2 & half D

Working up drawings from wire frames and point clouds needs a way to get 3D info into 2D space. Selective modelling provides a method for edge extraction of complex or repetitive details. The projection of curved or oblique alignments onto the reference plane involves getting views to hide the appropriate faces, solid modelling from edge profiles creates the depth needed.

Building a model is the easy way of getting the recession of the stair risers and handrail plotted:

Projection to 2D edge lines requires the model to be ‘clean’ with closed elements, solid profiling won’t find intersected edges between components unless they are unioned together.

cumulative error from adding the tread elements can be corrected by scaling the completed element although non uniform (using a different scale for Z axis to X,Y for example) scaling limits editing.


Digitising orthophotography is a successful route to delineation of detail although poorly surfaced models need a lot of interpretation. Orthophotos can present badly formed edges, SFM surfaces generated in reasonable time limits seem to melt definition more often than not. Their great value is the metric performance achieved from even fairly modest photo cover. This example required reference to templates which were positioned in x,y over the orthoimage.

Apse details are profiled from the model to give the correct projection of the ceiling ribs, window arcs and elipses:


These sectional elevations combine line-work from rectified and ortho imagery as well as notes and templates. The model dealt with the apse wall, spiral stair and ceiling projection. The decorative altar detail was digitised from an orthoimage in the centre and rectified photos on the side chapels, roof trusses developed from TST wire.


About billboyheritagesurvey

Heritage worker
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