{"id":3325,"date":"2021-09-30T11:04:37","date_gmt":"2021-09-29T22:04:37","guid":{"rendered":"http:\/\/blogs.lincoln.ac.nz\/gis\/?p=3325"},"modified":"2023-05-07T02:52:40","modified_gmt":"2023-05-07T02:52:40","slug":"profiles-in-curvage","status":"publish","type":"post","link":"https:\/\/blogs.lincoln.ac.nz\/gis\/profiles-in-curvage\/","title":{"rendered":"Profiles in Curvage"},"content":{"rendered":"

Here we look at creating elevation profile plots from high resolution LiDAR data.<\/em><\/p>\n

In a previous post<\/a> we looked at some elevation profiles using LiDAR data, like this one:<\/p>\n

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On the map we have a hillshade layer from a 1 m resolution DEM<\/a> derived from LiDAR<\/a> .\u00a0 The blue line was drawn from north to south and the elevation profile was\u00a0sampled from the DEM.\u00a0 These are nice ways of getting cross-sections through elevation data and in this post we’ll look at how this works.<\/p>\n

There are two ways to get an elevation profile.\u00a0 How they differ depends on what you’re using for the elevation.<\/p>\n

Let’s start with a new map that has the same hillshade<\/a> layer on it:<\/p>\n

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Remember, the hillshade layer doesn’t actually have any elevation data in it.\u00a0 While it was derived from a high resolution DEM, it’s purpose is to simulate the fall of shadows give the terrain.\u00a0 It’s a nice, effective way to show the topography.\u00a0 (I’ve actually got two hillshades on here – one for the peninsula and one for Kaitorete Spit – two separate datasets.)\u00a0 If you look in the Contents pane, I’ve got my hillshades and a basemap underneath (the NatGeo_World_Map).\u00a0 If I want to create an elevation profile, my option at this point<\/a> is found on the Analysis<\/span> tab under Ready To Use Tools<\/span>:<\/p>\n

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Perhaps another time we’ll look at some of these other options but for now we’ll focus on Profile.\u00a0 When the tool opens I’ve got a few parameters to set:<\/p>\n

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First off, I need a line for where I want my profile to be.\u00a0 I could use an existing layer or I can click the pencil and create a temporary one.\u00a0 With this option, a new layer gets added to the Contents and my cursor changes to cross-hairs – with this we can draw a line.\u00a0 I’ll go across the width of the Spit (ahem, barrier), clicking once at the start and anywhere I want the line to change direction, and double-clicking on the last point:<\/p>\n

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Clicking\u00a0Run adds a new layer to the Contents:\u00a0\"\"<\/p>\n

If I now right-click on the layer name and go to Create Chart<\/span> > Profile Graph<\/span> I’ll get a chart output:<\/p>\n

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Nice enough, yes?\u00a0 And it wasn’t too difficult.\u00a0 But I hope the astute amongst you (yeah, you!) are wondering where the elevation values came from.\u00a0 There’s nothing on my map that would give it those data, so what’s the deal?<\/p>\n

When using this tool, Pro goes off into the ether and uses a world wide elevation layer<\/a> with a roughly 60 m resolution courtesy of our friends at ESRI (Web services, anyone?).\u00a0 This should work just about anywhere in the world (Ed. Well, to be fair, only between 60\u00b0 north and 56\u00b0 south of the equator.<\/em>)\u00a0 But hang on, haven’t I got something better than this?\u00a0 Something on the order of 1 m?\u00a0 I most certainly do!\u00a0 So I can easily add this to my map:<\/p>\n

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(I wouldn’t normally show this but I will because it actually looks pretty cool, very windswept, eh?)\u00a0 Anyway, this isn’t going to do me much good really – there’s one more step I need do to take to take full advantage of these data.<\/p>\n

Recall that beside my hillshades and DEM, the only other thing on my map is a basemap.\u00a0 BUT, I can add the DEM as an elevation source which will allow me take advantage of all that detail.\u00a0 Do so from Map tab<\/span> > Add Data<\/span> > Elevation Source<\/span>.\u00a0 This adds a new group, Elevation Sources,\u00a0at the bottom of my Contents:<\/p>\n

