Tag Archives: mosaic

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:

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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

Streetscape Methodology

This is a screenshot from AutoCAD: The lower image is a series of rectified, scalable images of the east side of Augusta Street in Staunton, VA - with an aerial view of the same street grabbed from Google Earth inserted above.

This week I have been developing a methodology for capturing and representing a large volume of structures quickly as I will be doing a project charged with just this task next week in the Russian sea port city Vladivostok. Above is the sample streetscape I created using Augusta Street in Staunton as my test subject.  The photography work took about 15 minutes. I did not use a surveying rod as I typically do, nor did I collect control dimensions with my laser.  In stead, I scaled my model using dimensions extracted from satellite imagery available on Google Earth. I was looking to find a way to obtain accuracy sufficient for 1:200 prints that was also very fast.  To check my work I measured a distance from the model prepared in this way and checked it with the real world conditions. Incredibly, the error was 1/4″!

Scalable at 1/16"=1'-0" when printed at 300 dpi

Above is an individual image extracted from the group that comprises the streetscape. As noted the photo will behave like a measured drawing at the ratio of 1:192 when printed at 300dpi – which means that it is 1:96, or 1/8″=1′-0″ when printed at 150dpi and so on… My opinion is that this is very good considering how fast I made myself complete this work!

The streetscape in my test project represents about 1,000 linear feet. It will help me to extrapolate what can be achieved when tasked with documenting an entire historic district under tight time constraints.

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