GIS 1  Midterm
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Coverage
 the oldest vector format, developed for Arc/Info.
 contain multiple feature classes, which may store points, arcs, polygons, and polygon labels.
 also store topology, and the tables have several attribute fields reserved for this purpose.
 Do not use Windows to copy or delete coverages, shapefiles, and geodatabases. Always use ArcCatalog to delete or copy spatial data sets.

Aspatial data
Data that is not or only incidentally tied to a point on the Earth’s surface

Nodes
endpoints of the line

vertex
intermediate points between nodes

polygon
a group of vertices that define a closed area

feature class
 features grouped together into data sets
 only 1 kind of geometry per data set
 can include point features, line features, or polygon features but never a combination

attributes
 information stored about them, such as their names or populations
 This information is stored in a table

Feature ID (FlD) or ObjectlD (OID)
links the spatial data with the attributes

thematic mapping
one example of how linked attributes can be used to analyze geographic information

feature datasets
made up of multiple data classes all being related to each other in some way

spaghetti model
stores features of the file as independent objects, unrelated to each other

examples of topological features
 adjacency
 connectivity
 intersection
 overlap

Logical consistency
evaluates whether a data model or data set accurately represents the realworld relationships between features

large scale map
 a map in which the outcome will be large and the denominator is small
 Ex: a campus map is larger scale than a city map

Thematic accuracy
Accuracy of attributes associated with the map

Spatial resolution
At what distance interval measurements are taken or recorded. What is the size of a single pixel of satellite data?

Temporal resolution
How frequently measurements are taken

Precision
number of significant digits used to record a measurement or the statistical variation of a repeated single measurement

Steps for planning a drawing
 Determine the objectives of the map
 Decide on the data layers to be included.
 Plan the layout
 Choose colors and symbols.
 Create the map

Four properties of map features may be distorted by projections:
 area
 direction
 distance
 shape

Datum
a combination of an earth ellipsoid and a reference point to reduce mapping discrepancies

Map extent
the range of xy values currently displayed in the data frame. Zooming in reduces the map extent; zooming out enlarges it.

3 Ways to scale
 Automatic: image adjusts based on data frame
 Fixed scale: defined scale
 Fixed content: adjusts the scale but locks in the extent of the map

3 types of databases
 Flat file
 Hierarchical: multiple files with fixed relationships
 Relational: multiple files with flexible relationships

SQL
Structured Query Language

key field
The tables are combined using a common field called a key. The key field must be of the same data type in both tables.

cardinality of a relationship
 Dictates whether the tables can be joined.
 The Rule of Joining stipulates that there must be one and only one record in the source table for each record in the destination table.
 The destination table is the point of reference and comes first; that is, the relationship cardinality is reported as {destination} to {source}.

a geographic coordinate system, or GCS
Latitude and longitude system

3 types of map projections
 Azimuthal (flat)
 Cylindrical
 Conic

UTM
 Universal Transversal Mercator
 a secant transverse cylindrical projection
 distortion is negligible within a single zone
 convenient because users need only know the zone number and hemisphere
 has 60 northsouth zones, each 6 degrees wide
 World wide used
 Unit: meters
 UTM Zones: e.g. 14N

State Plane Coordinate System (SPCS)
 Includes an assortment of coordinate systems developed in the 1930s
 distortion is negligible within a single zone
 uses all 3 projections, depending on the zone and its orientation

spatial reference includes
 Coordinate system
 X/Y domain: range of xy coordinates
 Resolution: accuracy in coordinate values

Project tool
 Defines xy coordinates system, by adding a new feature and keeping the original data
 Should be used only with correct coordinate system data

Define projection
 Changes only the coordinate system labels
 Use only on data set with Unknown coordinate system

Gnomonic
light source is at the center of the globe

Stereographic
light source is at the point exactly opposite the point of tangency

Orthographic
at a considerable distance (infinite point). Light rays are parallel.

Projections Commonly Used in the US
 Albers Equalarea Conic
 Lambert Conformal Conic
 Transverse Mercator

Albers Equalarea Conic
 equalarea conic projection
 Well suited for large countries that mainly eastwest in extent
 Maximum scale error is approximately 1.25% over an area the size of the US.

