Laser scanning and LOD: standardising BIM
The construction world is moving to BIM (Building Information Modelling) at an accelerating rate. For the surveyor, the importance of 3D scans to the whole BIM model has moved up a level also. But understanding the acronyms inherent in BIM is not easy but it is important to do so. Here we will look at the various LoDs: Levels of Development and Levels of Detail in the context of what they mean in the BIM model and what “level of detail” actually means when you are scanning.
What is LOD?
At each stage of the project, BIM requires clear definitions for the information needed by the client or asset owner, and for the standards, methods, milestones and processes that will control its production and review.
In BIM, confusingly, both level of development and level of detail can be abbreviated as LOD, but they mean different things. LOD, where D stands for detail, talks about the graphical details that are added to a model and are the areas of interest for laser scanning.
The term “levels of development” was first used by the American Institute of Architects in 2008. In the UK, the CIC BIM Protocol defines what are the key development stages in the process. This is also abbreviated to LOD and is equivalent to the US definitions.
PAS 1192-2 (now incorporated in BS EN ISO 19650) defines two components which make up the LOD definition:
- Levels of model detail (also LOD — although can be LoMD), which relates to the graphical content of models — this is where scanned data fits.
- Levels of model information (LOI), which relates to the non-graphical content of models.
The level of model detail needed in a building information model increases as the project progresses, developing from scans to create a simple outline model through to detailed virtual construction, to an as-constructed Asset Information Model (AIM).
Why LOD is important
Unless the purpose of a survey is well understood, it can mean vital information is missed. It may also mean that the accuracy required isn't met, which can turn into huge costs during construction.
The nature of the proposed works will determine what should be captured, the level of detail that is modelled and the accuracy tolerance that is adhered to.
So, for a renovation project, it may be easier and cheaper to model a low level of detail initially from point cloud data, then raise the level of detail only in specific areas, pulling additional information from the point cloud that has already been captured.
For a construction project, the same holds true, but the project will require potentially more site visits to scan progress at different milestones within the project and therefore there is scope to change the resolution and detail of the scans carried out as the project progresses.
What always needs to be kept in mind when scanning is that lower file sizes make for easier processing and results which are faster to scan. One of the aims of BIM was to create just enough information at any particular stage of a project.
How to choose your LOD
Within BIM, the overall model breaks down to five stages. Each stage has a quality check associated with the stage. LOD will be decided by the need for a sufficiently detailed work-in-progress model available for the client at that stage.
When you start a BIM survey project, the first thing to do is to create a specification that details all information required by stakeholders, for example architects and engineers. This ensures you capture all required data when onsite surveying.
The Level of Detail or LOD contained in the model increases progressively through the stages.
LOD1 — Base survey mass model
This contains the basic outline of buildings and structures, but without any architectural details at a 1:200 traditional scale. The building 3D model is developed to represent the information on basic level. Parameters like area, height, volume, location and orientation are defined,
LOD2 — Structural components base survey model
The next level of model contains major structural components and structural openings at a 1:200 traditional scale
LOD3 — Principal basic architectural details and structures
Suitable to generate survey plans, elevations and sections at 1:50 or 1:100 traditional scale.
LOD4 —Detailed survey model
The next level — a LOD4 contains detailed architectural and structural features.
LOD5 — Detailed survey model with metadata
As with a level 4 model but enhanced with the addition of non-geometric/parametric data and record information
Note: the amount of time and therefore cost of capturing the data and creating the model will increase as the level of detail increases.
Is LOD the only thing you need to think about?
While Level of Detail has a major part to play in BIM — there are many practical considerations that will contribute to a good scanning outcome.
Can your software import the point cloud?
Make sure your registration software is compatible with all common data sources and formats, making it simple to integrate into the BIM workflow.
Will the point cloud be accurately aligned?
Scanners create an individual piece of the point cloud from each of the scanned positions, these pieces then must be placed together to create the finished product. If this isn't done well, you will see huge errors in the survey and risk of further errors throughout the BIM process.
There are ways to address these alignment issues — even inherent issues such as propagation errors. By using state of the art vector-based, multi-stage processing software you can improve quality and minimise alignment errors by:
- Automatically registering overlapping scans to achieve registration alignment, saving you time in the office.
- Minimising the time it takes to stage scans by eliminating the need to place artificial targets, saving you time in the field.
Will the point cloud have a good enough resolution to capture the detail you need?
You need to understand point density you require for the survey; this will also help establish how much time to spend on site.
Will the points have colour values?
Try to decide on specific areas where you need colour scans.
Will you be able to work with your point cloud efficiently once you’ve got it?
If a high-resolution point cloud is imported into a project at full resolution, it will potentially be difficult to work on even with high-end specification computers — although cloud-based registration software is making this process more manageable.
You should consider techniques for reducing the size of the point cloud — for example , how you can decimate the point cloud to a useful amount — ensuring good enough resolution while managing file size.
The interaction of laser scanning and BIM is becoming more important as standards are defined and the move from BIM Level 2 to Level 3 gets underway. However, definitions are still difficult to understand and, considering BIM is fundamentally about collaboration and communication, the multi-meanings of LOD are a short-term pain.
However, as BSI themselves pointed out; when applying BIM in practice, it is always paramount to focus on getting the job done as part of the delivery/process rather than its “name”.
From a surveying point of view, level of detail will always be about the quality and usability of the scan that is captured and ensuring it is registered into a fit-for-purpose point cloud and usable at all stages of the BIM process.
Tags: laser scanner