Tag Archives: targets

Photogrammetry of Ruins

Rosewell Ruins

Rosewell Ruins

In 2005 While I was running the digital documentation studio for Frazier Associates I enjoyed working on this challenging project for the Rosewell Foundation.  My task was to create accurate, architectural documentation of the  remaining masonry walls of this once grand house on the York river in Gloucester County, Virginia. You can read more about the house here.

Documenting ruins is a little different than a standing building.  The goal is less about presenting a description of building in its “platonic essence” and more about presenting it as it actually is at this moment in time.  Capturing the way a wall, tower, or chimney is leaning becomes rather important!  To this end, I prepared the building prior to the photo shoot by fixing special targets that could establish a level datum line (using a laser level).

Rosewell Documentation (interior sufaces)

Rosewell Documentation (interior sufaces)

The drawings and photographs produced for this project were used by architects Mesick Cohen Wilson Baker to create a “Blueprint for the Preservation of the Ruins at Rosewell” which consisted of a history, an structural analysis, an archaeological report, and stabilization plan.

Rosewell Ruins (exterior surfaces)

Rosewell Documentation (exterior surfaces)

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Photogrammetric Analysis

Photo with reseau and digitizing puck

Photo with reseau and digitizing puck

Analyzing stereo-pairs of photographs is a matter of locating the same precise point as it is captured in two or more photographs. In the old days, this meant using a digitizing puck to magnify and locate the x,y location of a point (relative to a set of cross hairs that were essentially coplanar to the film).  Today, with digital photographs, it is a matter of specifying pixels.  In both cases, a handful of these comparison points is needed to construct a geometric model for each camera station: each point becomes the terminus of a theoretical “ray” that passes through the focal point of the lens.  When the precise nature of how light passes through the focal point of the lens onto the film plane (or digital sensor) is understood mathematically, then this geometrical model can be quite precise, allowing for an accurate “back calculation” of the relative relationship of the camera from on photo station to another.

Its uncommonly difficult to write about – when you see it “happen” it is a little easier to understand!

wire frame diagrams of point cloud and camera stations

wire frame diagrams of point cloud and camera stations

Today, this whole process is becoming more and more automated, allowing for the creation of very rich point clouds when needed – or quickly creating smaller “smart point clouds” when the goal is a vector line drawing of a building, for example.

Photogrammetry and Stereoscopy

Ocean City City Hall (Left)

Ocean City City Hall (Left)

Ocean City City Hall (Center)

Ocean City City Hall (Center)

Here are two photographs from a project that IAT completed at the Ocean City City Hall in Ocean City, NJ.  These were shot using a Leica R5 which had been modified for use for photogrammetry. A thin glass plate with an array of cross hairs was inserted between the focal point of the lens and the film.  This was required for a good camera calibration. (Nowadays with digital SLR camera bodies the cross hairs – or the “reseau” – is no longer required.)

The basis of photogrammetry is “stereoscopy”, or simultaneously viewing the same subject matter from two vantage points.  So these two shots capture the same facade from positions to its left and center.  They are not shot simultaneously in fact, but sequentially – but in the life of a building they reflect more or less the same instant in time.

Additonal photographs from yet more unique camera locations can be included in a photogrammetric study, so in fact this technique goes well beyond stereoscopy which, technically, is limited to only two views. But for the sake of understanding what is happening in photogrammetry more easily I’ll only talk about two or three views for the moment.