\"\"<\/p>\n

Pro now knows about my high-res DEM and also makes a new tool available under Exploratory 3D Analysis<\/span> as the Elevation Profile<\/span><\/a> tool.\u00a0 This tool becomes active once an elevation source is added.<\/p>\n

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(To be honest, I didn’t even need to add the DEM to the map for this – the main thing was adding it as an Elevation Source.)\u00a0 \u00a0 This tool\u00a0works in the same way as the tool we used earlier:<\/p>\n

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I can either draw my own line interactively or use an existing layer.\u00a0 I’ll choose the latter as I’ve already drawn a line with the first tool.\u00a0 When it’s selected I can click Apply<\/span> and get a new output layer – this one goes straight to a profile:<\/p>\n

\"\"<\/p>\n

This one looks quite a lot more detailed than our first one –\u00a0it should as it now has access to\u00a0much higher resolution data from the LiDAR.\u00a0 As I move the cursor over the profile, I can see where that location is on the map plus get direct values for the elevation and the slope (in %):<\/p>\n

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And that’s not all.\u00a0 By adding in elevation sources of our own, our 3D scenes can better display the terrain.\u00a0 Here are a few quick examples.\u00a0 We’ll look at some data from\u00a0the Christchurch CDB\u00a0to illustrate this.<\/p>\n

In the 3D scene below, I’ve got a\u00a01m DSM (Digital Surface Model – this has ground elevations plus above ground features like buildings and trees) derived from LiDAR data draped over the built-in WorldElevation3D\/Terrain3D layer that automatically gets added to every 3D scene (the bottom item in the Contents pane).\u00a0 There are elevations in the that capture heights of buildings and trees but they are not being shown.<\/p>\n

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You can see a bit of relief around Hagley Park but the built in elevation model is a bare ground layer and doesn’t know about the buildings and trees.\u00a0 Following the same steps as above, I can add in my 1 m resolution DSM and use that for display purposes:<\/p>\n

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Yes, it looks a bit odd but that’s just Pro trying to display a blocky DSM as a continuous surface.\u00a0 Looking more closely (and using a bit of imagination) we can just make out features like the Cathedral:<\/p>\n

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Just for kicks, here’s the satellite image draped over this DSM – everything looks a bit melty, but the Cathedral actually comes out quite visible in 3D:<\/p>\n

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Trying to do things like this will also be a challenge in urban areas, where buildings are perpendicular to the ground surface (or supposed to be) but in more bucolic, rural areas, this can be quite effective.\u00a0 To understand the effect we’re seeing above, thing of the satellite image as a rubber sheet thrown over, say, a sofa and taking on the shape of that piece of furniture.\u00a0 There’s not an exact match, so we see some poor pixels spread out over many 10s of metres to form a continuous surface.<\/p>\n

So, the upshot of all this is that we can use our increasingly available high-resolution DEMs and DSMs to create elevation profiles and, to an extent, also use them for visualisation (provided we don’t zoom in too far).\u00a0 To be honest, if I were serious about doing 3D visualisations of urban areas, I might not choose GIS, or rather, if I did choose GIS, I’d probably spend a lot of my time creating 3D features (buildings, trees, etc) and building up a whole virtual world.\u00a0 Certainly can be done, but definitely a topic for another day.<\/p>\n

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

Here we look at creating elevation profile plots from high resolution LiDAR data. In a previous post we looked at some elevation profiles using LiDAR data, like this one: On the map we have a hillshade layer from a 1 m resolution DEM derived from LiDAR .\u00a0 The blue line was drawn from north to […]<\/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-3325","post","type-post","status-publish","format-standard","hentry","category-uncategorized"],"_links":{"self":[{"href":"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-json\/wp\/v2\/posts\/3325","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=3325"}],"version-history":[{"count":2,"href":"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-json\/wp\/v2\/posts\/3325\/revisions"}],"predecessor-version":[{"id":4068,"href":"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-json\/wp\/v2\/posts\/3325\/revisions\/4068"}],"wp:attachment":[{"href":"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-json\/wp\/v2\/media?parent=3325"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-json\/wp\/v2\/categories?post=3325"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-json\/wp\/v2\/tags?post=3325"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}