Lambert (Azimuthal) Conformal Conic
 Map is conformal, but not perspective, equalarea, or equidistant
 Distances are true only along standard lines and reasonably accurate elsewhere in limited regions
 Directions are reasonably accurate, and distortion of shapes and areas minimal at the standard lines

Transverse Mercator
 A horizontal cylinder
 Intersects the ellipsoid along a single northsouth tangent or two secant lines
 Has a band of low distortion, runs in a northsouth direction

Rectangle coordinate systems
 Universal Transversal Mercator
 State Plane

State Plane
 Used in US
 Each state in US are divided into several zones
 Devised by the US Coast and Geodetic Survey
 Unit: foot
 FIPS as zone number: e.g. Fips 4021
 Zone name: e.g. Louisiana South Zone

Reference systems
 Geoid: shape and size of earth
 GCS: Geographic Coordinate System

GCS
 Geographic Coordinate System
 Small circles: latitude circles other than the equator
 Grid circles permit to identify the shortest distance

Datum defined by:
 Coordinate system defines geographic system
 Ellipsoidal system approximates Earth’s shape
 Horizontal measures locations on earth

Geoid
 Threedimensional surface where the gravity is constant
 Difference in the density of the Earth cause variation in the gravity force
 Can be thought of as the level of an imaginary sea
 Why do we need Geoid?
 Elevation is typically the vertical distance to Geoid, also called orthometric height
 Height above the ellipsoid is referred to as an ellipsoid height
 Geoidal height: orthometric height – ellipsoid height not a mathematically defined surface. It is measured (e.g. using gravimeters) and interpolated.

datum
a set of parameters defining a coordinate system, and a set of control points whose geometric relationships are known, either through measurement or calculation

ArcGIS vector data formats
 Coverages
 Shapefiles
 Geodatabases

Encoding Grid Cell Value
 Centroid method: Each cell is assigned the value of the feature that passes through the center of the cell.
 Predominant type (winner takes all): Each cell is assigned the value of the feature that fills the majority of the cell.
 Most important type: Each cell is assigned the value associated with the features that have been specified as more important to the study.
 Edge separated: The ambiguity is solved by marking those cells as Edges

WhittakerShannon Sampling Theorem
 the cell size must be smaller than half of the minimum feature (minimum map units) that you intend to represent.
 The commonly suggested cell size is 1/5  1/7 of the minimum feature to be captured

Bands
A set of matrices of cells that represent multiple attributes of the same area

Data depth
 Known as pixel/bit depth
 How many bits are used to represent one pixel value
 Affects the data range (integer) and precision (floating point numbers)

NoData
 Where the phenomenon does not occur
 Do not confuse with value 0
 Use some specific values to represent

Advantages of Vector Model
 Spatial objects are represented based on precise x, y coordinates, and therefore measurements of area, perimeter and distance, and graphic representation are more accurate and precise.
 Data structure is more compact and less redundant, and thus less demanding for data storage.
 Besides geometric properties, topological relationships between spatial objects can be explicitly encoded and stored.
 Support a wide variety of advanced, topologybased analyses and well suited for representing and modeling linear features and network, such as, address geocoding, pathtracing, pavement management, bus routing, emergency response planning, pipeline planning, sales analysis and wildlife management.
 Encoded topological relationships facilitate error checking in vector database.
 Easy to do visual overlay analysis. Multiple vector layers can be overlaid together, or draped on top of raster data.

Limitations of Vector Model
 Complex data structure, and timeintensive data acquisition and input.
 Computationally intensive and complicated for some spatial operations, such as overlay, calculation of area, neighborhood analysis, etc.
 Not suitable for representing a gradual change (transition zone) between adjacent units. Many physical characteristics such as soil and vegetation types vary and have ‘fuzzy’ borders.
 Not suitable for representing continuous surface like terrain. Surface metric properties, like slope aspects, curvature, cannot be easily calculated from contour representation.
 Incompatible with digital image data. Manipulation and enhancement of remote sensing data are difficult in a vectorbased GIS system

Advantages of Raster Model
 Simple and straightforward data structuresmatrixlike 2D array. The easiest format to be dealt with Fortran, C, and other computer languages.
 Not only support the discrete (categorical) objects but also continuous geographical features. Highly varying surface like terrain can be effectively and efficiently represented in a raster format.
 Computationally efficient in some types of quantitative analysis: map overlay, map algebra, surface modeling and simulation, such as cutfill analysis, visibility, watershed modeling, slope and aspect calculation, and threedimensional display.
 Compatible to remotely sensed data and photogrammetric data. Traditional digital image processing techniques can be introduced for the manipulations of cellbased raster data.
 Compatible to modern high speed graphic input and output devices.

