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Frequently Asked Questions about: GIS Help Desk: 609-777-0672

  1. What is digital imagery?
  2. How are digital images generated?
  3. What kinds of digital image files are commonly used in a GIS?
  4. What makes a digital image in a GIS different than one that can be taken using a simple phone camera or personal digital camera?
  5. How is the coordinate information attached to the digital image file?
  6. What is meant by pixel size or image resolution?
  7. Is image resolution the same as image accuracy?
  8. What resolutions and accuracies are characteristic of the digital images found in NJ-GeoWeb?
  9. What types of digital imagery are available In NJ-GeoWeb?
  10. What is CIR imagery?
  11. Is any of the imagery available to the public?
  12. What can the digital imagery in a GIS be used for?



  1. What is digital imagery?

    Digital imagery is a computer compatible version of an aerial photograph, satellite photo or other map image. While there are many specific types of digital image formats, they all are examples of raster files. Rasters use a grid structure made up of small individual cells, or pixels (which is short for 'picture element'), to store and display information. In the case of a digital image, each pixel stores information on the wavelengths of light that are reflected off of the features being captured in the image.

    As with regular photographs, digital images can be either pan-chromatic (commonly referred to as black and white or gray scale images) or color. For a pan-chromatic image, the information stored in each pixel is a single value that a computer uses to display a gray shade for each pixel. For color images, each pixel stores information on the three color bands that make up the visible light spectrum—red, green and blue. Computers can use the values for the three bands to recreate the natural color of the object represented in each pixel when the digital image is displayed on a computer screen. Digital images can also store information from other light bands that are reflected from a feature, such as the infrared band, or other bands known as hyper-spectral bands, and these values can also be used by the computer to display the images. Whatever band values are being stored, when the digital image is viewed in a computer or GIS, the individual features in the images are represented by groups of many pixels with specific gray or color band characteristics that, when viewed together, allow each feature to be discerned in the display screen.

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  3. How are digital images generated?

    Digital images can be generated in several ways. Often, a digital image is made from a non-digital photo or map that exists in what is called hard copy format. The photo or map can be scanned in a digital scanner that converts the hard copy photo or map into a digital raster image file, made up of a large number of pixels as described above.

    Increasingly, however, digital images are now being directly generated using digital image cameras and sensors that output image files directly in digital image formats. The 2007 and 2012 statewide imagery layers available in NJ-GeoWeb are examples of digital imagery produced using this technology. These digital images were generated directly using a digital camera housed in a plane that flew over the state, with no photographic film being produced. Satellite systems, such as those used in the NASA/USGS Landsat program, have been using digital sensors to collect imagery for some time. This technology has now matured to a point where it is routinely used on most large scale image collection projects such as a New Jersey statewide overflight.

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  5. What kinds of digital image files are commonly used in a GIS?

    There are many types of digital image files which are reflective of the many types of software systems that can generate and view digital image files. Each of these can be identified by a unique file extension. The most common formats used in a GIS and their extensions are:

    TIFF: (Tagged Image File Format) (.tif)

    GEOTIFF: a TIFF file with geographic coordinate information imbedded in the image (.tif)

    JPEG: (Joint Photographic experts Group) (.jpg)*

    JPEG2000: An advanced version of the JPEG file (.jp2)*

    ERDAS IMAGINE: (.img)

    MrSID: (Multiresolution Seamless Image Database) (.sid)*

    ArcInfo Grid: Raster format developed by ESRI (no extension)

    The asterisk indicates a compressed image format. Images file sizes can be very large, often reaching several hundred megabytes for a TIFF or IMG file, or more. The JPEG, JPEG2000 and MrSid files are formats that have been developed to compress the original image files to more manageable sizes without losing too much detail of the image.

    Note that all of these formats can be used directly in a GIS. However, for many of the image layers available in NJ-GeoWeb, the images are converted to one of several ArcGIS formats, such as a geo-database raster, mosaic dataset, image catalogue or image service, to improve drawing these large data sets.

