- Количество слайдов: 71
Projections & GIS Data Collection: An Overview n n n Projections Primary data capture Secondary data capture Data transfer Capturing attribute data Managing a data capture project
Geodesy Basics for Geospatial Data n n Geodesy: The study of the Earth’s size and shape. or, more formally: ”A branch of applied mathematics which determines by observation and measurement the exact positions of points and the figures and areas of large portions of Earth's surface, the shape and size of the Earth, and the variations of terrestrial gravity. ”
Ellipsoid (Spheroid) n major axis q n half axis: semi-major axis (a) minor axis q half axis: semi-minor axis (b)
The History of Ellipsoids n n Because the Earth is not shaped precisely as an ellipsoid, initially each country felt free to adopt its own as the most accurate approximation to its own part of the Earth Today an international standard has been adopted known as WGS 84 q q q Its US implementation is the North American Datum of 1983 (NAD 83) Many US maps and data sets still use the North American Datum of 1927 (NAD 27) Differences can be as much as 200 m
Projections and Coordinates n There are many reasons for wanting to project the Earth’s surface onto a plane, rather than deal with the curved surface q q q The paper used to output GIS maps is flat Flat maps are scanned and digitized to create GIS databases Rasters are flat, it’s impossible to create a raster on a curved surface The Earth has to be projected to see all of it at once It’s much easier to measure distance on a plane
Geodetic Datums n Need a link between: q geoid -- ellipsoid -- sphere n How do we know where locations referenced in the geographic coordinate system are relative to the ellipsoid and geoid? Geodetic datums provide this link Datum defined: any numerical or geometric quantity which serves as a reference or base for other quantities q a geodetic datum is a reference for mapping n n
Geodetic Control Network n n n Horizontal datumhas connections from origin to other points network of these points at surveyed locations: geodetic control network UNC -Chapel Hill - main quad
Datum Info on a Map n Must have this information in order to utilize geospatial data n Why?
Latitude and Longitude n The most comprehensive and powerful method of georeferencing q n Metric, standard, stable, unique Uses a well-defined and fixed reference frame q Based on the Earth’s rotation and center of mass, and the Greenwich Meridian
Definition of Latitude n n Requires a model of the Earth’s shape The Earth is somewhat elliptical q q q The N-S diameter is roughly 1/300 less than the E -W diameter More accurately modeled as an ellipsoid than a sphere An ellipsoid is formed by rotating an ellipse about its shorter axis (the Earth’s axis in this case)
Geographic Coordinates n n spherical coordinate system unprojected! n expressed in terms of two angles (latitude & longitude) n longitude: angle formed by a line going from the intersection of the prime meridian and the equator to the center of the earth, and a second line from the center of the earth to the point in question n latitude: angle formed by a line from the equator toward the center of the earth, and a second line perpendicular to the reference ellipsoid at the point in question
Definition of longitude. The Earth is seen here from above the North Pole, looking along the Axis, with the Equator forming the outer circle. The location of Greenwich defines the Prime Meridian. The longitude of the point at the center of the red cross is determined by drawing a plane through it and the axis, and measuring the angle between this plane and the Prime Meridian.
Geographic Coordinates n latitude q q n positive in n. hemisphere negative in s. hemisphere longitude q q positive east of Prime Meridian negative west of Prime Meridian
Cartesian Coordinates n Computationally, it is much simpler to work with Cartesian coordinates than spherical coordinates q q q x, y coordinates referred to as “eastings” & “northings” defined units, e. g. meters, feet n common examples: n Universal Transverse Mercator: q n Cartesian coordinate system applicable nearly world-wide Many countries also have Cartesian systems… q q U. S. - State Plane U. K. - Ordnance Survey National Grid
Distortions n n Any projection must distort the Earth in some way Two types of projections are important in GIS q q q Conformal property: Shapes of small features are preserved: anywhere on the projection the distortion is the same in all directions Equal area property: Shapes are distorted, but features have the correct area Both types of projections will generally distort distances
Cylindrical Projections n n Conceptualized as the result of wrapping a cylinder of paper around the Earth The Mercator projection is the best-known cylindrical projection q q The cylinder is wrapped around the Equator The projection is conformal n n n At any point scale is the same in both directions Shape of small features is preserved Features in high latitudes are significantly enlarged
Conic Projections n Conceptualized as the result of wrapping a cone of paper around the Earth q q q Standard Parallels occur where the cone intersects the Earth The Lambert Conformal Conic projection is commonly used to map North America On this projection lines of latitude appear as arcs of circles, and lines of longitude are straight lines radiating from the North Pole
The Universal Transverse Mercator (UTM) Projection n n A type of cylindrical projection Implemented as an internationally standard coordinate system q n Uses a system of 60 zones q n Initially devised as a military standard Maximum distortion is 0. 04% Transverse Mercator because the cylinder is wrapped around the Poles, not the Equator
Universal Transverse Mercator (UTM) n n n 60 zones, each 6° longitude wide zones run from 80° S to 84° N poles covered by Universal Polar System (UPS)
Universal Transverse Mercator (UTM) n n Transverse Mercator projection applied to each 6° zone to minimize distortion
