Image Data from Scanning

Producer Field Guide

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Producer Field Guide
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Hard-copy maps and photographs can be incorporated into the ERDAS IMAGINE environment through the use of a scanning device to transfer them into a digital (raster) format.

In scanning, the map, photograph, transparency, or other object to be scanned is typically placed on a flat surface, and the scanner scans across the object to record the image. The image is then transferred from analog to digital data.

There are many commonly used scanners for GIS and other desktop applications, such as Eikonix (Eikonix Corp.) or Vexcel (Vexcel Imaging Corp., Boulder, Colorado). Many scanners produce a Tagged Image File Format (TIFF) file, which can be used directly by ERDAS IMAGINE.

SHARED Tip Use Import dialog to import scanned data. Eikonix data can be converted to ERDAS IMAGINE .img format using XSCAN™ Tool by Ektron and then imported directly into ERDAS IMAGINE.

Photogrammetric Scanners

There are photogrammetric high quality scanners and desktop scanners. Photogrammetric quality scanners are special devices capable of high image quality and excellent positional accuracy. Use of this type of scanner results in geometric accuracies similar to traditional analog and analytical photogrammetric instruments. These scanners are necessary for digital photogrammetric applications that have high accuracy requirements.

These units usually scan only film because film is superior to paper, both in terms of image detail and geometry. These units usually have a Root Mean Square Error (RMSE) positional accuracy of 4 microns or less, and are capable of scanning at a maximum resolution of 5 to 10 microns.

The required pixel resolution varies depending on the application. Aerial triangulation and feature collection applications often scan in the 10 to 15 micron range. Orthophoto applications often use 15- to 30-micron pixels. Color film is less sharp than panchromatic, therefore color ortho applications often use 20- to 40-micron pixels.

Desktop Scanners

Desktop scanners are general purpose devices. They lack the image detail and geometric accuracy of photogrammetric quality units, but they are much less expensive. When using a desktop scanner, you should make sure that the active area is at least 9 × 9 inches (that is, A3-type scanners), enabling you to capture the entire photo frame.

Desktop scanners are appropriate for less rigorous uses, such as digital photogrammetry in support of GIS or remote sensing applications. Calibrating these units improves geometric accuracy, but the results are still inferior to photogrammetric units. The image correlation techniques that are necessary for automatic tie point collection and elevation extraction are often sensitive to scan quality. Therefore, errors can be introduced into the photogrammetric solution that are attributable to scanning errors.

Aerial Photography

Aerial photographs, such as NAPP photos, are a common data source in photogrammetry. They can not be utilized in soft-copy or digital photogrammetric applications until scanned. Standard dimensions of the aerial photos are 9 × 9 inches or 230 × 230 mm. The ground area covered by the photo depends on the scale. Scanning resolution determines the digital image file size and pixel size.

For example, for a 1:40,000 scale standard block of white aerial photos scanned at 25 microns (1016 dots per inch), the ground pixel size is 1 × 1 m2. Resulting file size is about 85 MB. It is not recommended to scan a photo with a scanning resolution less than 5 microns or larger than 5080 dpi.

DOQs

DOQ stands for digital orthophoto quadrangle. USGS defines a DOQ as a computer-generated image of an aerial photo, which has been orthorectified to give it map coordinates. DOQs can provide accurate map measurements.

Format of the DOQ is a grayscale image that covers 3.75 minutes of latitude by 3.75 minutes of longitude. DOQs use the North American Datum of 1983, and the Universal Transverse Mercator projection. Each pixel of a DOQ represents a square meter. 3.75-minute quarter quadrangles have a 1:12,000 scale. 7.5-minute quadrangles have a 1:24,000 scale. Some DOQs are available in color-infrared, which is especially useful for vegetation monitoring.

DOQs can be used in land use and planning, management of natural resources, environmental impact assessments, and watershed analysis, among other applications. A DOQ can also be used as "a cartographic base on which to overlay any number of associated thematic layers for displaying, generating, and modifying planimetric data or associated data files" .

According to the USGS:

DOQ production begins with an aerial photo and requires four elements: (1) at least three ground positions that can be identified within the photo; (2) camera calibration specifications, such as focal length; (3) a digital elevation model (DEM) of the area covered by the photo; (4) and a high-resolution digital image of the photo, produced by scanning. The photo is processed pixel by pixel to produce an image with features in true geographic positions.

Source: United States Geological Survey, 1999b.