Tag Archives: point cloud

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…

Orthophotos from a Dense Point Cloud

45 degrees

45 degrees

RCP

RCP

on axis

on axis

45 degrees

45 degrees

on axis

on axis

Luzerne County Courthouse

Luzerne County Courthouse

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.

Measuring the deformation of masonry walls in an historic structure

Contour map of deformation SE

Contour map of deformation SE

For my project at the the Old Town Hall in New Castle, DE, I am first establishing the existing conditions of masonry walls – including their deformation. This documentation will help us to choose where to set targets for monitoring over the coming months/years.

Contour map of deformation NW

Contour map of deformation NW

see the project page HERE

A Comparison of Orthophotography and Mosaics of Rectified Photographs

In 2012 I prepared documentation of the masonry surfaces of this historic room in the Maryland Statehouse. This documentation provided a baseline of the existing conditions to allow for an investigation into how exactly the room was appointed in its heyday – when Thomas Jefferson was named ambassador to France etc…

The documentation that I created consisted of measured line drawings (in CAD format) augmented by a mosaic of photographs that had been rectified to match the real world size and shape of the various portions of masonry. here is what it looked like:

Measured Line Drawing

Measured Line Drawing

The image above ^ shows a screen capture of an accurate measured line drawing delineating the limits of the masonry surfaces and indicating the architectural features nearby.

Key to regions/individual rectified photographs

Key to regions/individual rectified photographs

This image ^ shows a drawing layer “thawed” to display a sort of key plan to the different regions of masonry for which individual rectified photographs were prepared. The next few images show some of the drawing layers containing these rectified photographs “thawed” and you can get an idea of how the composite whole is constructed like a mosaic.

A few rectified photographs thawed...

A few rectified photographs thawed…

a few more rectified photographs thawed...

a few more rectified photographs thawed…

A single rectified photo of the entire wall

A single rectified photo of the entire wall

So, the above image ^ shows how the entire wall could be captured, rectified, and brought into the measured drawing. (The reason that the other images were created in “panels” was to provide for two things: (1) enhanced resolution for the individual photographs, and (2) the ability to “see around” obstacles presented to a single point of view. For example, in the image above, portions of the masonry surface are obscured by some of the architectural detailing/millwork)

Below is a “zoomed in” version showing this condition in higher detail…

When a single image is rectified so that the masonry portions of the image match the real world size and shape of what is being depicted, features that are NOT co planar can be distorted, not matching real world conditions. This is one reason a mosaic approach was needed to cover all of the surfaces in question  accurately.

When a single image is rectified so that the masonry portions of the image match the real world size and shape of what is being depicted, features that are NOT co planar can be distorted, not matching real world conditions. This is one reason a mosaic approach was needed to cover all of the surfaces in question accurately.

So, in this approach, the line drawings carried the responsibility of delineating the wall’s size, shape and configuration of architectural elements while the rectified photos carried the responsibility of conveying the lay out and character of the masonry construction units.

Today, I might approach the project differently, using a true “Orthophoto” of the sucrose instead of a mosaic. The distinction is important. An Orthophoto is a planar projection of a dense point cloud or textured mesh 3D model. I was unable to create these back in 2012 when I did this work – but today I can do so -still using photogrammetry- and maintain the same level of accuracy demanded by such work.

An Orthophoto (constructed from images from various points of view) depicts the masonry surfaces as well as the architectural detailing/millwork accurately.

An Orthophoto (constructed from images from various points of view) depicts the masonry surfaces as well as the architectural detailing/millwork accurately.

This image ^ shows the orthophoto seated nicely behind the measured line drawings. As mentioned above, it is a projection from a 3D model, which can be looked at from a variety of angles and manipulated in 3D modeling software . below is a video clip showing the model these surfaces.

I am really excited to have this new set of tools at my disposal! The mosaic approach is still sound and may be more appropriate for some projects. In fact, the two approaches can coexist in the same set of documentation if needed. But the addition of 3D scanning and the creation of true Orthophotos to my toolbag will allow me to provide architects and engineers faced with complex preservation challenges with more options.

West Wall of the Old Senate Chamber - Orthophoto

West Wall of the Old Senate Chamber – Orthophoto

more background info here:

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.

Page Updates: Point clouds, F&M, Roxboro

I’ve updated my page on point clouds with some new images from Franklin and Marshall’s Goethean Hall that show some of the true color point clouds generated though photogrammetry. Likewise, I’ve update the page on the Roxboro House project to include some point cloud views.

Sharing: Middlebrook Avenue

Two more screenshots to share, this time of a streetscape project on Middlebrook Avenue here in Staunton, VA. The first shows a virtual aerial view of the the automatically created point cloud, the second of the assembly of photos used to create the point cloud. Both are screen shots from Elcovision software.

Middlebrook Avenue point cloud

Middlebrook Avenue Photos

Sharing: more Claude Ledoux, Quai Vauban & Besancon

Theatre (Besancon) Nicolas Claude Ledoux 1784

Here are some screen shots of the point cloud depicting the theater in Besancon designed by Ledoux in the 18th century.

Point Cloud of Theater

This was created with a few “snap shots” of the building taken from the street.

Photos used to create point cloud

In order to capture roof elements and such additional photos would be necessary. As a “piece” of the street however, it can be assembled along with additional scans to create a larger whole.

A few more screen shots from a separate locations in Bescanson, across form the Prefecture:

Screen capture from Elcovision

And an “aerial view” of a point cloud that encompasses the entire Quai Vauban:

The Quai Vauban