# GIS

## Terms

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- Ratio
- Data with known intervals between values that are based on a meaningful zero value, Population densities
- Choropleth
- A map that applies shading symbols to data or statistics collected for enumeration units such as counties or states
- Azimuthal
- Retains certain accurate directions, Uses a plane as the projection surface
- Sources of spatial data
- Existing: Federal, State, Local, Commercial. Create it yourself
- Suitability analysis
- Analyzing an area to see whether it is good to do whatever you want to do there
- Marginalia
- Title, Legend, Scale, Compass, Explanations
- Sources of uncertainty/error
- Spatial and informational, Derived sources (maps, secondary?), Measuring device, Bias/subjectivity, Sampling, Spatial autocorrelation (like values spatially close to each other), Interpolation (estimation)
- Datum
- Point of reference used to measure locations on the surface of the earth, Defines the origin and orientation of the lines of latitude and longitude, Always related to a spheroid
- Normalization
- The process of taking a table with all the attribute data and breaking it down to small tables while maintaining the necessary linkages between them in a relational database
- Equidistant
- Maintains consistency of scale for certain distances
- Decision support
- A computer program that includes data presentation and modeling tools that help people understand problems and find solutions
- Interval
- Data with known intervals between values, Temperature readings
- Components of GIS
- Hardware, Software, People, Data and Infrastructure
- Conformal
- Preserves local shapes
- Equivalent
- Represents areas in correct relative size
- Cylindrical
- Uses a cylinder as the projection surface
- Service Area
- Type of network analysis for determining the region that encompasses all streets that lie within a specified impedance, For example, the 20-minute service area for a fire station includes all the streets that can be reached within 20 minutes from that location.
- State Plane Coordinate System
- A coordinate system developed in the 1930s to permanently record original land survey monument locations in the United States. Most states have more than one zone based on the SPC27 or SPC83 system
- Nominal
- Data that shows different kinds or different categories, Land-use types or soil types
- Raster data model
- A spatial data model that uses a grid and cells to represent the spatial variation of a feature, Represent continuous features, Simple data structure with rows and columns and fixed cell locations
- Layout
- The arrangement and composition of various map elements, Focus, order and balance
- Visual Hierarchy
- The process of developing a visual plan to introduce the 3-D effect or depth to maps, Placing elements at different visual levels according to their importance to the map's purpose
- Dasymetric
- A map that uses statistics and additional information to delineate areas of homogenous values, rather than following administrative boundaries
- Buffer
- A GIS operation that creates zones consisting of areas within a specified distance of select features
- Other Considerations in Making a Map
- Purpose, Audience
- Polygon overlay
- A GIS operation that combines the geometries and attributes of the input layers to create the output
- Projected Coordinate System
- A plane coordinate system that is based on a map projection
- Ordinal
- Data that are ranked, Large, medium and small cities
- Geographic Coordinate System
- A location reference system for spatial features on the Earth's surface, Defined by longitude and latitude
- Descriptive models
- A model that describes the existing conditions of spatial data
- Earth-centered
- 1983 North American Datum (NAD83): The origin is very close to the earth's center of mass, Most commonly used datum for North America
- Isarithmic
- A map that uses a system of isolines to represent a surface
- Methods of data entry
- Convert data, Create data: Scanning existing maps, Digitizing, Tablet, Heads-up (softcopy), Image Processing, GPS, Surveying, Geocoding
- Model
- A simplified representation of a phenomenon or a system
- Lambert conformal conic
- A common map projection, Basis for the State Plane Coordinate system for many states
- Natural breaks
- A method of manual data classification that seeks to partition data into classes based on natural groups in the data distribution, Occur in the histogram at the low points of valleys, Breaks are assigned in the order of the size of the valleys, with the largest valley being assigned the first natural break
- Polygon-on-polygon overlay
- A GIS operation in which the output combines the polygon boundaries from the inputs to create a new set of polygons and each new polygon carries attributes from the inputs
- Standard deviation
- A data classification method that finds the mean value, then places class breaks above and below the mean at intervals, Values that are beyond three standard deviations from the mean are aggregated into two classes, greater than three standard deviations above the mean and less than three standard deviations below the mean
- Shortest path
- A network analysis approach that finds the path with the minimum cumulative impedance between nodes on a network
- Boolean
- A keyword such as AND, OR, XOR, or NOT that is used to construct compound expressions
- Vector data model
- A spatial data model that uses points and their x-,y-coordinates to construct spatial features of points, lines and areas, Ideal for representing discrete features: Points, Lines, Areas. May be georelational or object-based, May or may not involve topology, Nay include simple or composite features
- Equal interval
- A data classification method that divides a set of attribute values into groups that contain an equal range of values
- Histogram
- Bar chart that groups data into equal intervals, Uses bars to show the number or frequency of values falling within each class
- Quantile
- A data classification method that distributes a set of values into groups that contain an equal number of values
- Point-in-polygon overlay
- A GIS operation in which each point of a point layer is assigned the attribute data of the polygon within which it falls
- DEM
- A digital model with an array of uniformly spaced elevation data in raster format
- Transverse Mercator
- A common map projection, Basis for the Universal Transverse Mercator grid system and the State Plane Coordinate system, Central meridian is 90°W and the latitude of true scale is the equator
- Prescriptive models
- A model that offers a prediction of what the conditions of spatial data could be or should be
- Local
- Aligned so that it closely corresponds to the earth's surface for a particular area, 1927 North American Datum (NAD27): Uses Meades Ranch, Kansas as the point of origin for all measurements
- Location/Allocation analysis
- Solves problems of matching the supply and demand by using sets of objectives and constraints
- Metadata
- Data that provide information about geospatial data
- Conic
- Uses a cone as the projection surface