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[Point Cloud] Photogrammetry - Laser Scanning

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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

 

More lost blog posts: Arc de Triomphe

L'Arc de Triomphe

Re-posted here are some of the images collected this past summer of the Arc de Triomphe in Paris. These were made as a sort of demo project following some training sessions in which I was learning how to create rich 3D point clouds from photographs.

Axonometric view of point cloud

Looking at these images now, they don’t seem terribly “rich” as 3D point clouds – I’ve since learned how to do this work  better – but they are a good start. For one thing, they were the successful resolution of an automatic analysis module ( a big time saver).

"RCP" of point cloud

Individual photos with point measurements shown

Screen shot from Elcovision

Stone Barn at Morven Park

Below is a composite elevation of a portion of the stone barn at Morven Park in Leesburg, VA.

Composite Elevation with two of the three images faded to 50% opacity

The “elevation” is actually a composite of three images, each rectified to respect the scale of 1:48 (or a quarter inch equals a foot) when printed at 300dpi. The individual images are posted below.

partial elevation (LEFT) qtr scale at 300dpi

partial elevation (CENTER) qtr scale at 300dpi

partial elevation (RIGHT) qtr scale at 300dpi

Below is a screen shot of a point cloud in autocad that shows the exterior of a stone barn. I’ve found that the density of points has a strong correlation to the content of the photos. In this case, we see that the stone surfaces create a very high density while the painted doors and such read almost not at all.

Screen Capture of point cloud derived from photographs of the stone barn at Morven Park, as viewed in Autocad 2011 software

Documentation of a sculptural element using Photogrammetry

Test subject

In August I shot a variety of sculptural objects with a mind towards testing the capabilities of photogrammetric software to create useful point cloud descriptions of objects that are difficult to assess conventionally. This urn represents a little of both – parts of its form is sculptural, organic, non repeating, while parts are more classical or architectural in  nature.

Photo with magnifying glass (or ‘loupe’) in Elcovision software

For this study I processed 72 photographs which consisted of three sets of images: two rings around the urn at 16mm and a half arc at 50mm (spatial constraints made it impossible to go all the way around at 50mm).

Point Cloud and photo stations (in red)

60,000 points were generated to describe the urn itself.  When viewed orthographically the point cloud behaves like a drawing – or can be the basis for a measured drawing.  The sculptural relief of the figures still escapes precise documentation in the point cloud, I think.  Laser scans still do this fine detail better, it seems.  But this point cloud – even as it is – in combination with  the photographs do so much more than what photogrammetry was able to do even a year or so ago. I think its catching up to laser sourced point clouds and doesn’t require the huge equipment investment.

Orthographic View of Point Cloud

 

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Elevations scanning Town Hall in Klodzko
LOCATION
SURVEYING EQUIPMENT
ON-SITE TIME
COMPLETION TIME
FINAL PRODUCT
Kłodzko, Poland
ZF imaginer 5003; Leica TCR 405
3 days scanning 2 per; 2 days control points survery;
4 weeks 1 person
Elevations CAD drawings

Our task was to scan and produce Cad drawings of all elevations of the Town Hall building included in the National Heritage list. For this project we could only use ZF imaginer 5003, the range of this scanner was not satisfying, taking into account height of the building.
We decided to set up few scanner positions on a mobile crane.
In addition to Cad drawings we were asked to color scan points using RGB colors, the coloring was our pilot project, the results we present below.
Point cloud processing

Point cloud image

Point cloud in RGB

Point cloud in RGB

Registered point cloud

Registered point cloud

Elevation drawing pdf

Elevation drawing in pdf