Tag Archives: aaslestad

Franklin & Marshall Exterior Survey

Old Main at Franklin and Marshall

Old Main at Franklin and Marshall

Last spring I did some work at Franklin and Marshall College in Lancaster, PA.  I prepared a combination of measured drawings in CAD linked  to rectified photographs of the three oldest structures on the campus. These were then used as base documentation for an assessment of the existing conditions of the masonry surfaces across the extent of these buildings’ exterior facades.

The sequence of this work went roughly as follows:  I first shot photos of all of the buildings from points of view available on the ground (using a specially calibrated digital SLR).  Back in my offices in Virginia I used photogrammetric software to “back calculate” the camera stations and to make precise 3-D measurements of points on the three structures.  I used this dimensionally accurate point cloud as a reference to create rectified photographs of the surfaces visible from grade.

F and M Old Main Rear Elevation

F and M Old Main Rear Elevation

Using the point cloud and the rectified photographs I then created measured line drawings of the structures and laid them out onto tabloid size sheets for use in the field.  These were taken by hand up into an aerial lift so that conditions could be noted with a fair degree of accuracy once assessment were made both visually and manually.

tabloid size field sheet with notes

tabloid size field sheet with notes

Once the assessment was completed the data recorded on the field sheets was entered in the CAD drawings.  At this point the line drawings in CAD were enriched by a mosaic of rectified photographs visible inside of AutoCAD.  This allows for an accurate transfer of notes from the field to eventual construction documents.  A sketch of an area or region can be transferred to CAD and become dimensionally reliable.  This is a very important step because it provides for an accurate tally of areas to be treated in one way or another.

aerial lift

aerial lift

Since the main structure was rather tall, I also returned to the site to go up in the aerial lift to collect more photographs to further enrich my drawings with better rectified images of areas that were either blocked by vegetation, neighboring buildings, or were too foreshortened to provide good rectified images.

F and M Old Main Rear Elevation with photos

F and M Old Main Rear Elevation with photos

Discovering Photogrammetry in France

I first encountered the concept of architectural photogrammetry while working as an architect in Paris in 1990 -and was immediately intrigued. So much so that I decided to swap my job with a 45 minute commute into the city for a new job just a short bicycle ride away (in the town of Egly)  in order to learn how to do this work.  Simultaneously I had to learn how to draw with a computer; up until then I had only drafted with pen and pencil (plastic leads for working drawings).  It was a lot to take in, but I think its fair to say that I loved it.

digitizing puck

digitizing puck

Back then, the process was nearly completely analog: We shot film and had it developed into 8″x10″ prints.  These were then taped onto a large format digitizer so that points on the photos could be communicated to software running on a PC.  Then after a photogrammetric analysis of this information was completed, we made drawings that were plotted to scale – and it was these prints that were the “deliverable” product desired by our clients.

I remember seeing a number of photos taped onto a digitizing table and watching my colleague Herve use the digitizing puck to point to the same location on a building as seen from various points of view and then push a button and seeing the point location appear in 3-D coordinates in an CAD drawing on a computer screen.  It seemed magical.

As more and more points were measured in this way, I could start to see the shape of the building.  This became even clearer as points were connected into lines and polylines; the “point cloud” was becoming a “wire frame”.

When I first got out of architecture school and was working for a firm in New York City, the first thing I had to do was to learn how to make measured drawings of existing conditions.  It was messy, confusing work on site and often yielded error-ridden results in the office that required repeated return visits to understand and clarify what was really happening architecturally.  When I saw a dimensionally accurate 3-D model representing an existing structure being created from a handful of photos, I was hooked.  Nobody I knew was doing anything like this in the United States.

 

Triumphal Arch Jardin de Tuileries

Triumphal Arch Jardin de Tuileries