3369\" src=\"https:\/\/d-blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/Domain.jpg\" alt=\"\" width=\"517\" height=\"345\" srcset=\"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/Domain.jpg 517w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/Domain-300x200.jpg 300w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/Domain-330x220.jpg 330w\" sizes=\"auto, (max-width: 517px) 100vw, 517px\" \/><\/p>\n<p>Now I can assign a class for each polygon by selecting one of the pre-baked options, thus avoiding my inevitable misspellings.<\/p>\n<p>Next, up Shyams hand drawn map shows me that there&#8217;s some kanuka just next to this polygon.\u00a0 I could use the Polygon tool to create this one but there&#8217;s always an issue when creating a new polygon that shares an existing boundaries.\u00a0 With this area, it&#8217;s small enough that I could probably create an edge that matches the existing one, especially if I use snapping.\u00a0 But there&#8217;s a far easier way: <a href=\"https:\/\/pro.arcgis.com\/en\/pro-app\/latest\/help\/editing\/create-adjoining-polygons.htm\" target=\"_blank\" rel=\"noopener noreferrer\">Autocomplete Polygon<\/a>, <img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-3372\" src=\"https:\/\/d-blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/ACpoly.jpg\" alt=\"\" width=\"30\" height=\"27\" \/>.\u00a0 With this tool I start digitising <em>inside<\/em> the exiting polygon, then do all the vertices, finishing again inside the existing polygon.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"size-full wp-image-3373 alignnone\" src=\"https:\/\/d-blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/ACpolyD.jpg\" alt=\"\" width=\"534\" height=\"407\" srcset=\"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/ACpolyD.jpg 534w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/ACpolyD-300x229.jpg 300w\" sizes=\"auto, (max-width: 534px) 100vw, 534px\" \/><\/p>\n<p>Pro then automatically finishes with an exact, shared boundary, giving\u00a0me a nice result:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-3374\" src=\"https:\/\/d-blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/ACpolyD2.jpg\" alt=\"\" width=\"532\" height=\"458\" srcset=\"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/ACpolyD2.jpg 501w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/ACpolyD2-300x258.jpg 300w\" sizes=\"auto, (max-width: 532px) 100vw, 532px\" \/><\/p>\n<p>Using this tool becomes essential when you&#8217;re working with lots of polgyons with different shapes.\u00a0 If you try and match borders by hand you often end up with either &#8220;<a href=\"https:\/\/en.wikipedia.org\/wiki\/Sliver_polygon\" target=\"_blank\" rel=\"noopener noreferrer\">sliver polygons<\/a>&#8221; (worse than a paper cut!) or overlaps that will mess up your area calculations.<\/p>\n<p>Looking back a Shyam&#8217;s map, most of the catchment is classed as Mixed Pasture, so I next create a big polygon\u00a0to cover this area with <span style=\"color: #3366ff\">Autocomplete Polygon<\/span> and then I\u00a0can later chop it up into smaller bits of Mixed Scrub and Riparian.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-3375\" src=\"https:\/\/d-blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/BigPoly.jpg\" alt=\"\" width=\"899\" height=\"634\" srcset=\"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/BigPoly.jpg 899w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/BigPoly-300x212.jpg 300w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/BigPoly-768x542.jpg 768w\" sizes=\"auto, (max-width: 899px) 100vw, 899px\" \/><\/p>\n<p>Might as well <a href=\"https:\/\/pro.arcgis.com\/en\/pro-app\/latest\/tool-reference\/analysis\/clip.htm\" target=\"_blank\" rel=\"noopener noreferrer\">Clip <\/a>these polygons to the catchment boundary to tidy things up:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-3376\" src=\"https:\/\/d-blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/ThreePolys.jpg\" alt=\"\" width=\"1617\" height=\"856\" srcset=\"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/ThreePolys.jpg 1617w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/ThreePolys-300x159.jpg 300w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/ThreePolys-1024x542.jpg 1024w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/ThreePolys-768x407.jpg 768w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/ThreePolys-1536x813.jpg 1536w\" sizes=\"auto, (max-width: 1617px) 100vw, 1617px\" \/><\/p>\n<p>I&#8217;ve used the pre-cooked LandCover classes for my symbology here (that&#8217;s why there are so many shown) so you can see that 1) the extent matches my catchment boundary, and 2) we&#8217;re getting there.