Tag Archives: aaslestad

Overlapping Photogrammetric systems

View of Point Cloud from side/rear

Spending a lot of time seeing how different photogrammetric systems and point cloud software packages speak to one another. sometimes the interface is elegant, sometime not so much…

The Clocktower Building in Staunton, VA

Three views of a model of this iconic corner building

Three views of a model of this iconic corner building

Just a quick share of the results of a test project to evaluate some new technology. Check out the video below to see all three views above in their 3D context.

Progress at the North Carolina State Capitol

At this writing, Aaslestad Preservation Consulting, llc is in the middle of a project to document the NC State Capitol building (1840) in Raleigh. The exterior surfaces are being documented both in line drawing and hybrid imagery. When this is wrapped up, the interiors will be given a similar treatment. These drawings will then be augmented by a series of detail drawings capturing the many rooms’ unique door and window profiles, friezes and such. Stay tuned!

Line Drawing indicating the individual stones that compose the facade.

Line Drawing indicating the individual stones that compose the facade.

Hybrid Drawing incorporating rectified photographs of the individual stones that compose the facade into the line drawing.

Hybrid Drawing incorporating rectified photographs of the individual stones that compose the facade into the line drawing. Note that the patches of sunlight shown here are a little confusing. This will have to be re-shot under better light conditions.

Some obstruction with regard to adjacent vegetation...

Some obstruction with regard to adjacent vegetation…

 

In this sheet, a new shot of the column will be required...

In this sheet, a new shot of the column will be required…

The dome is not safely accessible on all 8 sides; shown here are the three types of elevation (beneath the dome itself) that are repeated)

The dome is not safely accessible on all 8 sides; shown here are the three types of elevation (beneath the dome itself) that are repeated)

 

 

Photogrammetry, point clouds and stained glass

Last weekend I met with Jules Mominee of Mominee Studios [nationally renowned designers of fine stained glass and restorers of historic art glass] to conduct a work shop demonstrating how photogrammetry can add value to his work. We visited Trinity Episcopal Church in Staunton, VA to choose a test subject from their rich collection of stained glass windows – and selected the triptych behind the allar which was designed by the celebrated Tiffany Glass and Decorating Co. of New York in 1897.

My goal was to demonstrate how photographing the windows with a calibrated camera+lens combination could produce a valuable documentary record of these important heirlooms above and beyond standard photography. I would show how we could use the photographs as the basis for rectified scale-able photographs (with all lens and parallax distortion removed). I also wanted to show how we could “go into” the photographs and extract precise 3D point measurements as needed to create measured drawings and such.

This blog post will try to cover what we did.

Photographing the triptych with telescoping tripod. Note surveyor’s rod (to establish real world distance in the photos) and a white balance target.

The first step involved shooting overlapping photographs of the subject with a different lenses. Some of the shots captured the scene in its entirety while others captured  smaller regions in greater detail (for use later as pieces of a mosaic).

The variety of images shot loaded into photogrammetric software

Next up, we processed the photographs using software that automatically calculates the relationship of the camera stations to one another and creates a point cloud describing features in common captured by the photographs.

Point Cloud representing the stained glass (in true color) and the relative 3D locations for each photograph used.

The point cloud is essentially flat (due to what it is depicting) but nonetheless consists of an agglomeration of precise 3D measurements. Here is an animation showing its three dimensional nature:

An animation showing the point cloud depicting the stained glass triptych and the camera stations (in red)

Then we chose a handful of “smart points” relating to specific locations on the glass in order to establish a meaningful coordinate system. These points are shown on the images below.

Location of “smart points” on center window (lower portion)

Location of “smart points” on center window (upper portion)

Location of “smart points” on right window

Once these ponts were chosen and used to define our principal plane, we recalculated the model (with “smart points” on our surveying rod to establish real world dimensions).  Here are the x, y, and z values for our smart points:

Object Point Calculation Table

If you look closely at these values you’ll find that the average error value for this small batch of points is calculated to be about one one hundredth of an inch. Photogrammetric analysis (esp. when using controls and targets) can greatly exceed this level of accuracy – but this is already well beyond what would be required to replicate this design.

