Eric Tate
May 1998

REMOTE SENSING WITH DIGITAL ORTHOPHOTOS

Introduction

In recent years, many technological advances have been made in the field of photogrammetry, the process of making maps using photographs. Perhaps the most significant achievement has been the refinement of methods to produce digital orthophotographs. Digital orthophotos are scaled aerial photographs, which can be used (among other things) as a base map in a GIS or as a tool to revise digital line graphs and topographic maps. The procedure used to create digital orthophotos, called ortho-rectification, requires aerial photographs and a digital terrain model as inputs. In the following paragraphs, the procedure is described.

Aerial Photography

The first step in digital orthophoto production is to take aerial photographs of the land surface. While taking the photographs, the plane meanders over a certain area, such that it's covered by overlapping photographs. Typically, the photos are taken with a camera with a 6-inch focal length lens at an altitude of 15,000 feet; this generates photographs with a scale of 1:30,000. The film diapositives are later scanned with a precision image scanner to create a raster image file.

Digital Terrain Model

The digital terrain model (DTM) can either come from an existing source or it can be developed from the aerial photography. The aerial photos are taken using a stereoscopic camera, with which two pictures of a particular area are simultaneously taken, but from slightly different angles. The overlapping area of the two resulting photos is called a stereo pair. Using a computer called a stereoplotter, the stereo pair can be viewed as a single image with the appearance of depth or relief. Ground control points are established based on ground surveys or aerial triangulation and are viewed in the stereoplotter in conjunction with the stereo pair. In this setting, the image coordinates of any (x,y,z) point in the stereo pair can be determined and randomly selected and digitized. These points, in conjunction with the control points, comprise the data points for the DTM. The accuracy of the final digital orthophoto will depend in large part on the point density of the DTM.

For a digital representation of the terrain, a triangular irregular network (TIN) model is often used. In the TIN model, a triangular mesh is drawn on the control and determined data points. To form the TIN, a perimeter around the data points is first established, called the convex hull. To connect the interior points, Delaunay triangulation is used, in which a surface approximation is generated where triangles are created with all internal angles as nearly equiangular as possible. Each resulting triangle is a planar surface. By integrating all of the triangles over the domain, a surface is created. Additional elevation data such as spot elevations at summits and depressions and break lines are also collected for the TIN model. Break lines represent significant terrain features like a lake or cliff that cause a change in slope. TIN triangles do not cross break lines. One reason the TIN model is used is that it requires a much smaller number of points than a gridded DTM does in order to represent the surface terrain with equal accuracy.

Orthophoto Rectification

Conventional aerial photographs have limited use in GIS because they are not true to scale. When you look at the center of an aerial photograph, your view is the same as if you were looking straight down from the aircraft. But as you look toward the edges of the photograph, the view of the ground is no longer straight down, but from an angle. This is called a central perspective projection; scale is true at the very center of the aerial photograph, but not elsewhere. In order to create a scale correct photograph that can be accurately measured, an orthographic projection is necessary, in which the view is straight down over every point in the photograph.

The TIN surface is used to orthogonally rectify the scanned image file. By combining the two data sources, each image pixel has a known position and intensity value. In the rectification process, the intensity value for each pixel is re-sampled using a space resection equation, removing image displacements caused by central perspective projection, camera tilt, and terrain relief. The individual photographs are then clipped and seamlessly joined together over the entire study area. The result is a digital image that combines the image characteristics of a photograph with the geometric qualities of a map--a true to scale photographic map. The typical digital orthophoto is a quarter quadrangle image cast in the Universal Transverse Mercator projection. Ground/pixel resolutions can be as fine as 1 meter.


Alief, NE DOQQ (2.5-meter resolution)

Benefits

Digital orthophoto quarter quadrangles (DOQQs) have many uses in the GIS environment. Some DOQQ benefits include the following:

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