<\/p>\n<p>According to Shyam&#8217;s map, there are smaller patches within the Mixed Pasture area that need to carved out for both Mixed Scrub and Riparian.\u00a0 Surprisingly, it&#8217;s rather difficult using the <span style=\"color: #3366ff\">Modify Features<\/span> tools to add new polygons <em>into<\/em> an existing one.\u00a0 It&#8217;s easy enough to create new ones <em>on top<\/em> of an existing one, but then I won&#8217;t have a true representation of the areas when I go to tally up hectares of land cover.\u00a0 So I&#8217;ll use one of the less often used boundary operations: <span style=\"color: #3366ff\"><a style=\"color: #3366ff\" href=\"https:\/\/pro.arcgis.com\/en\/pro-app\/latest\/tool-reference\/analysis\/update.htm\" target=\"_blank\" rel=\"noopener noreferrer\">Update<\/a><\/span>.\u00a0 With this tool, I intend to create the rats and mice polygons for Riparian and Mixed Scrub areas and then essentially force them in to the existing poly.\u00a0 That means creating a new layer with these polgyons in them and then joining that to the existing layer.\u00a0 It&#8217;s a simple tool to use &#8211; you just have to be mindful about which layer is which:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-3377\" src=\"https:\/\/d-blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/Update.jpg\" alt=\"\" width=\"295\" height=\"312\" srcset=\"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/Update.jpg 295w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/Update-284x300.jpg 284w\" sizes=\"auto, (max-width: 295px) 100vw, 295px\" \/><\/p>\n<p>With this result:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-3378\" src=\"https:\/\/d-blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/Updated.jpg\" alt=\"\" width=\"1616\" height=\"852\" srcset=\"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/Updated.jpg 1616w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/Updated-300x158.jpg 300w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/Updated-1024x540.jpg 1024w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/Updated-768x405.jpg 768w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/Updated-1536x810.jpg 1536w\" sizes=\"auto, (max-width: 1616px) 100vw, 1616px\" \/><\/p>\n<p>Please note that I&#8217;m being really rough here to demonstrate things for this post &#8211; in the final version I paid a lot more attention to exact shapes and extents of areas.<\/p>\n<p>I&#8217;m not all that happy with the patch of Mixed Scrub\u00a0at the top of the Riparian area &#8211; I\u00a0haven&#8217;t quite gotten the right boundary\u00a0when I look at the satellite image, so I want to adjust that polygon a bit.\u00a0 <span style=\"color: #3366ff\">Edit Vertices<\/span> seems like a good way to go &#8212; but when I use this, I notice something a bit odd:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-3379\" src=\"https:\/\/d-blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/EditVertices3.jpg\" alt=\"\" width=\"655\" height=\"609\" srcset=\"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/EditVertices3.jpg 655w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/EditVertices3-300x279.jpg 300w\" sizes=\"auto, (max-width: 655px) 100vw, 655px\" \/><\/p>\n<p>I&#8217;ve added a new vertex (with a right-click) and moved it to the south &#8211; but in the process, the adjoining Mixed Pasture polygon hasn&#8217;t shifted, so I&#8217;m left with a hole!\u00a0 What to do?\u00a0 Fear not, there&#8217;s a way to address this.\u00a0 First, on the <span style=\"color: #3366ff\">Edit<\/span> tab, set the map to use <span style=\"color: #3366ff\">Map Topology<\/span>:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-3380\" src=\"https:\/\/d-blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/MapTopo.jpg\" alt=\"\" width=\"569\" height=\"267\" srcset=\"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/MapTopo.jpg 569w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/MapTopo-300x141.jpg 300w\" sizes=\"auto, (max-width: 569px) 100vw, 569px\" \/><\/p>\n<p>Topology is all about how features are spatially related to each other in space, especially those that are connected in some way, as in our polygons sharing boundaries.