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  7. What makes a digital image in a GIS different than one that can be taken using a simple phone camera or personal digital camera?

    While digital images that are taken with your phone or hand held digital camera are similar to those in the GIS in that they are all raster files made of millions of small pixels, images used in a GIS have some other special characteristics. GIS digital images have been processed so that they have a specific ground coordinate system associated with them. In other words, the images have been given real world coordinates that the GIS can use to place the image in its real world location in relation to other mapped data.

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  9. How is the coordinate information attached to the digital image file?

    There are several processes that can be used to give digital images coordinate referencing. The simplest process is known as geo-referencing. In this process, a digital image without coordinates is given coordinate referencing either by using coordinate information that is collected by field surveying locations covered in the image, or by comparing the image with another digital image of the same area that does have coordinates, using a computer program. In the first case, coordinates of several ground features visible in the image are determined by surveying those locations using standard survey techniques or global positioning systems (GPS). The surveyed features are then located when the digital image is displayed on a computer screen and assigned the coordinates determined during the field survey. A computer program then uses the coordinates from all of the surveyed points to generate a real coordinate space for the entire digital image, with every pixel in the final image having real world coordinates generated for it.

    In the second case in which an un-referenced image is compared with a referenced image, several features identifiable in both images are selected to be used as link points. As the link points are selected, a computer program transfers the real coordinates of each feature from the referenced image, to the same features in the un-referenced image. The computer then uses this information to generate a coordinate solution for the entire image being referenced.

    A more robust process that is used in some digital image processing is known as ortho-rectification. Ortho-rectification uses steps similar to those used in simple geo-referencing. But the process also includes additional steps that further remove image displacements that occur because of terrain relief. In ortho-rectification, a 3-D model of the ground surface is generated using a digital elevation model (DEM) of the area covered by the image. By accounting for terrain displacement, the true coordinate space of the image can be generated more accurately, resulting in the processed image having a consistent scale throughout the entire image.

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  11. What is meant by pixel size or image resolution?

    Image resolution refers to the actual ground area each pixel represents, in units of the coordinate system used to reference the image. For example, the 2012 statewide digital imagery available in NJ-GeoWeb, which is referenced in New Jersey State Plane Feet, has a resolution of 1 foot, meaning each pixel in the image represents a 1 square foot area of ground surface. Image resolution is an extremely important characteristic of the imagery, since the higher the resolution, that is, the smaller area each pixel represents, the more detail that can be seen in the image.

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  13. Is image resolution the same as image accuracy?

    Image accuracy and image resolution are two different characteristics of digital imagery. As described above, resolution refers to the ground area represented by each pixel. Image accuracy is actually a reflection of how close coordinates of features measured from the image after it is referenced are to the actual ground coordinates of those features determined from ground surveys, or from other images that have a tested accuracy level. For example, the 2012 digital imagery has a tested accuracy of +/- 4 feet, meaning that coordinates extracted from the digital image will be within + or – 4 feet of the coordinate values determined if one were to go out and survey the location of that feature. Images can have different pixel resolutions and the same accuracies, as well as the same pixel resolution and different accuracies.

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  15. What resolutions and accuracies are characteristic of the digital images found in NJ-GeoWeb?

    Unfortunately, not all of the digital image layers used in NJ-GeoWeb have a ground accuracy level measured for them. This is true of the layers made from historical photos and maps, such as the 1930, 1970 and 1977 imagery, which generally had no coordinate information collected at the time the map or image was generated. A representative accuracy can be determined by making some measurements in GIS when viewing each historical image layer, but since the accuracy may vary from place to place across each image, this measurement can only be used as a general guide.

    For other data sets, however, particularly all of the ortho-rectified imagery, a tested accuracy is available for each image layer. The first ortho-rectified dataset produced for NJDEP, the 1995 imagery CIR imagery, was produced to meet a USGS accuracy for images produced at a scale of 1:12000, which means an accuracy of +/- 33 feet. For later imagery such as the 2002, 2007 and 2012 image layers, the tested accuracy is +/- 4 feet. So there is a great range in the accuracy of the various image sets. If an accuracy test has been performed, it will be listed in the metadata for that image layer.

