Topography: Converting Point Data and Kerb Modelling

Topography: Converting Point Data and Kerb Modelling

As the use of Revit in refurbishment and existing building project increases, the requirement has naturally spread to the areas…

As the use of Revit in refurbishment and existing building project increases, the requirement has naturally spread to the areas around the superstructures. As such, there has been a notable increase in the demand for the traditional two dimensional topographical surveys to be instead delivered as three dimensional models. This post takes a broad look at what can be done to deliver this, and the overcoming the reoccuring problem which is kerb modelling.

Methods already available

As any proficient user of Revit will already know there are a few tools available to add and manipulate basic topography in a model. Central to this function is the Toposurface tool which can create anything from basic to complex 3D forms, intelligently acting as the surface of the earth. Surfaces can also be created by the ‘Create from import’ tool – allowing imported information such as CAD plans (with spot height or ‘Z’ values) to be converted into a Revit surface. Both these options will provide the basis for a simple 3D surface addition to a Revit model.

Converting Scan data to Toposurface

Despite an increase in the buildings being delivered in full 3D via Revit, the uptake for topography to be also in 3D has lagged behind – with architects, contractors and the like still requesting a traditional ‘flat’ topo plan (usually delivered in CAD). For the surveyor this can often pose the issue of duplication of data gathered; the area covered on a laser scan often covering that required for a topographical survey. Instead, the point data itself can be converted inside Revit using the native tools and some basic techniques.

Creating a grid and placing points: this method is a very accurate and quality assured way of creating a topo surface, but quite labour intensive. The pointcloud is inserted into Revit as standard and an artificial grid created over the top of it using reference places.

The basic boundary of the topo surface is then applied, and points placed at the grid intersections.

A thin section line is then created along each grid line with the view used to measure the height between datum and pointcloud.

The height value at each point can then be transferred to the topo surface (in edit mode) to give the surface its 3D form.

Here’s a 3D perspective example of the typical end results (with pointcloud shown)

Feature extraction:  Existing only in principle at the time of writing, feature extraction technology takes the point data and transposes it directly into a Revit element. Autodesk themselves brought out a basic feature extraction plug-in for Revit in early 2012, only to abandon it in favour of a developing more comprehensive software with a developer used to working with point clouds. As the key barrier of the software being able to read the point data has been overcome, it’s likely only a matter of time before it becomes more widely available.

Limitations to both:  The prime limitation to the manual, ‘grid based’ method is obviously the time involved in transposing the information – feature extraction in theory can take the same given area and transpose the cloud in a tenth of the time. The problem with early feature extraction technology is filtering the cloud so that the likes of vegetation are not identified as ground level and create anomalies in the data.

Also, the Toposurface tool is only designed for the kind of relatively flat and uncomplicated topography found on 99% of construction sites; which isn’t typically a problem given the rarity for a site to have harsh topography such as cliff faces. However, it does pose a problem when wanting to add smaller vertical surfaces / boundaries – the prime example being road kerbs.

Kerbs

At present, there is no native tool that will define the combined boundary and sharp change in level that a kerb presents in real life – as such, the current tools need to be adapted to suit. Naturally, as an adaption needs to be made there is no one ‘right’ way of doing this – and there is number of methods that can be depending in the level of fine detail required. This is a quicker method, generally used for a standard level of detail required:

Start with two topo surfaces at their correct height – here we have a basic road and grass combination.

Next, both surfaces need to be split along the edge they will adjoin using the Split Surface tool in the Site ribbon. The combined width of the two strips will need to represent the notional width of the kerb – i.e. if the kerb is 100mm wide, 50mm strips will need to be cut either side.

Now these two thin strips need to be merged using the merge surfaces tool. Note: the highest surface must be selected first to ensure the correct profile is created in the merge.

The merging creates a very basic splayed kerb profile – albeit showing only the exposed face.

As mentioned earlier, there are alternative ways to create kerbs; although this is probably the most time efficient way currently available. The lack of native tool to insert kerbs remains a fairly large oversight by Autodesk – particularly considering the problem has been noted frequently on forums for a number of years now. For now at least, this basic guide should be able to help others overcome the problem to a reasonable extent.

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