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Data Storage and Representation

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815Data Storage and RepresentationComputer cartography demands a familiarity with the tools and limitations of digitaltechnology. In Chapter 4 we examined the conversion of map data to digital form usinggeocoding. When a map consists solely of numbers, the cartographer is faced with manydecisions about how to store the map data and how to select methods of representationthat allow the data to be used for mapping. In Chapter 2 we surveyed the various types ofdevices used in computer cartography, especially graphic input and output devices, butwe explicitly left out storage devices. The ultimate purpose of geocoding and choosing arepresentational method and data structure for cartographic data is to store the digital mapon some kind of permanent storage device. Ideally, the data should be easy to get backfrom storage and should also be stored in a way that is both permanent (usable for morethan 10 years, at least!) and transportable to other computers. The means of storage is of-ten also the means of distribution. In the days of paper maps, one could purchase the pa-per over the counter or through the mail, and the information was contained in the printedinks on the paper’s surface. Digital map data have to be distributed also. In this chapterwe deal with the means of storage, both physical and logical. In Chapter 6 we examinemore closely the role of distribution of digital map information.Over time, improvements in storage technology have drastically reduced the cost andspace requirements for storage, and permanence has improved. A key issue for storageremains portability between computers, although distributed computing and networkshave relegated this problem to one of archiving. During the initial years of computing,the most widely used storage methods were the punched card and paper tape. These stor-age methods were not permanent, and they suffered from additional problems, such assequencing and tearing. Disk and magnetic tape storage replaced cards and paper tape.Fixed or hard disks are usually part of the computer and as such are not transportable,unlike their smaller but portable equivalent, the removable disk. Removable disks, how-ever, are fragile and bulky and can be damaged by strong magnetic fields. 5.1 STORAGE MEDIAData Storage and Representatio nChap .582Floppy disks, introduced when microcomputers became available, are relatively rig-id, so a preferable term in use is diskette. Older disks were enclosed in a plastic envelopecontaining the thin magnetic sheet that holds the data. Early types were 133 mm (5.25inches) and 203 mm (8.0 inches) in diameter, but they are rapidly being replaced by 89-mm (3.5-inch) disks, which have a more rigid plastic case and can support higher storagedensities. Such a disk has been used to distribute the software and data for use with thisbook, and is enclosed in a plastic pocket at the back of the book.Probably the longest surviving and most reliable storage medium is magnetic tape,which comes both as loose reels and tape cartridges. Different lengths of tape and differ-ent numbers of bits per inch (BPI) of storage density allow different amounts of data tobe stored. The cost, reliability, transferability, and high density of magnetic tape madethis the normal medium for data and software distribution for large computers. The ad-vent of workstations has led to several new tape formats, including the 4-mm tape car-tridge and the 8-mm tape. The latter are highly flexible and extremely compact and arecapable of storing between 2.3 and 5.0 gigabytes of data, depending on blocking andstructure, at a cost of only a few dollars. This change clearly indicates that basic level dig-ital cartography now faces few physical limitations of storage volume, even on micro-computers.More recently, mass storage has become possible using optical disk technology.Many optical disks are read-only and can play all or part of a record but cannot record.The first generation of optical disks used compact discs as read-only memory, and disktemplates had to be written in much the same way that a book is printed. More recent op-tical media allow both reading and writing. The advantage of optical media is the size ofthe storage, sometimes approaching gigabytes of storage even for a microcomputer. Al-most all topographic map coverage for a single state, both current and historical, wouldfit onto a single optical disk. Several companies now distribute atlases, gazetteers, andeven street maps in CD-ROM, such as the Map Explorer software and data by DeLorme.The disks themselves are comparatively fragile and require special holders for placementinto a drive. Nevertheless, the prices are now so low that they are a favored means of dataand software distribution. The per item costs for disks that are sold in volume can ap-proach the cost of cartridge tape. For example, many government agencies distribute CD-ROM data sets for about $30. Recently, optical disk “juke-boxes” have become available. These devices allow theuser to select, through software, which CD-ROM is to be loaded onto the drive to be read.Stacks of CD-ROMs are therefore available to the user, making hundreds of gigabytesaccessible. So large have storage volumes now become that discussions use the terms ter-abyte (1,000 gigabytes) and even petabyte (1,000,000 gigabyte). The archive of Landsatdata at the EROS Data Center in Sioux Falls, South Dakota, for example, now containsabout 40 terabytes or 0.04 petabytes. Perhaps a petabyte of storage for a desktop carto-graphic workstation may be possible, perhaps sooner than we think! Such a situation en-sures a vast redundancy in the supply of digital map data, guaranteeing its survival in thesame way as publishing ensures the survival of the information content of a book. Simi-larly, every cartographer may eventually have access to all the maps and images ever cre-ated.Sec. 5.2 Internal Representation83Even reliable storage such as disk memory has to be backed up to a more permanentstorage medium to guard against failure and loss. Backups are either full-system or incre-mental, in which every new or changed file on the entire system is copied. All files arecopied onto magnetic tape, and the tapes are archived. If anybody inadvertently deletes afile, or if something goes wrong with the computer, data can be retrieved as they weresaved on a given day. Magnetic tape readers are available for microcomputers and areusually used to back up a hard disk on magnetic


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