Digital Orthophotos

The growth of image-processing technology has spawned the emergence of digital orthophotos: computer-rectified aerial photographs that provide raster images of ground features in their true map positions. Digital orthophotos—which have come on strong as a new and different base map—offer a complete, accurate foundation for a Geographic Information System (GIS). Digital orthophotos are a proven alternative for applications ranging from infrastructure management to appraisal mapping.

 

An orthophoto is a photograph with images of ground features in their true map coordinates, created photogrammetrically from aerial photography. Think of an orthophoto as a picture and a scaleable map.

 

Orthophotos have a consistent scale because the effects of tilt, relief, and lens distortion have been removed through a photogrammetric process that creates a new image with the original images in corrected positions.

 

 

A digital orthophoto is simply a computerized version of a conventional orthophoto: a raster image of ground features in their true map positions. A raster image is a grid of computer pixels. Each pixel has a row and column “address” (an X,Y value) and an intensity value ranging from 0 to 255.

 

A digital orthophoto is a continuous-tone raster image; all pixels are “on” but at varying intensities of black, white, and gray. (By contrast, binary raster image would produce no gray tones; pixels with a binary value of either 0 or 1 would be either “off” or “on.”)

Creating Digital Orthophotos

Digital orthophotos are created in a five-step process:

 

Aerial Photography. First, aerial photography is taken during conditions favorable for producing clear and crisp photographs and at a scale appropriate for the accuracy and resolution required. The aerial camera should be equipped with Forward Motion Compensation (FMC), which ensures precise exposure and sharply defined photos.

 

Ground Control. Next, sufficient horizontal and vertical ground control is acquired to orient the photographs to known coordinates and ground features. Fully analytic triangulation is performed to mathematically densify the ground control.

 

Image Scanning. Third, the aerial photographs are scanned, producing continuous-tone, digital raster images of the photography.
 

Digital Elevation Model (DEM) Production. Fourth, aerial photographs are used to compile a DEM, a series of X,Y,Z coordinates that accurately depict ground elevations.

 

The DEM must be both accurate and dense enough to adequately define the terrain. Two kinds of points are input to create a DEM: breaklines, which indicate an abrupt change in elevation, and spot elevations in a grid-type pattern (mass points). The result is a dense grid of mass points that accurately define the terrain, the more variations in the terrain, the more dense the grid.

 

During the DEM production, the stereoplotter can capture certain planimetric detail cost-effectively since features, such as roads and hydrologic structures; are also captured as breaklines.
 

The following figures are illustrations depicting the differences between DEM data for orthophoto rectification and DTM data for the generation of contours of TIN files for various engineering applications.

Image Rectification. Finally, the raster images are overlaid with the DEM and corrected, based on ground coordinates, so that image displacements can be removed. The result is accurate, rectified raster images of the aerial photography.
 

In the past, costs prohibited users from digitizing every visible ground feature in a vector format. Digital orthophotos provide complete, accurate images of ground features and can often be produced at a lower cost than photogrammetrically created vector maps.

 

Digital orthophotos, along with the accompanying DEM, are useful for many of the same applications as line maps. These include engineering design, planning, water resource management, industrial and commercial site management, and other applications requiring an accurate base map. In addition, the DEM has 3D capabilities, which allow the user to perform engineering calculations such as volumetrics, earthwork computations, cross sections, and profiles.

Quality, Resolution, and Accuracy of Digital Orthophotos

 

The quality of digital orthophoto imagery depends on factors such as the image-processing software and the resolution of the aerial camera, photography, scanner, workstation screen, and output device.

 

The resolution (not to be confused with accuracy) of the final orthophoto imagery is mainly affected by the quality of the original aerial negative, scanning device, and density of scanned images. Images can be scanned at various densities; the scanning device selected should have a resolution of at least 1,000 dots per inch (dpi), or ±25 microns per pixel. A smaller pixel produces dense, sharp images; a larger pixel (although it generates smaller file sizes) produces images that are less sharp, which affects the quality of digital orthophotos.

 

The following table is a general guideline for image scanning resolutions in relation to the original aerial negative and final output scale of the digital orthophotos.

Guideline for Image Input Scanning Resolution

Digital Orthophoto Output Scale	Original Aerial Photography Negative Scale	Image Scanning Resolution
1”=50’	1”=400’	0.5 foot per pixel
1”=100’	1”=800’	1.0 foot per pixel
1”=200’	1”=1,600’	2.0 feet per pixel
1”=400’	1”=3,200’	4.0 feet per pixel

 

The accuracy of digital orthophoto imagery is a result of accurate ground control, triangulation, and DEM data rather than the scanned image resolution.