I spoke recently at the Digital Construction Week Show in London on the subject of Topographical Surveys… As-Built BIM for Outside. Okay, not the snappiest of titles but at least most of the audience seemed to stay awake during my delivery! I thought it might be a good idea to share some of the points I brought up which will hopefully give a little bit of guidance to anyone who wants to model complex topography or needs to commission a Revit topographical survey. If you have any comments or want to share your own thoughts on this subject then I’d love to hear from you.
Firstly, just in case you don’t already know... what is a topographical survey?
In a nutshell, a topographical survey gathers information about the natural and man-made features of a piece of land. This information consists of the position and level of each feature along with its description. The data collected is used to produce a highly detailed digital map of a site that gives architects, engineers, planners and designers a full understanding of the land’s existing condition.
Moving on from 2D CAD to Revit
A lot has been written over the last few years about producing as-built information in Revit using Scan to BIM techniques – but nearly all of that has been about laser scanning and modelling buildings and their contents. Most architects and engineers are well acquainted with using topographical surveys in the form of 2D CAD drawings as the base for their design work, but what about when they want that same topographical information so they can work in Revit’s 3D environment?
|A 3D Revit Topographical Survey model|
How would someone capture the necessary information for them, how would it be modelled and what would they need to ask for when they need a 3D Revit topographical survey?
Capturing the 3D topographical information on site
There is a strong (I’d say far too strong!) association with laser scanning and the production of as-built Building Information Models. Laser scanning may be the best way of capturing the internals and externals of a building, but especially when working outside to gather topographical information, it has its limitations. Always remember that laser scanners can only capture objects to which they have a direct line of sight, so if something like a car or a pedestrian passes in between the scanner and the target then those things will appear in the point cloud instead of whatever it is you were trying to capture. Also, objects such as manhole covers that may be flush with the ground are often very difficult or impossible to identify in a point cloud. That all means that other ways of surveying are always needed to make sure everything on a site has been fully recorded.
|Using precise GPS system to capture a kerb line hidden from the scanner by parked cars|
Luckily, as well as having scanners to hand to pick up all the ‘sticky-up’ site information, a well-equipped land surveying company will have total stations and precise GPS systems that will not only efficiently capture the data a scanner may miss, they will also help establish accurate site control and place the survey on the correct co-ordinate system. And that’s a really important thing to remember – capturing all that existing information in the real world is not just about scanning. Laser scanning gets an awful lot of coverage because it’s the latest sexy technology, but other data capture techniques (some of which have been around for quite a few years) are not only often more efficient, they are also very often a lot more cost effective. So always trust your surveyor to use the best methods available to capture everything you need on your site instead of insisting on just having it laser scanned.
After the site work is finished - processing the survey
Once a site has been surveyed, the GPS and total station data needs processing and the data captured by the laser scanner has to be registered onto the same coordinate system as the rest of the survey. All that information is combined to produce a 3D points file for importing into Revit so a Topo Surface can be created. Additionally a ‘traditional’ 2D CAD version of the topographical survey is produced that can be used as an overlay to help form an accurate model.
That 2D topographical survey should always be supplied to the client along with the 3D deliverable - it’s still as much an essential to the design team as it has ever been. Some parts of a topographical survey are just far easier to see in 2D than they are in the full model. 3D is great but 2D will still be around for a long time yet!
Creating the Revit Topo Surface
The ground modelling capabilities of Revit are actually fairly poor, even compared with software being used by surveyors and engineers as long as 30 years ago! Autodesk, (if you’re reading this) please come up with something better very soon!
In most other environments when you form a digital ground model you can control it by using breaklines. Breaklines are 3D string lines that force a triangulated digital surface to form in way that truly represents the real ground profile – essentially you can tell the software where things like the tops and bottoms of banks are.
|This Revit Topo Surface was created from a 3D coordinate file consisting of points gathered by total stations and laser scanners|
Revit uses a different ground modelling system that just drapes a surface over a series of 3D points - it ignores all those important things like bank lines and other sharp edges! This doesn’t matter on a relatively flat site with no significant sudden changes in level, but Revit has no way of automatically creating an accurate ground model for areas that may be a lot more complicated. In order to get around the Topo Surface problems Revit likes to throw at us some clever modelling techniques are needed so we can accurately represent a site's topography in 3D.
Splitting the surface
Once the Topo Surface has been formed in Revit it has to be split into different surface types and we have to try to force it to more closely represent a site's slopes and other features such as kerb lines. Without those all-important breaklines this is never an easy process and the results are seldom absolutely perfect.
|We can see here that a few creases are often visible in an edited Topo Surface. These are hard to avoid, particularly when trying to model something like a curved kerb line that isn’t on a perfectly flat surface.|
Adding the detail
The next process is to add all the other topographical features – manhole covers, lampposts, trees, road signs etc.
Again, this is a nice automated process when producing 2D output through our survey processing software, but we’re yet to see any software vendors managing to get geospatial information straight into Revit in an effective way, so we still have to resort to a lot of manual manipulation to get things right.
A particular problem at this stage comes to light when we want to place manhole and other service covers onto a Topo Surface that might be on a gradient.
|As this image shows, if you insert a horizontally level cover it will often sit partially below the surface instead of sitting nicely in it. The best way to overcome this is to add some additional 3D points to the surface adjacent to its 4 corners. If these new points are at the same level as the cover it will no longer be half submerged.|
So, what should I ask for when I need a topographical survey for BIM?
- The most important thing is…EXPERIENCE! Make sure you chose your provider carefully. If someone doesn’t know what they’re doing then you’re like to end up with something that isn’t fit for purpose. You provider will need to have access to total stations and GPS and have the skills to use them if you’re to get a survey you can rely on.
- Don’t just ask for a laser scan or a point cloud. Make sure you request a full topographical survey and make sure a detailed 2D CAD topographical survey drawing will be supplied along with your model.
- Know the difference between a 3D points file used to create a Topo Surface in Revit and a point cloud. You don’t need a point cloud if you just want to create your own Topo Surface – a surveyor can supply you with a 3D coordinate file containing all the site levels quickly and usually at a lower cost if that’s all you need.
- Always specify what’s important to you! Use a standard survey specification such as the one supplied by The Survey Association or the RICS. Whether you want 2D CAD or 3D Revit you still need to ensure all the necessary information has been captured to whatever accuracy you need.
- And finally… LoD – that all important Level of Detail. Decide exactly how much detail you need for every element - if you don’t need something then don’t have it modelled. The more unnecessary modelling your surveyor has to do, the higher your final bill will be and the longer it will be before you’ll have a model in your hands that you can start using for your design work.
Finally, remember that a quality BIM project always needs quality information.
Bury Associates have been supplying high quality topographical and measured building surveys as Revit models to many of the UK’s leading architects and engineers since 2007
Steve Bury November 2015