‘Reality capture’ is only the begining. I’m untangling some drawings derived from laser scans and the lack of clear notes of architectural form and detail compromises the work.
Once again I’m forced to reflect on the powerful measurement technologies at our disposal and how they conflict with the need for line drawings to direct works.
There are no short cuts from the point cloud to the line drawing, 3 decisions have to be made for each line that’s plotted:
- Measurement: Is the required scale met by the precision of the measured data?
- Selection What defines the edge? which lines are needed to convey the information required?
- Communication How to clearly present it within the convention for depection, what line weight is apropriate & which planes conform to the plan section and elevation required to effect the intervention required?
Different data sources feed the process. I post here a mix of site sketch, photo and CAD work as an attempt to show CAD is not a drawing standard but, like the drawing board of old, a means to transmit clear line work for the purpose of both record and works. A measured drawing has a purpose and the abstraction from reality needed to arrive at the depiction required reflects that purpose. The origin of the measured information is of no consequence other than as proof of measurement.
It should not matter how humble the structure, the draughtsperson has a duty to present the measured work as clearly defined planes, in a meaningful projection such that material, form and function are evident.
We should not be afraid of the ugly sketch, it is the imformation it conveys that is important, showing the measured edges is key.
Clarity is all: if we need to show historic development let the line drawing do the work, conveying phases by annotated photograph or point cloud flat shot is possible but often it’s the carefuly prepared drawing that records the interpretation best:
Informed observation and note taking often provide a starting point for the selection of what needs to be measured and its depection. As a base for condition mark up and work direction a line drawing is still the norm.
Scale plays a big part in determining how much detail gets drawn. 1:20 sets a high order of inclusion, 1: 50 less so. Getting a grip on just how much needs to go into a plot is something of a balance between recording the significance of the object and it’s size. This wall monument is 1.5 m high so it will appear 30mm tall on the plot:
The significance is recorded in the inscription (the monument was erected by government order) but texture, material and condition are indicated by the base data, in this case an orthophoto component scaled by TST trace:
By conveying the subject as linework without texture and colour the lines need to show the architecural form at the scale of reproduction:
In this example font matches were used rather than letter by letter tracing for the inscriptions. Mtext editing was able to get reasonably close to the stone cut letter spacing:
The final deliverable. Scale drawings are abstractions derived from metric sources, the draughtsperson needs to be able to convey architectural value in a metric frame work. This is not a simple matter and the process of delineation and its interpretive aspects can be a matter of trial and error as too many lines can detract from the utility of the drawing whereas to few degrades the process to the point of approximation.
Showing spatial relationships by means of sectional elevations adds readability to the record in that (for example) tracing water ingress can be done with confidence in the relative heights of interior and exterior features.
Line weights make changes of plane clear, the separation of components by outlining aids clarity.
It’s different in BIM. Abstraction and level of detail (LOD) have a different relationship in BIM. It is only at LOD 5 (500 in the USA) the same parity between scale drawing and metric performance occurs.
“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.”
Delineation remains a problem. It is ironic that as we achieve ever greater capacities in 3d capture the process of drawing production remains difficult, it still requires judgement on delineation. Digitising from point clouds is laborious, automation in modelling has come on in leaps and bounds but feature extraction is as difficult as ever, particularly if the client requires scale line drawings.