GIS Blog

Profiles in Curvage

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 .  The blue line was drawn from north to south and the elevation profile was sampled from the DEM.  These are nice ways of getting cross-sections through elevation data and in this post we’ll look at how this works.

There are two ways to get an elevation profile.  How they differ depends on what you’re using for the elevation.

Let’s start with a new map that has the same hillshade layer on it:

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

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

First off, I need a line for where I want my profile to be.  I could use an existing layer or I can click the pencil and create a temporary one.  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.  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:

Clicking Run adds a new layer to the Contents: 

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

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

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

(I wouldn’t normally show this but I will because it actually looks pretty cool, very windswept, eh?)  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.

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

 

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

(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.)    This tool works in the same way as the tool we used earlier:

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

This one looks quite a lot more detailed than our first one – it should as it now has access to much higher resolution data from the LiDAR.  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 %):

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

In the 3D scene below, I’ve got a 1m 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).  There are elevations in the that capture heights of buildings and trees but they are not being shown.

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.  Following the same steps as above, I can add in my 1 m resolution DSM and use that for display purposes:

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

 

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:

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.  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.  There’s not an exact match, so we see some poor pixels spread out over many 10s of metres to form a continuous surface.

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).  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.  Certainly can be done, but definitely a topic for another day.

C

 

 

 

• 30/09/2021


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