Geographic Information Systems

Box 2: Resolution Has Various Meanings.


In remote sensing and GISs the term resolution has many meanings. The most common use of the term resolution refers to the area on the ground represented by one cell or pixel (picture element) of a raster-based system. For raster data collected with satellite-based digital sensors, the size of the cell is a function of characteristics of the sensor and orbiting altitude of the satellite. Landsat MSS data have a spatial resolution of approximately 0.64 ha (80 X 80 m). Landsat TM data have a spatial resolution of approximately 0.09 ha (30 X 30 m). The panchromatic data from SPOT have a spatial resolution of 0.01 ha (10 X 10 m).

The ability to separate various cover classes or features with data from satellite-based digital sensors is not totally a function of spatial resolution. Spectral resolution often is as important as spatial resolution in selecting a particular satellite for mapping land cover. Spectral resolution refers to the wavelengths of the electromagnetic spectrum selected to be measured by the sensor. Landsat TM data have been able to successfully differentiate among various cover classes that could not be separated by Landsat MSS data. Not only has the improved spatial resolution of TM data accounted for the improved accuracy of classification, but the increase from four to seven spectral bands has had a great impact in the successful use of TM data for mapping cover types. The number of spectral bands and the width of the spectral bands selected make up the spectral resolution of a sensor.

Resolution can also refer to the range of data collected by the satellite. For example, for MSS data collected on Landsats 1-3, the intensity of energy received for each band was measured in the range of 0 to 127. The TM sensors on Landsat 4 and 5 measure the energy received from each band in the range of 0 to 255. The number of computer bits used to store the range sets the maximum limit on this use of the term resolution.

Resolution can also describe the capabilities of some of the hardware used with GISs. When resolution is used to describe display monitors, it refers to size and number of screen pixels. A high-resolution monitor may display 1,024 elements by 1,024 lines, and each pixel is as small as 0.3 mm. A lower resolution monitor may be able to display only 512 elements by 512 lines. The resolution of scanners and plotters is often measured in dots per inch. For plotters, dots per inch is a measure of the number of pixels plotted per linear inch. A high-resolution plotter can display 400 or more pixels per inch, whereas a lower-resolution plotter may be able to display only 200 pixels per inch.


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