Disadvantages of Raster Model
 Unable to explicitly representing the topological relations, therefore does NOT support network type of analysis.
 Data redundancy in homogenous areas and corresponding large volume of data.
 Limited accuracy of location and corresponding area and distance measurements. The resolution and accuracy depends on the size of the grid cells.
 The output of graphics is less aesthetically pleasing because irregular lines and boundaries tend to be a blocky, jagged, staircase like appearance rather than the smooth lines.

Triangulated Irregular Network (TIN)
 The major problem of vector data structure is the representation of continuous surface model such as elevation, precipitation, etc.
 TIN is a masspoint dataset with lines linking the points and constructing triangles

Data frames
Boxes containing layers to be viewed and analyzed together

Jenks Natural Breaks
 The “best” classification method
 Exploits natural gaps in the data
 Good for unevenly distributed or skewed data
 Default method, works well for most data sets
 Can be very slow if the data has many values

Graticule
 Spherical grid or geographic grid
 the network of parallels and meridians on the earth’s surface

Standard line
 line of true scale
 The line along which projection surface touches or intersects the globe are called standard lines
 There is one standard line when a planar surface intersects the globe, or a cone or cylinder is tangential to the globe.
 There are two standard lines when a cone or cylinder intersects the globe.
 Along the standard line, the map has no distortion, and the map scale is identical to the nominal globe.
 In general, geometric distortion increases with the distance to the standard lines.

Azimuthal (direction)
 A straight line drawn between the central point (standard point) to all other points shows the true greatcircle route and azimuthal direction from the central point to all other points.
 Directions from points other than the central point (standard) to other points are not accurate.

The primary use of the map and desired geometric properties to be preserved
 Equalarea cylindrical projections are often used to show the worldwide distribution of a variety of geographic phenomena.
 Presentation maps are usually conformal projections, although compromise and equalarea projections can also be used.
 Navigational maps are true direction and/or equidistant.

The locations of the area to be mapped
 Azimuthal projection is often used to map polar region
 Conic projection is often used to map midlatitude regions;
 Cylindrical projection is often used to map equatorial region.

Predominant orientation of the area to be mapped
 Albers Equalarea Conic, Lambert Conformal Conic are often used to map the countries that are mainly eastwest in extent.
 Transverse Mercator is used to map the states that are mainly northsouth in extent
 Azimuthal projection is better for mapping the area with a circular shape.

The Extent of the area to be mapped
 As the mapped area increases to subcontinental, distortion becomes a significant problem.
 As mapped areas become smaller in extent, the selection of the projection becomes less critical. Potential scale errors begin to drop off considerably.

Projections Commonly Used in the US
 Albers Equalarea Conic
 Lambert Conformal Conic
 Transverse Mercator

Albers Equalarea Conic
 This is an equalarea conic projection having two standard parallels, but not conformal, perspective, or equidistant.
 Well suited for large countries that mainly eastwest in extent.
 USGS uses 29.5N and 45.5N as the standard parallels to map the conterminous US. Maximum scale error is approximately 1.25% over an area the size of the US.
 Meridians are straight lines that intersect parallels at right angle. Parallels are concentric circles, making construction relatively easy.

Lambert Conformal Conic
 Map is conformal, but not perspective, equalarea, or equidistant.
 Distances are true only along standard lines and reasonably accurate elsewhere in limited regions.
 Directions are reasonably accurate, and distortion of shapes and areas minimal at the standard lines.
 Also used to show other countries or regions that is mainly eastwest in extent.

Transverse Mercator
 A horizontal cylinder
 Intersects the ellipsoid along a single northsouth tangent or two secant lines
 Has a band of low distortion, runs in a northsouth direction