Tag Archives: Orthophotography

Jackson Square – Vieux Carré – New Orleans

Orthophoto of the Cabildo - St. Louis Cathedral - Presbetère facing Jackson Square

Orthophoto of the Cabildo – St. Louis Cathedral – Presbetère facing Jackson Square

Neglecting the website/blog has gone on too long. I’ve been doing a lot of work since last fall but none of it has made it onto this page! So, just to break the ice, so to speak, I decided to grab share an orthophoto from a favorite place to many.

This orthophoto was composed of infrared shots of this famous trio of buildings in the heart of New Orleans. I grabbed these when I was visiting the city and looking at the possibility of creating a high res (both spatial and temporal) 3D model of the entire French Quarter for use as a planning and preservation tool.

Anyway, stay tuned and I’ll try to share some of the recent work soon.

Detail

Detail

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

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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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

Starting to fill in content to this site

I’ve started to add content to this site now that its structure is more or less laid out. I’ve added sample photographs to the Photography|Travel section

Plus I have added content to these projects:

Stone Barn at Morven Park

Here I am posting 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 1:96 (or a quarter inch equals a foot) when printed at 300dpi. The individual images are posted below.

partial elev left qtr scale at 300dpi

partial elev center qtr scale at 300dpi

partial elev right qtr scale at 300dpi

Tomb of the Unknowns – part 2

Here are shown four rectified images of the Tomb of the Unknowns (reduced in size/resolution).

The Cosmos Club Ballroom

Ballroom at the Cosmo Club

Here is a new project that I shot last week: documentation of the existing conditions of the interior surfaces in the ballroom at the Cosmos Club in Washington DC (formerly the Townsend Mansion). This is going to be a great opportunity to show what the photo + measured line drawing approach to documentation can yield (with a limited budget).