Assignment 5

Index

A data structure, usually an array, used to speed the search for records in a database or for spatial features in geographic datasets. In general, unique identifiers stored in a key field point to records or files holding more detailed information.

Attribute Table

A database containing information about a set of geographic features, usually arranged so that each row represents a feature and each column represents a feature attribute. In raster datasets, each row of an attribute table corresponds to a certain zone of cells having the same value. In a GIS, attribute tables are often joined or related to spatial data layers, and the attribute values they contain can be used to find, query, and symbolize features or raster cells.

binary file

A file that contains data encoded as a sequence of bits (ones and zeros) instead of plain text. A binary file, such as a DLL or an executable file, contains information that can be directly loaded or executed by a computer.

row

A horizontal group of cells in a raster, or pixels in an image.

continuous raster

A raster in which cell values vary continuously to form a surface. In a continuous raster, the phenomena represented have no clear boundaries. Values exist on a scale relative to each other. It is assumed that the value assigned to each cell is what is found at the center of the cell. Rasters representing elevation, precipitation, chemical concentrations, suitability models, or distance from a road are examples of continuous rasters.

discrete raster

A raster that typically represents phenomena that have clear boundaries with attributes that are descriptions, classes or categories. Generally, integers are used for the cell values. In a raster of land cover, for example, the value 1 might represent forestland, the value 2 urban land, and so on. It is assumed that the phenomena that each value represents fill the entire area of the cell. Rasters representing land use, political boundaries or ownership are examples of discrete rasters.

raster data model

A representation of the world as a surface divided into a grid of cells. Raster models are useful for storing data that varies continuously, as in an aerial photograph, a satellite image, a surface of chemical concentrations, or an elevation surface.

band

A set of adjacent wavelengths or frequencies with a common characteristic. For example, visible light is one band of the electromagnetic spectrum, which also includes radio, gamma, and infrared waves.

raster

A spatial data model that defines space as an array of equally sized cells arranged in rows and columns, and comprised of single or multiple bands. Each cell contains an attribute value and location coordinates. Unlike a vector structure, which stores coordinates explicitly, raster coordinates are contained in the ordering of the matrix. Groups of cells that share the same value represent the same type of geographic feature.

spatial database

A structured collection of spatial data and its related attribute data, organized for efficient storage and retrieval.

JPEG

Acronym for Joint Photographic Experts Group. A lossy image compression format commonly used on the Internet. JPEG Is well-suited for photographs or images that have graduated colors.

Multispectral image

An image created from several narrow spectral bands.

digital image

An image stored in binary form and divided into a matrix of pixels. Each pixel consists of a digital value of one or more bits, defined by the bit depth. The digital value may represent, but is not limited to energy, brightness, color, intensity, sound, elevation, or a classified value derived through image processing. A digital image is stored as a raster and may contain one or more bands.

data

Any collection of related facts arranged in a particular format, often, the basic elements of information that are produced, stored or processed by a computer.

DEM

Digital Elevation Model. The representation of continuous elevation values over a topographic surface by a regular array of z-values, referenced to a common datum. DEMs are typically used to represent terrain relief.

FTP

File Transfer Protocol. A protocol that allows the transmission of files between computers over a network.

NoData

In raster data, the absence of a recorded value. NoData does not equate to a zero value. While the measure of a particular attribute in a cell may be zero, NoData value indicates that no measurements have been taken for that cell at all.

Pyramid

In raster datasets, a reduced resolution layer that copies the original data in decreasing levels of resolution to enhance performance. The coarsest level of resolution is used to quickly draw the entire dataset. As the display zooms in, layers with finer resolutions are drawn; drawing speed is maintained because fewer pixels are needed to represent the successively smaller areas.

metadata

Information that describes the content, quality, condition, origin, and other characteristics of data or other pieces of information. Metadata for spatial data may describe and document its subject matter, how, when, where, and by whom the data was collected, availability and distribution information, its projection, scale, resolution and accuracy, and its reliability with regard to some standard. Metadata consists of properties and documentation. Properties are derived from the data source (for example, the coordinate system and projection of the data), while documentation is entered by a person (for example, keywords used to describe the data).

attribute

Nonspatial information about a geographic feature in a GIS, usually stored in the attribute table and linked to the feature by a unique identifier. For example, attributes of a river might include its name, length, and sediment load at a gauging station. 2. In raster datasets, information associated with each unique value of a raster cell. 3. Information that specifies how features are displayed and lableled on a map, for example, the graphic attributes of a river might include line thickness, line length, color, and font for labeling.

database

One or more structured sets of persistent data, managed and stored as a unit and generally associated with software to update and query the data. A simple database might be a single file with many records, each of which references the same set of fields. A GIS database includes the data about the spatial locations and shapes of geographic features recorded as points, lines, areas, pixels, grid cells or TINS, as well as their attributes.

hillshading

Shadows drawn on a map to simulate the effect of the sun's rays over the varied terrain of the land. 2. The hypothetical illumination of a surface according to a specified azimuth and altitude for the sun. Hillshading creates a three-dimensional effect that provides a sense of visual relief for cartography, and a relative measure of incident light for analysis.

File name extension

The abbreviation following the final period in a file name that indicates the file's format, such as .shp, .zip, .tif. File name extensions are usually one to four letters long.

cell size

The dimensions on the ground of a single cell in a raster, measured in map units, Cell size is often used synonymously with pixel size.

File name

The name that distinguishes a file from all other files in a particular directory. It can refer to the name of the file by itself (harold), the name plus the file extension (harold.shp), or the whole path of a file up to and including the file name extension (C:/mygisdata/shapefiles/harold.shp).

byte

The smallest addressable unit of data storage within a computer, almost always equivalent to 8 bits and containing one character.

Bit

The smallest unit of information in a computer. A bit can have one of 2 values, 1 and 0, that can represent on and off, yes and no, or true and false.

cell

The smallest unit of information in raster data, usually square in shape. In a map or GIS data set, each cell represents a portion of the earth, such as a square meter or square mile, and usually has an attribute value associated with it, such as soil type or vegetation class.