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  17. What types of digital imagery are available In NJ-GeoWeb?

    There are several types of digital imagery used in NJ-GeoWeb. Some of the imagery layers, such as the 1930 imagery, are pan-chromatic images, which means they are displayed only as gray scale images. Other imagery, such as the 2013 imagery, is available as true color imagery. Still other imagery, such as the 1995 and 2002 imagery, is available as color infrared (CIR) imagery.

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  19. What is CIR imagery?

    Normal color imagery is collected using the visible part of the light spectrum, with all colors really only a combination of the three primary colors—red, green and blue. A true color digital image is really made up of three bands, one for each of the primary colors. When the three bands of the image are displayed on a computer, the result is the true color of the object based on how much red, green and blue light reflects from the object.

    For CIR, or color infrared, imagery, the camera or sensor used to collect the imagery can also sense light in the non-visible portion of the light spectrum, specifically in the near infrared portion, by using special filters. This infrared data is collected in a separate band which can be substituted for one of the primary color bands to generate a false color or color infrared display on a computer. Generally, the three bands of a CIR image display are the infrared band, the red band and the green band, with the blue band not being used. What this band arrangement results in is a shift in the colors used to display various features. For instance, since blue is not used, anything that was blue true color generally shows up as black in a CIR image. In addition, since the red color gun in the monitor is being used to display the green color band, features normally shades of green in a true color image show up as shades of red and pink in a CIR image. Although the color balance is not what one would expect to see, CIR images are very useful in doing many types of analysis since many characteristics of features are more defined when seen in CIR images than in true color images. This is particularly true of vegetation since characteristics such as canopy arrangement, leaf structure, water content, and general growth vigor, among others, are more discernable using infrared reflectance.

    Note that some of the image layers, particularly the 2007 and 2012 imagery, have both a true color and a CIR version of the imagery available. This is because the imagery for these data sets was collected using a digital sensor that could collect data for the red, green, blue and infrared bands all at the same time. All four bands are stored as part of the image file, and a GIS can be used to simply select the three bands to display to produce a true color or CIR image set.

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  21. Is any of the imagery available to the public?

    Yes. There are several ways for the public to access most of the imagery available to DEP staff. Many of the imagery layers are available in the public versions of NJ-GeoWeb. The same image layers are also available through NJGIN which is a site hosted by the Office of GIS of the NJ Office of Information Technology. There are image services available from the NJGIN site that can be added to most GIS mapping software applications. So outside parties that have their own GIS can go to the NJGIN site and add the image layers directly to their own applications. The NJGIN site also has some of the image files available for download, so that users that cannot take advantage of the image service format, can still get some of the image files. The NJGIN site has directions both for ordering the image files and for accessing the image services.

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  23. What can the digital imagery in a GIS be used for?

    Since all of the image layers available in NJ-GeoWeb or through the NJGIN services or downloads have coordinate referencing associated with them, most importantly, they can be used as a base to overlay other referenced digital data. This would include every data set database available through the NJDEP GIS data download site. In addition, any other data set that has a coordinate reference defined for it can be added to a GIS application with the imagery for analysis and comparison.

    Data can also be generated using the digital imagery as a base, and much of the data available through the NJDEP data download site is generated using heads-up data generation techniques with the digital imagery.

    Lastly, measurements and coordinate locations can be determined using the imagery as a base. Distance measurements, determination of the area of a polygon, and the X and Y coordinates of point features can all be determined from the referenced imagery sources. Since the 2002, 2007 and 2012 image layers are fully ortho-rectified layers with a tested accuracy level, they would be the best sources to uses to determine coordinate locations or length and area measurements. But similar measurements can be made using all referenced image layers.

 

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