Digitising, what is it good for?

Given the choice between a good ortho-image and a line drawing which would you choose?For this record survey of an historic floor a mosaic of rectified images was assembled to provide a brick by brick record commensurate with 1:20 scale. Site capture was 2 days work for the 15mx5m floor comprising 36 12Mp photos by radio controlled PAP and 1,431 control points as 3D wire-frame by reflectorles TST in TheoLt.

Rectifying the images and blending the mosaic was about 3 days work…a job made straight-forward with PhoToPlan for the rectification and PhotoShop for the montage. Then, once the whole montage was complete ….

…digitising the photomosaic took 5 days and 9,444 elements were recorded.The digitising process is selective and decisions have to be made on how to select the edges. The digitised record is differentiated and the photo record is undifferentiated. The selectivity necessary for the extraction of the vectors has, in my view, significantly changed the value of the record. This raises 2 important questions:

1. What is the value of the digitising?

Well, being paid to trace is easy money…but it’s very boring work and the decisions made to select edges, define form and indicate condition are a daily compromise: the selectivity of the process is evident in every line:

2. What is the optimum record?

The answer to the second is, of course, irrevocably linked to the information need: if my client says they want a given devilerable for an agreed price then this shall be so. In mapping the abstraction of information is the norm: it is very unusual to present a photomap without some form of feature extraction (even Google Earth provides basic vectorised information in its Primary Data base)I can’t see why anyone would want anything less than the photomap, it records texture, condition, material and colour so well …but it also records, lighting effects, shadow, camera performance and camera position too!The digitised lines will provide a metric base for mapping condition and intervention and this is where the effort of tracing will no doubt pay off. I’m sure there are strong arguments for using a point cloud from a scanner to record the surface of the floor but the ‘light touch’ approach on control makes rectified photography achieve a very cost-effective result and the tracing process would have been much the same regardless of the source control data in the end.

For me the satisfaction in using a simple metric toolset (camera, total station, PhotoPlan and PhotoShop) to get the job done is part of the joy of the work. The conservation team know they will get the information they need quickly and their confidence in my selection of line is professional validation of the very best kind!

More about PhotoPlan here

More on TheoLt here

About billboyheritagesurvey

Heritage worker
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