Universal Transverse Mercator (UTM) n Units: meters n Each 6° zone subdivided into North and South zones N and S zones have separate coordinate systems n n x-origin set 500, 000 m east of central meridian n N zone y-origin: Equator S zone y-origin: 10, 000 m south of Equator n
State Plane Coordinates n Defined in the US by each state q q n Some states use multiple zones Several different types of projections are used by the system Provides less distortion than UTM q Preferred for applications needing very high accuracy, such as surveying
U. S. State Plane Coordinate System n Each U. S. state composed of one or more zones n Zones trend predominantly N-S or E-W n Each zone has separate coordinate system and appropriate projection
Data Collection n One of most expensive GIS activities Many diverse sources Two broad types of collection q q n Data capture (direct collection) Data transfer Two broad capture methods q q Primary (direct measurement) Secondary (indirect derivation)
Data Collection Techniques Raster Vector Secondary Digital remote sensing images GPS measurements Digital aerial photographs Primary Survey measurements Scanned maps Topographic surveys DEMs from maps Data sets from atlases
Stages in Data Collection Projects Planning Evaluation Editing / Improvement Preparation Digitizing / Transfer
Primary Data Capture n n Capture specifically for GIS use Raster – remote sensing q q n e. g. SPOT and IKONOS satellites and aerial photography Passive and active sensors Resolution is key consideration q q q Spatial Spectral Temporal
Typical Reflectance Signatures
Vector Primary Data Capture n Surveying q q q n Locations of objects determines by angle and distance measurements from known locations Uses expensive field equipment and crews Most accurate method for large scale, small areas GPS q q Collection of satellites used to fix locations on Earth’s surface Differential GPS used to improve accuracy
Secondary Geographic Data Capture n n Data collected for other purposes can be converted for use in GIS Raster conversion q q Scanning of maps, aerial photographs, documents, etc Important scanning parameters are spatial and spectral (bit depth) resolution
Vector Secondary Data Capture n n Collection of vector objects from maps, photographs, plans, etc. Digitizing q q n Manual (table) Heads-up and vectorization Photogrammetry – the science and technology of making measurements from photographs, etc.
Tablet Digitizing & Scanning n Developing data from analog (paper) maps -q q n Both approaches require good quality source maps q q q n free of physical distortion (wrinkling, shrinkage) coordinate information visible on map statement of projection, coordinate units, datum, etc. Tablet digitizing q q n convert information from the analog map into digital form process called digitzing, accomplished using: n tablet digitizer -or- scanner “trace” map from tablet assign attributes Scanning q q q scan map to create digital “picture” trace picture on-screen or using vectorization software assign attributes
Tablet Digitizer & Software n n n contains fine (. 01” -. 001”) mesh of electromagnetically charged wire common grid resolutions & pucks lead to accuracies ranging from. 05 mm to. 25 mm. Puck- recognizes position on tablet relative to wire mesh. records coordinates of location tablet in “digitizer units” (e. g. inches, mm). Digitizing software accepts coordinate information from digitizer& converts from digitizer coordinates to map coordinates. assembles digitized coordinates into geographic data objects (points, lines, polys).
Digitizing Geographic Features n n Generally digitize one "layer" (set of related features) from the map at a time q e. g digitize roads separately from hydrography, etc. q each digitized set of features becomes a separate vector data layer in GIS “Trace” the features from the map using the digitizing puck q digitize a single x-y location for point feature q digitize a series of points to form a line feature n n endpoints of lines are nodes points defining shape along lines are vertices digitize a series of lines to form a polygon Feature digitizing issues: q coordinate entry mode: “point” mode -vs. - “stream” mode q common polygon borders n n n treat arcs/lines forming common boundaries as separate entities? or enter common arcs/lines only once? major topological consequences. . .
Automation During Tablet Digitizing n n n n Digitizing is tedious and error prone… Software can “help” by automating certain steps during digitizing increase efficiency and reduce error Examples: node snapping - automatically join ends of lines (nodes) together if they fall within a specified distance tolerance node-line snapping - automatically join end of one line (a node) to an existing line if the node falls within a specified distance of the existing line intersection detection - automatically detect when two lines cross and create a node at the intersection point
Scan Digitizing n Alternative method for digitizing. . . sometimes called “automatic digitizing” …. but it isn’t necessarily very “automatic” n Equipment q q scanner n "large-format" scanners available as flat-bed or roller scanners n scanner "takes picture" of map -- creates a raster image software n capabilities to read scanned image n display image on screen for "heads-up" digitizing or to do automatic vectorization
Factors Affecting Accuracy n Source map q inherent spatial resolution of source map (dependent on map scale) q q n positional & attribute coding errors present in source map physical condition of map Digitizing or scanning process q care with which map is affixed to digitizing tablet (digitizing) q q q n accuracy of coordinate registration from tablet coordinates (digitizing) or image coordinates (scanning) to real-world coordinates operator error while digitizing, or while vectorizing scanned image operator error while assigning attribute codes to digitized/scanned spatial data features Post-processing q effects of generalization, edge matching, rubber sheeting, etc.
