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Orthophotography

Digital Orthophotos

The growth of image-processing technology has initiated the emergence of digital orthophotography: computer-rectified aerial photographs that provide raster images of ground features in their true map (geographical) 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 an image of ground features in their true map coordinates, created photogrammetrically from aerial photography. Think of an orthophoto as a picture and a scalable map.

 

Orthophotos have a consistent scale because the effects of camera 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 (or red, yellow and blue for color). By contrast, binary raster images 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:

1.    Aerial Photography.   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.

2.    Ground Control.   Sufficient horizontal and vertical ground control is acquired to orient the photographs to known coordinates and ground features.  Airborne GPS coordinates may also be used to supplement the ground surveyed control.  Fully analytic triangulation is performed to mathematically densify the ground control.  

3.    Image Scanning.   The aerial negatives are scanned producing a continuous-tone digital raster image of the photography.

4.    Digital Elevation Model (DEM) Production.   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 are linear vectors that indicate an abrupt change in elevation, and mass points in either a random or grid-type pattern.  The result is a dense grid of elevation points that accurately define the terrain.  The more variations in the terrain the more dense the grid. 

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


 

                                                                                                 DEM Data





                                            DTM Data


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, real estate management, and other applications requiring an accurate base map.  In addition, the DTM has 3D capabilities which allow the user to perform engineering calculations such as volumetrics, earthwork computations, cross sections, and profiles.


Quality, Resolution, 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.  More modern scanners can produce pixel resolutions down to 12 microns.  A smaller pixel produces dense, sharp images but results in much larger file sizes.  Conversely, a larger pixel will produce images that are less sharp (but smaller file sizes) but will affect the quality of the digital orthophotos. 

The following table is a general guideline for image scanning resolutions in relation to the original 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.

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