Tag Archives: as-built

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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Survey Documents of Fort Sumter Masonry Completed

Exterior Masonry at Fort Sumter, Charleston, SC

After shooting a series of photographs of the old Southern stronghold in October I’ve just completed a set of forty seven tabloid size sheets that capture and represent the existing conditions of the exterior walls and courtyard elevations. These will be used later in the year as the basis for a study that will clearly distinguish different periods of masonry and masonry infill in order to create a viable preservation plan.

Scal-able Photographs of the Entry to Fort Sumter

Rectified Photographs of the Fort's interior elevations

The drawings and rectified images (hybrid drawings) are laid out for plotting at the architectural scale of 1/4″=1′-0″ – but can be printed at larger scales if needed.

Detail View (can be enlarged further)

Scope of Work / Key Plan

Envelope Plans

Envelope Plan

An envelope plan is a delineation of the extent of a building’s mass taken at a fixed elevation. It establishes a precise limit to the outermost profile of the building. Envelope plans are easily derived from photogrammetric surveys and point clouds that decribe the exterior conditions of a building.

The value of the envelope plan should not be overlooked. When charged with creating plan drawings of a building, the envelope plan is the first place to start. It provides a reliable framework against which any data from an interior survey can be compared – significantly reducing the occurrence of human error when constructing a detailed floor plan drawings depicting interior layouts.

When possible, I use data from photogrammetry to prepare envelope plans before I go inside a structure to collect additional observations. By establishing accurate envelope conditions, previously mysterious internal conditions that might have been otherwise unmeasurable (such as wall thicknesses) can then be reliably calculated.

Developed floor plan

An envelope plan is especially valuable for non orthogonal structures. For some projects when I am collaborating with a team of architects who are close by to a project site, I prepare the envelope plan using photogrammetry and then hand it off to my colleagues who will complete the drawing during subsequent visits to the building. It is a great way to eliminate questions that arise amid a wealth of (sometimes) confusing field notes and measurements.

The Carnegie Library - Atlantic City, NJ