Unlearning line: what is LOD 500?

Can a survey scale relate to a BIM* LOD?

“LOD 500: The Model Element is a field verified representation in terms of size, shape, location, quantity, and orientation. Non-graphic information may also be attached to the Model Elements.”

Ref AIA, Guide, Instructions and Commentary to the 2013 AIA Digital Practice Documents.

The Level of Development (US) or Level of Detail and sometimes Level of Definition (UK) required for BIM does not stipulate a scale performance.  As the function of objects in the model will differ according to the design or construction stage involved it can be assumed the scalar performance (precision) will vary too.  I have searched in vain for a clear description of the BIM LODs in terms of drawing scales and the table below is as close as I can get based on my experience to date as a drawing service provider:

Approximate LOD to drawing scale equivalence

BIM function

Building drawing contents

Building services drawing/layer contents

Approximate 2D scale(s)

Concept Stage

LOD 2

To provide visual indication of the proposal. Identifying key requirements such as component type, access & maintenance zones.

Information to be suitable for zonal spatial coordination of primary systems

‘General Arrangement’ or ‘Planing application drawing’

Outline plan and elevations, site plan location, contour and orientation of site etc

Floor areas indicated to scale

3D cell diagram

1:500 /1:200 (site)

1:100 (GA for planning ap)

Development Stage

LOD 3

To provide visual representation of proposal at a design development stage and to allow spatial co-ordination

‘Draft scheme’

Indicates component type & approximate size as a plane section and elevation drawing.

Indicates size and location of plant/ cable and pipe by block

1:50 ‘outline’

Technical Stage

LOD 4

To provide a visual representation of proposals at a technical design stage supporting full spatial co-ordination

‘As proposed’ ‘Building plan’

Indicates component type, geometric location & actual size as plan, section and elevation drawings.

Room heights and heights relative to datum shown.

Switches, outlets & fittings shown as symbol.

1:50 ‘detail’

Construction Stage

LOD 5 (or 500 in US)

To provide sufficient information for construction / installation of the appropriate products

‘As built’ & ‘Details sheet’

Indicates component type, geometric location & actual size as plan, section and elevation drawings.

Material type annotation included.

Path of services shown. With wiring and plumbing shown.

1:50, 1:20,

1:10 &1:5

‘Full size’ details

Adapted from:

https://toolkit.thenbs.com/articles/levels-of-definition/#alistair

For a good while now 3D CAD models have been far behind drawings as the normal means of information exchange in construction but this is changing, if ony at the drawing production end as 3D capture lends itself to transmission in 3D form.  For me (and my clients) the 2D drawing remains key to the processes they seek to achive: planning consent, buiding development and conservation condition mapping. A 3D model may be the means to a 2D end but the cost of BIM, even if only as a bare bones 3D frame work, is steep.

Why scale matters. If an ‘as built’ LOD 5 is required there are no short cuts: all components must be shown at full size which sets a tough scale standard to work to- I get tolleraces of around +/- 5mm per point by TST so  working  at 1: 50 scale can absorb this easily, a point cloud from a typical scanner (e.g. Faro Focus 70) has a tollerance of  around +/-3mm per point and point density will come into play at in the capture to model path, if I take full ‘full size’ to mean as measured to scale I need to get agreement on the scale of the work to be caried out and this is where the confusion between BIM LOD and survey precision begins.

progress-17012017-2

LOD 5 model built up from TST trace, measured drawing and photography: the scale tolerance is equivalent to 1:50.

3D costs. A clean 3D model of a building takes 3 to 5 times longer to prepare than a set of 2D drawing sheets: capture is quick but data extraction takes time and care. The captured measurement must be clear and precise, this plan took around 3 hours to capture by tape and Disto…

sit-note

…cheaper than a scanner, but oh so slow. Lost a clip a while back..ping.. The supporting TST trace of the exterior took around half an hour per station…

3d-wire-a-go-go

… with 4 stations capturing the exterior edges of the building as CAD lines. Photographs were taken of all elevations internal and external as well as general views. Internal measurement was plotted ‘on the go’ for the principal rooms with details added from the field sketch: the plan was ready within 3 hours of return to the office:

progress-18012017-1In contrast to the rapid 2D work getting the model to LOD5 took days. Adding the surfaces for the roof, developing the ridge, deatailing the opeinings, cills, eaves and dormer, whilst keeping to the scale tollerance all took a great deal of time…

progress-14012017-1   …this is working the BIM path backwards: the ‘as built’ is a deconstruction of the existing building…

progress-17012017-4

from the plan and TST wire the model is built up…

detail-clash_01

…resolving component clashes in 3D takes a good while…

detail-clash_02

choices have to be made as to treatment of material, component and geometric precision: does the LOD require each tile to be modelled or will a simple surface indicate the array?

progress-18012017-2

The 2D elevations are worked up from projected profiles from the model, not the neatest method but better than a complete redraw.

As the tools for it’s capture and dissemination become more acessable client expectations of 3D data are high, I have heard it said 3D is good because you can cut the model any how and get drawings: how I wish that were true! A model is only ever as complete as it needs to be. These things are such hard work many believe clients will be happy with a point cloud and do their own modelling and find their own way of getting drawings out of it…but point clouds are heavy on mamory and the processing costs dear in terms of software.

Capture for this model and the drawings produced

  • a wire frame of 48kb
  • an archive of 30 photos amounting to 65 MB of data,
  • a CAD model of 4.1Mb
  • and the extracted 2D drawing sheet at 276Kb

TST, measured drawing and rectified photography might be slow but, for this surveyor at least, they are the tools that clients can afford for small domestic projects like this one.

progress-18012017-3

*I’ll never forget the hour long lecture on BIM when, at the end, students were asked the usual ‘any questions?’ …and a lone voice piped up from the back of the room: ‘What is BIM?’ to which the lecturer responded by starting over from the beginining; I was impressed by his patience but I’m afraid for me it was well past beer o’clock … it is, of course, a Bulding Information Model.

Happy surveys,

B

About billboyheritagesurvey

Heritage worker
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