Vector to Raster spatial resolution q finer resolution = better representation of the converted vector data q coarser resolution = more information loss! Method used to determine cell values How do we know what is “in” each cell? We choose: n cell center (centroid) n majority weighting n weighted values based on priority/importance
Raster to Vector (Vectorization) Points & polys - relatively simple q points: if cell=value, then a vector point is created at cell centroid with attribute=value q polygons: polygon with attribute=value is created for all adjoining cells=value; poly boundary follows exterior of cells Lines - more complex q must somehow determine: n n n start/end/intersection points (nodes) for lines shape points along lines (vertices) topological relationships
Raster to Vector n information loss in result:
Mismatches of Adjacent Spatial Data Sources
Managing Data Capture Projects n Key principles q n Fundamental tradeoff between q n Quality, speed and price Two strategies q q n Clear plan, adequate resources, appropriate funding, and sufficient time Incremental ‘Blitzkrieg’ (all at once) Alternative resource options q q In house Specialist external agency
TIGER/Line Files n n n Nominal scale: 1: 100, 000 Data "layers": Enumeration units - blocks, block groups, tracts/block numbering areas, counties, cities/MA, etc. ; multiple hierarchies Voting districts; used for Congressional redistricting; Supporting geography roads/streets/highways basic hydrography point & area landmarks TIGER designed to: support pre-census functions in preparation for Census of Population and Housing supports census-taking efforts evaluate success of the Census provide geographic framework for analysis of Census data
TIGER Polygon & Landmark Data n n Point and poly landmarks Census geography (tracts, blocks, etc. ) used for reporting Census data q ID linkage from polygons in TIGER/Line data to Census attribute data
TIGER Line & Address Data n n n Linear features. . . Form polygon boundaries Roads q n attributes include basic road type, address ranges also hydro features, etc.
Definitions n n n Database – an integrated set of data on a particular subject Geographic (=spatial) database - database containing geographic data of a particular subject for a particular area Database Management System (DBMS) – software to create, maintain and access databases
Advantages of Databases over Files n n n Avoids redundancy and duplication Reduces data maintenance costs Applications are separated from the data q q n n Applications persist over time Support multiple concurrent applications Better data sharing Security and standards can be defined and enforced
Disadvantages of Databases n n Expense Complexity Performance – especially complex data types Integration with other systems can be difficult
Distributed GIS: Outline n n Introduction Distributing the data The mobile user Distributing the software: GIServices
Distributing a GIS n The component parts can be at different locations q q q n The user The data The software The network links all of the parts together
The Role of Standards n Distributed GIS relies on the adoption of common standards q q To allow the various components to operate together Such standards have been developed by various national and international bodies, aided by the Open Geospatial Consortium
Distributing the Data n It must be possible to find remotely located data q q Data documentation, or metadata, provides the key to successful search The U. S. Federal Geographic Data Committee devised a much-emulated standard for geographic data description n The Content Standard for Digital Geospatial Metadata
Major Features of FGDC Metadata 1. 2. 3. 4. 5. Identification Information: basic information about the data set Data Quality Information: a general assessment of the quality of the data set Spatial Data Organization Information: the mechanism used to represent spatial information in the data set Spatial Reference Information: the description of the reference frame for, and the means to encode, coordinates in the data set Entity and Attribute Information: details about the information content of the data set, including the entity types, their attributes, and the domains from which attribute values may be assigned 6. 7. 8. 9. 10. Distribution Information: information about the distributor of and options for obtaining the data set Metadata Reference Information: information on the currentness of the metadata information, and the responsible party Citation Information: the recommended reference to be used for the data set Time Period Information: information about the date and time of an event Contact Information: identity of, and means to communicate with, person(s) and organization(s) associated with the data set
Geolibraries and Geoportals n A Geolibrary is a digital library containing georeferenced information q n Its search mechanism uses geographic location as the primary key A Geoportal is a digital library of geographic data and GIServices q A one-stop shop for information relevant to GIS
The Mobile User n It is increasingly possible to obtain the services of a GIS through hand-held and wearable devices q Some cell phones can be used to generate maps n Such maps can be centered on the user's current location
Map showing Wi. Fi hotspots in the area surrounding the user's current location (the White House, 1600 Pennsylvania Avenue NW, Washington DC)