Tag Archives: preservation

Like a Cat Scan of a Building

View of “internal organs”

Using a medical analogy to understand new ways of accessing point cloud data… The slide above shows the point cloud in RECAP software with the exterior envelope layer turned off, revealing the cluster of internal spaces.

The slides below show a series of “slices” taken vertically through the buildings at 1 meter intervals, from north towards the south and then from east proceeding towards the west.

a series of “slices” ex. 1

a series of “slices” ex. 2

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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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Photogrammetry > Laser Scanning

I’d argue that for preservation work, photogrammetry can often provide a richer sort of architectural documentation than laser scanning techniques.  There are merits to both techniques and their products, of course – each has its strengths vis-a-vis the other.

When comparing the products of photogrammetry and laser scanning, I find it interesting to see how they are converging and looking more and more alike as each respective technology continues to advance. Simply put, photogrammetry is producing richer and richer point clouds (a strong point for laser scanning) while laser scanning is producing higher fidelity imagery than ever before (but still far from the photographic quality required for sensitive preservation work).

But in some cases, photogrammetry wins the argument as to which technique is more appropriate to the task because it can perform in conditions that render laser scanning impossible. This is even more true when one factors in what it costs to get a project from start to finish.

An ocean facing portion of Fort Sumter shot with a long lens

Take for example the work completed at Fort Sumter in Charleston Harbor by Aaslestad Preservation Consulting late last year.  In order to precisely map the layout, composition and condition of the fort’s exterior masonry walls, Aaslestad shot photos from a pitching boat!

Above is one of the shots used in the survey.  It was shot with a 200mm lens from a small craft that the Park Service provided Aaslestad to circumnavigate (as much as possible) the fort.  Later that day during the peak of low tide, Aaslestad was able to scramble around the the perimeter of the fort to collect a series of 16mm shots as well, see below.

The same ocean facing portion of Fort Sumter shot with a wide angle lens

So the versatility of using a handheld camera for ‘data capture’ can make some jobs possible through photogrammetry that would otherwise be either impossible or much more time intensive and expensive. To be sure, a laser scanner is a fabulous piece of equipment that can produce incomparable results for some applications – but it needs a stable platform from which to operate (therefore can not be used from a pitching boat!).  Repositioning a laser scanner around the perimeter of Fort Sumter (on these slippery rocks shown above) during the relatively small window of opportunity of extreme low tide would also be unfeasible, or at the very least impractical and time consuming/expensive.

Another example of the versatility of using camera equipment for data capture with preservation in mind is the use of a telescopic tripod.  The shot below was taken using a remote shutter release while the camera was suspended 25′ above grade on a a tripod. Gaining points of views such as this can sometimes make the difference between be able to document a surface or not – or at the very least of enhancing a survey through greater quality of coverage.

The courtyard at Fort Sumter from atop a telescoping tripod

Looking into the future we may see devices the size of an iPhone hovering around a structure like a miniature drone collecting 3-D scan data and high resolution digital imagery – maybe even sonography or thermography as well – but until then I’m very happy to rely on the versatility provided by a calibrated SLR.

Survey Documents of Fort Sumter Masonry Completed

Exterior Masonry at Fort Sumter, Charleston, SC

After shooting a series of photographs of the old Southern stronghold in October I’ve just completed a set of forty seven tabloid size sheets that capture and represent the existing conditions of the exterior walls and courtyard elevations. These will be used later in the year as the basis for a study that will clearly distinguish different periods of masonry and masonry infill in order to create a viable preservation plan.

Scal-able Photographs of the Entry to Fort Sumter

Rectified Photographs of the Fort's interior elevations

The drawings and rectified images (hybrid drawings) are laid out for plotting at the architectural scale of 1/4″=1′-0″ – but can be printed at larger scales if needed.

Detail View (can be enlarged further)

Scope of Work / Key Plan

Staunton Train Bridge

A pedestrian bridge that connects Staunton’s downtown to Sears Hill and Woodrow Wilson Park has been moved by the city as part of an effort to restore or repair this important piece of urban fabric.  (more info and images of the bridge being moved can be found here.) These photographs (among others) serve to document the bridge and its access stair as they were prior to the bridge being moved.  They will prove invaluable should the site be affected while the city raises money to replace the bridge.

Bridge Stair

A selection of photos of the stair

Below is a partial point cloud scan of the access stair. This particular view is looking down onto the bridge without any perspective distortion – in other words a plan view that can serve as the basis for measured drawings!

Plan of Stair

Plan view of point cloud

Progress

Streetscape images overlaid onto plan of Vladivostok

I’ve slightly adjusted my methodology for creating streetscapes thanks to a new clipping tool. The aim here is to to make the streetsacpes as clear as possible so as to be immediately useful upon delivery – while still being able to be produced quickly.

Streetscapes in Vladivostok

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