In this definition, primary devices refer to the computer's main storage device, such as an IDE or SCSI hard drive. Secondary devices, under this definition, are any non-primary storage device, such as a tape drive, writeable CD-ROMs or removable flash drives.
Each type of storage media has different applications. Hard drives are the best choice for large volume, primary data storage. Regardless of the hard drive technology, the hard drive sacrifices speed for storage capacity and density when compared to volatile storage devices such as RAM (Random Access Memory) . RAM, on the other hand, is not typically used for file system data storage because of its non-permanent nature. Due to its high-speed connection to the computer's CPU, RAM makes the best choice for primary storage. RAM typically can store and retrieve data at speeds that exceed 4 to 5 times that of the fastest hard drives.
Writeable CD-ROM media, tape drives and flash drives make good choices for secondary storage devices (using either definition). These devices are often removable and, with the exception of magnetic tapes, offer higher reliability than hard drives that are dependent on moving parts. Computer users often depend on these devices to store backup or archive information, or to carry data that must be easily portable from site-to-site.
"A character file may be transferred to a host for one of three purposes: for printing, for storage and later retrieval, or for processing. If a file is sent for printing, the receiving host must know how the vertical format control is represented. In the second case, it must be possible to store a file at a host and then retrieve it later in exactly the same form. Finally, it should be possible to move a file from one host to another and process the file at the second host without undue trouble. A single ASCII or EBCDIC format does not satisfy all these conditions." (DATA REPRESENTATION AND STORAGE)
Flash drives, tapes or Zip disks. Unlike secondary storage, primary storage is volatile and all primary storage information is lost when the computer is powered down. Secondary storage devices are non-volatile and store information not currently being processed by the computer. Secondary storage devices do not lose information when the device is not powered.
The real world objects that are included must be represented by an object defined in the GISystem software.
For example, a road network of a town comprises the road surfaces, footpaths, kerbing (and other structures). In a GISystem it will usually be represented by a network of lines defining the centre lines of the roads. In a raster model GISystem, a connected series of cells (rather than a line) would represent a road. The objects that are represented in a GISystem will have defined boundaries. Real world features like forests or soil parcels, do not have sharp boundaries in the real world, however in a GISystem, they will be assigned boundaries. The likely uses of the data will again be decisive in determining the form of representation. At small scales, roads are usually represented by line networks defining the centre lines of the roads. For engineering uses, larger scales are employed, and the objects represented will include the kerbing and footpaths, the exact shapes of the curves, etc., but not just single lines. Thus,