\u00a0 Once set, the <span style=\"color: #3366ff\">Edit Vertices<\/span> tool now gives us a choice between <span style=\"color: #3366ff\">Features<\/span> and <span style=\"color: #3366ff\">Edges<\/span>:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-3381\" src=\"https:\/\/d-blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/EditFeatures.jpg\" alt=\"\" width=\"789\" height=\"530\" srcset=\"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/EditFeatures.jpg 789w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/EditFeatures-300x202.jpg 300w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/EditFeatures-768x516.jpg 768w\" sizes=\"auto, (max-width: 789px) 100vw, 789px\" \/><\/p>\n<p>Clicking over to <span style=\"color: #3366ff\">Edges<\/span> means I can now move the vertices around and maintain the contiguity (after I add in a new vertex):<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-3382\" src=\"https:\/\/d-blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/EditEdges.jpg\" alt=\"\" width=\"801\" height=\"530\" srcset=\"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/EditEdges.jpg 801w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/EditEdges-300x199.jpg 300w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/EditEdges-768x508.jpg 768w\" sizes=\"auto, (max-width: 801px) 100vw, 801px\" \/><\/p>\n<p>So we&#8217;re getting there &#8211; one last thing I want to show.\u00a0 Chatting with Shyam some more it was clear that we need to also highlight areas of Snow Tussock high in the catchment.\u00a0 The rough guidance from DOC was that tussocks come in above roughly 1000 m in elevation, so I added in the 20 m contours and selected the 1000 m one to give me an indication of where it is:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-3383\" src=\"https:\/\/d-blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/contours.jpg\" alt=\"\" width=\"1287\" height=\"827\" srcset=\"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/contours.jpg 1287w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/contours-300x193.jpg 300w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/contours-1024x658.jpg 1024w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/contours-768x494.jpg 768w\" sizes=\"auto, (max-width: 1287px) 100vw, 1287px\" \/><\/p>\n<p>The easiest way to add this dividing line in is to use the <span style=\"color: #3366ff\">Split<\/span> tool from <span style=\"color: #3366ff\">Modify Features<\/span>.\u00a0 I&#8217;ve got two options to use this tool &#8211; <span style=\"color: #3366ff\">Interactive<\/span>, whereby I draw a line where I want to split the polygon, or<span style=\"color: #3366ff\"> By Feature<\/span>, where I get to use an existing feature to do the splitting.\u00a0 <span style=\"color: #3366ff\">Interactive<\/span> is just another instance of digitising, so let&#8217;s try <span style=\"color: #3366ff\">By Feature<\/span>.\u00a0 With this tool open I need to explicitly say which are the input features (the ones used to do the splitting &#8211; the 1000 m contour) by selecting them (it) and which are the\u00a0ones that will be split (the Target Features):<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-3384\" src=\"https:\/\/d-blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/Split.jpg\" alt=\"\" width=\"290\" height=\"358\" srcset=\"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/Split.jpg 290w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/Split-243x300.jpg 243w\" sizes=\"auto, (max-width: 290px) 100vw, 290px\" \/><\/p>\n<p>Running this tool gives me some new polygons (turned on some labels here to help):<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-3386\" src=\"https:\/\/d-blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/tussock2.jpg\" alt=\"\" width=\"846\" height=\"788\" srcset=\"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/tussock2.jpg 846w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/tussock2-300x279.jpg 300w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/tussock2-768x715.jpg 768w\" sizes=\"auto, (max-width: 846px) 100vw, 846px\" \/><\/p>\n<p>Ah, well, I thought I was mostly done, but now I see I&#8217;ve created a bit of untidiness&#8230;the contour line has nicely split things but I&#8217;ve got two polygons for Snow Tussock when I should really have just one.\u00a0 The contour crossing over the Mixed Scrub poly has given me that extra little bit.