On to image rectification… The next step is to use these same 3D coordinates to define theoretical planes onto which the individual photographs will be projected so that the resulting images match precisely the real world conditions of the glass surface.

Defining a rectification plane with four or more points

The window above shows a plane formed by points 5, 6, 7 and 8 that has a maximum error value of about a sixteenth of an inch (which means that this portion of the window is pretty flat – if there were buckling and such, as will happen with windows over a hundred years old, this value would be greater…). So this will be the spatial plane onto which we will rectify the photo of the center window, lower portion.

Next up, we made a lasso of the area of the photo that we want to rectify since not all of the image corresponds to our rectification plane.

Lasso indicating extent of image that is coplanar to the rectification plane.

Then we were ready to create our rectified image of the triptych in its entirety by creating a mosaic of four smaller rectified images. In the way that we shot this example, we were able to create a rectified image that would respect the graphic scale of 3″=1′-0″ (1:4) when printed at 150 dots per inch. This ‘resolution’ can be increased by shooting more images that are in closer range to the surface being documented.

Creating a mosaic of individually rectified regions

And here is an overview of our finished result:

with some additional images “zoomed in”:

100% crop

400% crop

So this is the level of detail available across the entire surface of the three windows. If the image were printed on several sheets full size, we would produce, effectively, the same type of document as if we were able to make a high quality “rubbing” of the window – with out having to remove it and in a fraction of the time (and in color!!!)

Continuing on, I showed how the image could also be brought into a CAD program  (such as AutoCAD) in order to create a highly detailed measured drawing in vector format. In this scenario one can directly query the image to get real world dimensions.

Overview of Triptych in CAD

Closer up view in CAD

Detail view in CAD

So, in the end we showed the value of photogrammetry as a high quality AND cost effective tool for documenting heritage artifacts such as stained glass both for restoration purposes as well as for insurance purpose to provide a reliable document in the event of catastrophic loss. It also can provide a way to share the unmatched artistry of these windows to any who would like to have a closer look.

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Cosmos Club Ball Room

just a quick post to share some images from a project completed in 2010. I understand that the restoration of the ballroom is fully complete now.

Ceiling of the Cosmos Club Ballroom

Centerpiece of Ballroom ceiling

Detail

Elevation of Cosmos Club Ballroom

a closer look...

...and a little closer

In these elevation drawings won can see black lines overlaid atop the rectified images. These are dimensionally accurate vector lines in AutoCAD.

New Page Updates

The following project pages have been updated:

some rearranging going on

Staunton from atop the Stonewall Jackson Hotel

if you are looking for my photo blog  PICTURES OF STAUNTON (which used to be located at this address) please follow this link:

http://picturesofstaunton.wordpress.com/ (the blog has moved)

Meanwhile check back at this address which will be home to a site/blog that consolidate my photographic work with my architectural work and so on.

Thanks,

Peter

Mosaic Images from multiple view points simultaneously

Maria Mitchell Hybrid Drawing

Maria Mitchell Hybrid Drawing

Above is what I like to call a “hybrid drawing” of the historic Maria Mitchell House located on the island of Nantucket. A hybrid drawing is one of two things, or possible both: It is a photographic image that behaves like a measured drawing (it is scalable and can provide quantifiable data), or it is a measured drawing that is rich in the way a photographic image is (materials, colors, actual as-found existing conditions are depicted photographically).

Line drawing with key to individual rectified images

Line drawing with key to individual rectified images

Above is a view of the line drawing with all of the rectified images that compose the hybrid drawing “frozen”. In other words, in the CAD drawing, these layers of information are turned off and made invisible so as to see just the line-work itself and a series of polygons that correspond to bit map/raster images that are referenced by the drawing.

Below are a series of images showing each individual rectified photographs as it is situated in the context of the drawing.  These added together make the composite image at the top of this post.

Maria Mitchell 10Maria Mitchell 9

Maria Mitchell 8Maria Mitchell 7Maria Mitchell 6Maria Mitchell 5

Maria Mitchell 11

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