Orthographic Projections

Orthographic projections represent three-dimensional objects with two-dimensional images, such as a drawing on paper.  In theory an object is viewed from infinity and projected onto the plane with lines perpendicular to the plane.  Complete spatial depiction of an object may require multiple images, such as top, front and side views.  Architectural plans, elevations and section drawings of buildings are orthographic projections.  Perspective distortions are thereby removed, and the scale is constant throughout the image.

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Control Background Drawings

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The three-dimensional metric spatial data that had been collected during the Total Station field surveys were then processed.  Orthographic elevation line drawings of the building facades were first created, which were then transformed into the control backgrounds for photomosaic image placement.

Equipment – The work was accomplished on an Apple iMac G5 (System 10.4.11).

Orthographic elevation drawings – The point coordinate data files were processed with a 3D computer aided drafting application (AutoCAD and VectorWorks were both used).  The 3D survey points were not immediately available in an orthogonal orientation and required rotation along two axes:

  • The CAD application was opened, a drawing scale of 1:25 selected and the digital file of the survey coordinates imported.
  • The first image seen was a top view looking down on the plane of the facade with the points clustered in a line.
  • The points were selected and rotated as a group until the line of points was exactly horizontal on the screen
  • The CAD orientation was then rotated from the top view to a front view in order to display orthographically the points as they were recorded on the facade.
  • The appropriate points (facade outline, doors, windows, etc.) were then connected with a polyline tool to create the orthographic line drawing of the facade that could be used as the control background.
  • A 10.00 m long scale line was added to the bottom of the drawing as well as any appropriate corrections or notes from the field journal entries.
  • The “address” number of each building entrance was added to the drawing.
  • The completed drawing was “Saved As” and exported as a Photoshop (bitmap) image file with a native resolution of 72 ppi.

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Control backgrounds
- The final photomosaic control background drawings were then created:

  • The exported 72 ppi bitmap elevation drawing was opened in Photoshop image editing application.
  • The scale was checked to assure it was exactly 40.00 cm long, producing the desired scale of 1:25.
  • The drawing was resampled (Resample Image tool) at 300 ppi to create the final control background.
  • A hard copy then was printed for quality review and filed in a binder with other jobs.

The completed image represented an orthographic line drawing of the surveyed plane of the building facade compatible with the previously developed technical standards of a 1:25 scale and a 300 ppi resolution.  The control background would form the base layer of a photomosaic, and any incorporated photographs would be automatically imported by the Photoshop application at the same resolution of 300 ppi.

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