Glossary
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ANSI
- American National Standards Institute
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C & C++
- C is probably the computer programming language used most to write professional-level software, though programs written new today are usually done with its object-oriented successor, C++. Both are best suited to well-trained programmers with the knowledge, tools, and time to avoid some major pitfalls.
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collar
- The white margin around a DRG topo map that includes "marginal information" which can be quite useful, such as the map's date and rotation, and the names of adjoining maps (sometimes wrong on older DRGs, as encountered with southeastern Utah).
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datum
- An Earth-shape (ellipsoid) reference model upon which points on the Earth's surface are calculated. For instance, NAD27 is common for UTM 1:24000 DRGs and 30m and 10m DEMs, though corresponding DOQs may be in a different datum, such as NAD83. Draping data from one datum, such as images or DLGs, onto DEMs in another datum, will result in noticeable misalignments.
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DDF
- Data Descriptive File - This is the file name extension used for the multiple files stored in a single USGS SDTS transfer .tar.gz archive such as for a DEM or DLG.
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DDR
- Data Descriptive Record - The first record in a DDF file, it carries the information a DDF-reading program needs to know how to read the following data records.
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DEM
- digital elevation model - This term is used very generally to describe a great variety of terrain files and file formats. There are also a number of mutually incompatible file formats that use the .dem file name extension.
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DLG
- Digital Line Graph - A USGS format, with several variations, for descriptions of individual data types such as transportation (e.g., roads), hydrography (streams, lake shores), and contours.
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DOQ
- digital orthophoto quadrangle - Aerial photography of the Earth's surface, usually provided in GeoTIFF form, but also seen in other formats. A DOQ for a 7.5-min., 1:24000-scale quad is huge, thus the common use of DOQQs--digital orthophoto quarter quadrangles.
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DRG
- Digital Raster Graphic - A scanned or digitally-created topographic ("topo") map just like you may have taken hiking. These usually come as indexed-color GeoTIFF files, sometimes in a .tar.gz file that includes an accompanying .tfw world file.
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FGDC
- U.S. Federal Geographic Data Committee
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FIPS
- U.S. Federal Information Processing Standards
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GeoTIFF
- A special expanded version of the TIFF6 standard used for various purposes, especially for DRGs, that provides for embedding topographic information. Embedded geo-referencing removes the need for a companion world file, though one may be provided for compatibility.
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GIS
- geographic information systems
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grayscale
- Grayscale "raster" files come in two flavors: 8-bit and 16-bit, where possible value ranges can be from 0 (black) to, respectively, 255 or 65,535 (white).
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GUI
- graphical user interface
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interpreted
- A scripting language, such as Basic and its many "flavors," as well as Python, Tcl, and Javascript, has its programming code "interpreted" into computer instructions every time a script is run (at "run time"). An executable program, one that comes complete unto itself, has had all of its computer instructions "compiled" just once. C/C++, Pascal, Delphi/Kylix, and Java are examples of compiled languages.
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ISO
- International Standards Organization
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LOD
- level of detail - To decrease memory impact and to speed rendering, a terrain or other object in a 3D scene may be represented by substitutes of different detail according to proximity to the camera - low detail when far away and high detail when seen close.
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object-oriented
- Programming languages and techniques where data carries with itself the "methods" (also known as "functions") used to handle that data. An OO programmer, for instance, can write a statement such as "object.print()" without having to be concerned about what kind of object will be involved at "run time" or what its printing method is. Object-oriented code is both more flexible and more organized, so it is far easier to write, read, and change than procedural code. OO code is also incomparably better for writing programs to run in interactive windowing environments.
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NAD27
- North American Datum 1927, CONUS - Also called NAS-C (or sometimes just NAS), this datum is being superceded by NAD83 and WGS84, but is still very commonly found used for USGS 1:24000 DRGs and DEMs.
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NAD83
- North American Datum 1983, CONUS - Also called NAR-C.
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NIMA
- National Imagery & Mapping Agency - This was formerly the DMA, Defense Mapping Agency.
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Python
- A freeware open-source cross-platform interpreted scripting language from www.python.org.
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SDTS
- Spatial Data Transfer Standard - A USGS format used for data such as 1:24000 DEMs and DLGs. An SDTS "transfer" consists of multiple files using the .ddf file name extension that usually are packed together into a single .tar.gz file.
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Tcl
- Tool Command Language - A freeware open-source cross-platform interpreted scripting language from www.scriptics.com. Tcl's Tk interface toolkit is embedded in the Python Tkinter module.
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tuple
- a set of values - In Python, a "tuple" is used to describe a set of two or more values which do not have to be of the same type. Python 2.0 tuple members cannot receive direct assignment, but an easy workaround is to regenerate the set to incorporate a changed value. Tuples are very useful for passing multiple values from one function to another in one step. A good example is a Tkinter font tuple: 'Helvetica',10,'bold' for 10-point Helvetica Bold, where the face and style are of type text but the point size is of type integer.
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USGS
- U.S. Geological Survey - www.usgs.gov
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UTM
- Universal Transverse Mercator - A rectangular metric mapping coordinate system used instead of latitude and longitude.
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UTM grid zone map: www.dmap.co.uk/utmworld.htm.
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WGS84
- World Geodetic System 1984 - A worldwide datum.
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world file
- A map or image (DRG, DOQ, etc.) companion file that includes basic plain-text information about location and scale (geo-referencing). World files share the image file name but use a different extension. For a TIFF, the companion world file uses the extension, .tfw. World files for JPEG and BMP files use .jpw and .bpw, respectively.
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You also may encounter .met metadata files that contain much more information than world files.
For an exhaustive list of GIS acronyms, with expansions though not definitions, visit www.lib.berkeley.edu/EART/abbrev.html.
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