Vertical transformations in GIS

You know those moments where you are sat in a pub, the office or at a friends house and you say something, then suddenly wonder why you haven’t thought of it before?….I had that very issue in the Brew Dog pub near Spitalfields in London, UK a couple of weeks ago. You see, I was discussing the use of web mapping in 3D and writing a specification for the data being hosted and then it came out….

“I wonder if 3D GIS systems use 3D transformations”…...

Before I even finished the sentence I was starting to get palpitations at the thought of throwing all my hard work over the last few months away. Okay, so maybe that is a bit of an exaggeration. I do take into consideration the vertical transformation when working between data and am a bit of a tide datum nerd, though the question was now out there….

3D GIS…a quick reminder

First, let me address the “3D GIS” elephant in the room. Is there such a thing? My current stance is yes. Having used many different GIS over the last few years, I would say that many would tell you that they are, but they would almost all be wrong. Take for example CesiumJS, it is a fantastic 3D webgl viewer but can you edit the data on screen? Can you transform data between coordinate systems? (The answer you are looking for is no) Other systems which I have discounted are QGIS, MapInfo, CADCorp, Google Earth, ArcScene, GRASS, OpenGIS and also AutoCAD Map¹, purely due to them not having the capability of showing multiple coordinate system 3D data on a globe. Yes, I understand that you can pre-process the data so that it is all in the same coordinate system and then process the datum so that it is all consistent, but NOT on the fly.

There is only one “3D GIS” at the moment, the much overlooked ESRI ArcGIS Pro.

flight-paths-for-wind-farm-analysis

Compared to the 3D capabilities of the other GIS, asking it to handle vertical transformations on the globe as well as the most up to date horizontal transformations would be a bit of an ask, but you know what?…..It is pretty good…..

Disclaimer time, I am currently running the Beta version of ArcGIS Pro 1.4, though I am told it shouldn’t matter, also I have been talking with a few of the developers on functionality but this is a 3D GIS which is out on the shelves now and works. For the OCD spatial nerd, it ticks the boxes, I can use the provided international transformations or I can create my own, though more importantly, I can work with my survey (lidar) data alongside my topographic data, which is alongside my hydrographic data- so I can easily QA the data and share the results quickly.

For the n00bs, why do you need vertical transformations?

Simply put, a horizontal datum is a reference system for specifying positions on the Earth’s surface, so that the coordinate system is of uniform measure, likewise, a vertical datum is a reference to the height of the surface. All simple at the moment….a datum is based on an ellipsoid/spheroid (these geo people can’t decide what shape round they want to call it, so interchange all the time) Each ellipsoid is the best approximation of the Earths surface at any point on the Earth, you may have seen “Clarke 1866, GRS80 or WGS84” in the coordinate information of your data

clarke1866_wgs84_grs80

So you can start to see, just transforming your 3D data from one coordinate system to another isn’t quite so simple as the 2D Helmert you’ve been using. Relax, I’m not going to now go into a geoids and equipotential surfaces (unless you want me to) I am merely pointing out that we need vertical transformations in out lives, otherwise, when you transform your beautiful 3D village model from WGS84 to British National Grid, you will be wondering why it is floating in the air.

This might not matter much to the casual 3D GIS dabbler, as most of the work may be small and the effects won’t be large enough to be noticeable, though when it gets to city size, you will certainly start seeing issues with the buildings at the periphery not sitting right (when you edit the centre to sit right).

In summary

Since joining Garsdale Design in January, I have been doing a LOT of 3D GIS and putting my geodetic knowledge to ensuring things are correct. In the last 6 months I have been having to use ArcGIS Pro more and more, partly due to functionality and partly due to data formats which clients use. My opinion is that this software is becoming the one stop shop for 3D GIS and it is capable of supporting the whole project lifecycle of the project….it could turn out to be the BIM solution that everyone has overlooked. More importantly though, is that ESRI have done their homework and are supporting vertical datum transformations….okay, there are a few missing here and there BUT the list is growing and they are putting these vertical transformations into the geoprocessing tools of ArcGIS Desktop too.

verticaltransforms
Screenshot of (a small portion of) the vertical transformation list

If you know of another 3D GIS, please let me know, as I’d be eager to try it. Please feel free to either contact me direct or through comment below.