Category: 3D GIS

3D GIS is game changing, it can change the way you view your analysis, it can provide insights which you may have overlooked….but there is a little problem which I may had not shared in my deluge of blogs about how great 3D GIS is…..

…Sometimes it can be hard work. What I mean by this, is that it can sometimes need a lot more consideration than standard methods. Let’s look at some of the major culprits when using Esri ArcGIS Pro 1.4 & Desktop 10.5, keeping in mind that there isn’t many (or any) other 3D GIS you can do this kind of work in.

Editing

Once you have converted or built your 3D model into a multipatch polygon you may find yourself struggling to edit or adjust your model.

Split

Having built entire cities, one issue that I’ve come across a fair bit is removing parts of multipatch polygons. For example, when adding multiple models, you may find that two features might overlap and need to remove part of one. In a 2D, standard GIS, you would simply turn the editing on and split the offending polygon.

This doesn’t work in a 3D GIS…Think about it, how does the GIS know where all the planes you require breaking lie with a 3D plane? 3D GIS can do some pretty clever assumptions but I am yet to find a way to remove part of a complex multipatch polygon.

True, within ArcGIS Pro 1.4 there are some editing tools for 3D multipatch features but it is early days at the moment. For simple cubes it is quite easy to adjust and manipulate the model a little but you don’t stand a chance if you have a curved edge.

 

Don’t despair, don’t give up on the 3D just yet, as you know, 3D isn’t new and there are many “workarounds”. One of my favourites is using the ArcGIS Pro “Replace Multipatch” tool. If you want to make multiple edits to a model (feature) you can export the multipatch to a collada format or keyhole markup language (kml) format, edit it in Sketchup, Blender, Meshlab or any of your favourite modelling suites and then import it again without affecting the other features in that layer.

 

If you are extruding simple 2D polygons with the intention of creating 3D mulitpatch polygons, it is a good idea to keep the conversion to multipatch until you explicitly need to. This way, you can edit, split and reshape your 2D polygon within normal ArcGIS Desktop without any issues at all and then draw up and extrude when you are 100% sure it all fits and works okay.

 

In my experience, pre-planning and clarity of the end goal means that you can be prepared for these slight niggles in advance.

Overlay analysis 

So, I’ve built out an entire city of 3D buildings, it looks amazing, last thing to do is clip them by the city boundary….oh, I forgot, the 2D polygon doesn’t truly intersect the multipatch polygon in 3D space, the “clip” tool doesn’t work, the “intersect tool” nor the “merge” tool…in fact you can ignore using modelbuilder.

Clip features

I learned the hard way that 3D features work best with 3D features. Unless the boundary polygon is a 3D feature, then you won’t be able to use spatial analysis to use any kind of overlay querying.

But, there is always a way to trick the system…

Although the multipatch polygon feature is a 3D object, you can still open it in ArcGIS Desktop, meaning that you can use the good ol’ fashioned “select by location” tool. No, it shouldn’t work but it does, furthermore, you can then export your selected data to a new multipatch feature. It begs the question why you can do this and not the “clip” as, in geoprocessing terms, they are the same thing (the clip tool is just the separate bits combined into a single script).

Let’s not get too hung up on it, as it works,

Volumetric Analysis

If you thought that the  “volume” tool was just put there to look clever, think again. It can be a great tool for easily representing floor space or calculating the tonnage of aggregate that needs extracting from the ground to lay a new pipeline.

There is a slight problem though…it can be a little hit and miss, especially when using sourced models.

Calculating Volume

Try adding a model you built in Sketchup, Blender or Meshlab to ArcGIS Pro and then calculate volume (using the add z information tool), Nothing, right? But why? The reason is that it doesn’t like “open” multipatch 3D features, it isn’t fully enclosed. Even if there is a slither of a gap in the polygon and it isn’t 100% enclosed, the tool cannot calculate the volume.

There are other methods whereby you can use a raster surface, like the “surface volume” tool but this isn’t quite as accurate as using your super detailed vector multipatch.

You could try the “Enclose Multipatch” tool, as this closes the multipatch and then running the volume tool BUT you need to consider that unless the multipatch is cut to the surface, for example, where a building sits on a hill and the base isn’t perfectly flat, the volume will not be ideal. So please consider using the data as a high resolution TIN which is merged with the terrain to provide a more accurate volume result.

Oh, last point on this – make sure you use a projected coordinate system that uses metric for your data, a geographic coordinate system will leave you with your volume in degrees….is that even possible?!

Which brings me nicely to –

Vertical referencing

I distinctly remember my first adventures into 3D through Google Earth, trying to create some of Romsey, UK as 3D buildings using Sketchup. The first hurdle was always figuring out whether the building was “Absolute height”, “On the Ground” or “Relative to the ground”…I mean, what does that mean anyway? I just drew it to sit on the floor, why does it need to ask me more questions about it?

Correct Height Placement

Right now, if you are a regular reader of xyHt or a 3D Geoninja, you will be calling me a muppet. In reality though, you wouldn’t believe how often I am asked about this, especially now that Digital Surface Models (DSMs), Digital Terrain Models (DTMs), bathymetry and other elevation data are so readily available.

Elevation is never easy, worst still, there are either too many or too few options. With the Esri ArcGIS Pro, I am 100% confident that I know where my data sits within 3D space but it is only because I’ve worked with this day in and day out and understand the limitations and data sources.

Let’s consider the Esri “scene” – it’s a cool 3D map and as you zoom into that lovely globe you can see lovely mountains and valleys all popping out of the surface, my question to you is, what elevation data is it using? what is the resolution of that data? You see, I love that Esri provide a detailed and complete coverage elevation surface for the entire globe but the flip side of it is that you cannot know the exact limitations of that surface easily (the information is provided by Esri but it is not a simple “point & click” exercise).

My words of advice here are to use your own terrain when placing 3D multipatch features. Therefore you are in control of both the vertical datum and the resolution of the height.

While I’m here, I want to also point out that there isn’t a “snap to ground” feature in the editing tools within ArcGIS Pro either. This becomes an issue when you bring a model in which isn’t vertically reference, has no vertical datum because you then need to sit it on the surface. Even when your model is a captured point cloud and accurate to 0.5cm, you have no way to accurately place it on the ground. You can adjust it up and down and sit it by sight, though you cannot “snap” it.

The big takeaway here is that firstly, you need to ensure you are confident and know your elevation data if you plan to work in the 3D scene views and secondly that you need to set up your x,y & z coordinate systems correctly from the start to ensure that all the work you do is as precise as possible.

…and yes, I now know the difference between “absolute”, “relative to the ground” & “on the ground”….maybe an interesting blog for another day, though feel free to contact me if you need quicker answers!

And everything else

There are still many things I have not had a chance to mention, for example the complexities of cartographic representation using 3D models in a GIS, or ways of minimising the clashing of overlapping data plus other 3D centric issues such as shadow and light. Maybe a blog for another day?….

Dragons8mycat