Photo credit: Douglas Broughton

As most everyone will be aware, the Burns Building is slowly being demolished. I doubt that many will mourn its passing, but I’m no architectural critic. What I can be critical about is views. In my new digs in far reaching Forbistan (Ed. you mean the Forbes Building, right?), I was checking out the view for our fourth floor tea room – as things progress, this is what I can see:

From this vantage point, the Port Hills have long been blocked by Burns and of course my delicate spatial condition got me thinking about Viewsheds (Ed. Counselling is available, please seek help.) With the right data, we can model visibility from points on the landscape – this has applications to the best locations for telecommunications towers, military strategies, and visual impact assessments of all kinds. We’ve been using them in the GIS courses recently to evaluate how visible wind turbine and fire tower locations are. I was wondering how our view would change once the demolition was complete.

To do viewsheds, we need elevation data as a key input. We also need to know where we’re determining the visibility from, usually in the form of points. First to elevation.

As we have seen previously, there are (at least) two kinds of elevation models: Digital Elevation Models (DEMs) and Digital Surface Models (DSMs), usually in raster form. A DEM is a “bare earth” model, so just ground level elevations with no trees or trees or anything on the ground surface. A DSM captures both the bare earth elevations as well as features on the surface. To do proper viewsheds, one should really use DSMs if they are available as those buildings and trees can have a huge impact on visibility.

With the advent of LiDAR and high resolution elevation data, these are becoming easier to get our hands on. So to start this analysis, I went to the LINZ Data Service and searched around for some relevant data. The best I could find was a 2023 DSM covering much of the Selwyn District (the DEM is also available). I was only interested in a small subset of the overall dataset so cropped those data and downloaded what I needed:

Once downloaded and massaged a bit, I could add this to a map. DEMs on their own aren’t that easy to interpret so I also made a Hillshade layer to better visualise these data:

Here we’re zoomed into the Lincoln campus and you can make out the outlines of buildings and larger trees, plus a lot of other features. Note that I’ve also added a red point that is my rough approximation of the tea room window location. Before I can run my viewshed I need to take into account that I’m up on the 4th floor, not at ground level. We can do this by adding in a special field to the point attribute table called OffsetA. Then I can add in my elevation above the ground and the viewshed will be determined from this level. My rough approximation of this was 12.5 m. I was also interested in limiting the field of view. After all, my view is from inside the building so I can’t see all 360 degrees around me. I can take this into account with two additional fields: AZIMUTH1 and AZIMUTH2. The first tells Pro what compass direction to start from and the second tells it where to stop (see here for more details on these settings). I set these as below (degrees from north):

Now we’re ready to run the Viewshed tool. The elevation layer is called SelwynDSM.tif:

Here’s my output with for the immediate area on the hillshade layer:

The areas visible from the vantage point are orange – compared to the photo above I thought it did a pretty good job of capturing what I can see from the tea room. You can clearly see how Burns blocks much of the view. Zooming out, we can see the wider visibility of the hills:

A bit hard to see it all at this scale but it does show how much of the view up Early Valley Road is blocked.

So what will it look like after Burns goes away? To do this I had to remove the Burns Building from the DSM data. That’s a little bit complicated and I’ll cover it in another post, but here’s the upshot in my hillshade layer:

Et voila! Bye bye Burns. I think I might put a quote in to take over the demolition work – I totally removed the whole building in about 10 minutes. Now, let’s rerun the viewshed with this new DSM:

Visible areas are now purple. Not surprisingly, there’s much more visibility across the road. And the wider view:

We’re gonna have a grand view of the hills! To finish this off, here’s that viewshed on a 3D satellite image – nice to know that we’ll be having such a lovely view when all the dust finally settles.

C