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[Archived] AskInga Wiki Understanding raster file sizes and more
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    Understanding raster file sizes and more

    AskInga logo

    Original Article Date: Feb. 5, 2001

    Need to calulate the compression ratio, dots per inch or displacement size of your raster images? Give this article a read.

    Dimensions of the image
    To determine the dimensions in working units of the area covered by the raster document:

    X number of pixels * Pixel size = X dimension
    Y number of pixels * Pixel size = Y dimension

    For example, if you have 10 pixels and each pixel covers 2 feet on the ground then the dimension for that axis would be 10 x 2 = 20 feet.


    Image Resolution
    Is used to define the number of dots that fit into one inch of the image. To determine the size in inches of each dot:
    1 inch / Dots per Inch = Image Resolution

    For example, you have an image scanned in at 600 DPI, what's the image resolution?
    Image Resolution = 1 / 600 DPI = 0.00166666


    Terrain Resolution
    Is the 'ground distance' covered by each image dot and is calculated by:
    Scanned Scale / Dots per Inch = Terrain Resolution

    For example, you've got an image which was scanned at 600 DPI at a scale of 1:500, how much ground does one dot cover?
    Terrain Resolution = 500 / 600 DPI = 0.833333 inches


    Output Resolution
    Is the maximum output resolution of the image when printing at a given size.
    Output Resolution = Image width in pixels / Paper width in inches

    For example, an image has 4800 pixels across it's width and needs to be plotted on 42" paper. What's the maximum output resolution? Output resolution = 4800 / 42 = 114dpi


    To calculate image file size in bytes: Using sample data: 10 inch x 10 inch photo, Scanned at 600 DPI.

    8 bit gray scale Width of image in inches X length of image in inches X DPI X DPI = Size in bytes
    Sample data: 10 x 10 x 600 x 600 = 36,000,000 bytes or 36mb

    24 bit RGB width of image in inches X length of image in inches X DPI X DPI X 3= Size in bytes
    Sample data: 10 x 10 x 600 x 600 x 3 = 108,000,000 bytes or 108mb

    1 bit Binary (width of image in inches X length of image in inches X DPI X DPI) / 8 = Size in bytes
    Sample data: (10 x 10 x 600 x 600) / 8 = 4,500,000 bytes or 4.5mb

    AskInga Article #43

    • Raster Images
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    • Hannah Blye Created by Hannah Blye
    • When: Tue, May 27 2008 3:08 PM
    • Inga Morozoff Last revision by Bentley Colleague Inga Morozoff
    • When: Mon, Jun 14 2010 3:36 PM
    • Revisions: 4
    • Comments: 0
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