Tag Archives: Photogrammetry

U.S. Mint Philadelphia, now Philly Community College

US Mint Street View

A few snapshots from a 3D point cloud / 3D scan of this structure- in support of a rendering by a third party. (Snapshots are screen grabs from Autodesk RECAP)

VNITED

Scroll, window surround, and cornice

Left side of the entrance stair (symmetrical design)

Scroll at Entrance Stair

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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

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…

Aerial Photography > Dense Point Clouds

Since 2009 I’ve done a fair amount of work with aerial photography, all the while continuing to explore architectural photography and photogrammetry while on the ground. I’ve tried in the past with some success to merge the two things but have found the results to be lacking something. That something, I think, is the hyper dense point cloud – and now I have it pretty much figured out.

Below are tow animated GIF images showing first the point cloud describing our local hospital (which I was contracted to cover with aerial photographs a few years back) and the second is a textured 3-D model of the scan.

Its so amazing to think that the flight over this structure can be reconstructed virtually a few years later with even more options to “fly around”…

Dense Point Cloud

Dense Point Cloud

Textured 3-D model

Textured 3-D model

Tall Structures and Focal Lengths

The Carillon in Byrd Park - Richmond, VA

The Carillon in Byrd Park – Richmond, VA

I am going to share a quick study I did of the Carillon in Richmond’s Byrd Park and use photographs of this tower structure to demonstrate how different lens focal lengths can work together to access a structure when it is freely accessible (visually) from points of view on grade. In essence, I am trying to answer the question from one of my clients who asks “what kind of resolution can I expect to get for rectified photography of a tall structure (up to 200′ high)?” -and, correspondingly, “what kind of access would be required to obtain these photos?”

So, for starters, the photograph above was shot with a 40mm lens on a full frame DSLR about 250′ back from the face of the tower. It serves to capture the entirety of the structure – but not with particularly high resolution once you “zoom in”, see below:

crop on 40mm shot "zoomed in"

crop on 40mm shot “zoomed in”

Below is a diagram that shows the position of the camera and image capture relative to the tower both in side view and front view.

c40-132

In order to gain sufficient resolution to clearly see the architectural features and construction units with a lens this length on must position the camera a lot closer to the structure – more like 50′ or so away, such as is shown here:

c40-18

This yields a photo that obtains a desirable level of resolution in terms of detail, but only covers a portion of the overall surface. Below I’ll post the “close range”  photo before and after rectification and then I’ll post an additional image that shows something closer to a “100% crop” of the rectified image (since these images are much reduced in size) .

40mm shot ~50' from surface

40mm shot ~50′ from surface

40mm shot ~ 50' from surface - rectified to principal plane

40mm shot ~ 50′ from surface – rectified to principal plane

Detail of Rectified Photo showing a sufficiently rich image resolution to document architectural elements in elevation

Detail of Rectified Photo showing a sufficiently rich image resolution to document architectural elements in elevation

So, in order to cover the fell extent of the tower’s surface, one needs to tak a multiple of images from a variety of points of view and blend them together as a sort of mosaic of rectified images which can be collectively tied together by a measured line drawing.

d118

Shooting overlapping photos from different points of view is easy enough, as we move around the structure laterally… But how do we maintain this level of resolution VERTICALLY? Some answers to this question include gaining higher points of view for additional photographs by using scaffolding, an aerial lift/cherry picker, shooting from an adjacent structure, even a remote controlled helicopter/drone etc… These are each workable solutions – but each carry additional complications and costs. Sometime these types of solutions are required because space around the structure is limited (imagine a tall building on a narrow street in Manhattan). But if a structure is sufficiently free standing, a consistently adequate level of resolution can be obtained by moving away from the building and shooting photos using longer focal lengths. In such cases this can be a very cost effective solution.

The question, then, is how far away do you have to go and what focal lengths need to be used?

Below I’ll post a number of diagrams similar to the one shown above that each include a 200mm expo taken at a variety of distances from the structure.

200mm shot ~400' from structure

200mm shot ~400′ from structure

200mm shot ~200' from structure

200mm shot ~200′ from structure

200mm shot ~75' from structure

200mm shot ~75′ from structure

I tried to shoot the same masonry surface in each case – the center being roughly 150′ or more above grade. In the diagrams’ side views you can see how the amount of foreshortening increases with shots closer to the building. This will be apparent both in the un-rectified shots as well as the rectified shots, shown here:

rectified image from a 200mm shot taken ~75 from the structure

Rectified image from a 200mm shot taken ~75 from the structure

detail of rectified image from a 200mm shot taken ~75 from the structure. Very pronounce foreshortening  and resulting distortion of elements no coplanar with principal place.

Detail of rectified image from a 200mm shot taken ~75 from the structure. Very pronounce foreshortening and resulting distortion of elements no coplanar with principal place.

rectified image from a 200mm shot taken ~200 from the structure

Rectified image from a 200mm shot taken ~200 from the structure

detail rectified image from a 200mm shot taken ~75 from the structure showing much less distortion.

Detail rectified image from a 200mm shot taken ~75 from the structure showing much less distortion of non co-planar elements.

detail from a rectified image from a 200mm shot taken ~400 from the structure. Distortion is reduced even further but distance from the structure starts to have an effect on the image's resolution.

Detail from a rectified image from a 200mm shot taken ~400 from the structure. Distortion is reduced even further but distance from the structure starts to have an effect on the image’s resolution.

In the last image, one can see how as the camera moves away from the building the distortion continues to be mitigated -but the resolution of the image starts to suffer. So, as you move away from the building you have to increase your calibrated focal length in order to maintain a desirable resolution.

Below, I’ll share a number of “slides” that try to summarize what I’ve been trying to describe. Each shows a plan view of the Carillon along with a front and side diagram at the lower corners, with a snap shot of the image captured in the upper left corner. First, two 40mm shots and then a batch of 200mm shots getting progressively  closer to the building.

40mm 50' from building

40mm 50′ from building

40mm 220' from building

40mm 220′ from building

200mm 500' from building

200mm 500′ from building

200mm 400' from building

200mm 400′ from building

200mm 320' from building

200mm 320′ from building

200mm 250' from building

200mm 250′ from building

200mm 200' from building

200mm 200′ from building

200mm 100' from building

200mm 100′ from building

200mm 75' from building

200mm 75′ from building

In fact, this method involves trade offs, as I mentioned in the captions above… I think the most successful arrangement are the shots at about 200′ away – they combine a reasonable amount of “distortion reduction” while maintaining sufficient resolution without having to move too far away from the building. This can be important (not having to go too far from the structure) when shooting tall structures that are adjacent to water or a similar obstacle such as one would finde when shooting LIGHTHOUSES (see below). At 200′ from the building sufficent visual access should be available without the need to rent a boat, even for the bayside of the Cape Lookout Lighthouse.

Cape Hatteras Light

Cape Hatteras Lighthouse

Cape Lookout Lighthouse

Cape Lookout Lighthouse

The Carillon at Byrd Park.

The Carillon at Byrd Park.

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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Photographing the NC State Capitol

So finally work is underway to document this little gem of a statehouse from 1840. Below are three shots of my assistant raising the camera up on the telescoping tripod to gain important points of view for photogrammetric work on the building’s exterior.

raising the tripod…

… a little more

…and a little more…

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.

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.