\u00a0 No worries &#8211; last step is to use <span style=\"color: #3366ff\">Modify Features<\/span>\u00a0&gt; <span style=\"color: #3366ff\">Construct<\/span> &gt; <span style=\"color: #3366ff\">Merge<\/span>.\u00a0 First I select both polys, and then click Merge:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-3387\" src=\"https:\/\/d-blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/Merge.jpg\" alt=\"\" width=\"925\" height=\"687\" srcset=\"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/Merge.jpg 925w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/Merge-300x223.jpg 300w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/Merge-768x570.jpg 768w\" sizes=\"auto, (max-width: 925px) 100vw, 925px\" \/><\/p>\n<p>Eh voila!<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-3388\" src=\"https:\/\/d-blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/Merged.jpg\" alt=\"\" width=\"297\" height=\"263\" \/><\/p>\n<p>Job mostly done.\u00a0 There were a few refinements from here, including adding a transition zone from to the Snow Tussock line, modifying existing polygons and also adding High and Low Producing Pasture classes, but the steps outlined here were used to get us to our final results for Cameron Gully:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-3389\" src=\"https:\/\/d-blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/FinalCamerons.jpg\" alt=\"\" width=\"845\" height=\"612\" srcset=\"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/FinalCamerons.jpg 845w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/FinalCamerons-300x217.jpg 300w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/FinalCamerons-768x556.jpg 768w\" sizes=\"auto, (max-width: 845px) 100vw, 845px\" \/><\/p>\n<p>And just to finish off, the other two:<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-3390\" src=\"https:\/\/d-blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/Others.jpg\" alt=\"\" width=\"769\" height=\"620\" srcset=\"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/Others.jpg 769w, https:\/\/blogs.lincoln.ac.nz\/gis\/wp-content\/uploads\/sites\/3\/2021\/10\/Others-300x242.jpg 300w\" sizes=\"auto, (max-width: 769px) 100vw, 769px\" \/><\/p>\n<p><em>Aside: another option for this\u00a0effort could have been to make a copy of the catchment boundary polygons and start carving them up with <span style=\"color: #3366ff\">Split<\/span> and <span style=\"color: #3366ff\">Update<\/span>.\u00a0 Six of one, half dozen of the other, I say.<\/em><\/p>\n<p>There was quite a lot of back-and-forth between me and the experts to get this right.\u00a0 Quite a lot.\u00a0 We now have an essentially bespoke land cover classification at a roughly 1:5,000 scale, just right (hopefully) for Shyam&#8217;s analysis.\u00a0 Along the way, I hope you&#8217;ve gotten a taste for the wide range of editing tools that are available (and can see why people might opt for image classification to do this sort of thing).\u00a0 It&#8217;s a painstaking process and I think I may now need to visit my optometrist (and psychiatrist for that matter), but that&#8217;s the cost of trying to get things right.<\/p>\n<p>C<\/p>\n","protected":false},"excerpt":{"rendered":"<p>We look at different editing tools for working with vector polygons in this post. Digitising (creating and editing vector features) is a key GIS skill.\u00a0 Most find that it&#8217;s also one that can be more than a bit frustrating. Yes, we&#8217;ve all been there&#8230; At the risk of damage to your machine (and psyche), we&#8217;ll [&hellip;]<\/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-3353","post","type-post","status-publish","format-standard","hentry","category-uncategorized"],"_links":{"self":[{"href":"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-json\/wp\/v2\/posts\/3353","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=3353"}],"version-history":[{"count":2,"href":"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-json\/wp\/v2\/posts\/3353\/revisions"}],"predecessor-version":[{"id":4965,"href":"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-json\/wp\/v2\/posts\/3353\/revisions\/4965"}],"wp:attachment":[{"href":"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-json\/wp\/v2\/media?parent=3353"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-json\/wp\/v2\/categories?post=3353"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-json\/wp\/v2\/tags?post=3353"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}