1 Projections & GIS Data Collection: An Overview Projections Primary data capture Secondary data capture Data transfer Capturing attribute data Managing a data capture project
2 Geodesy Basics for Geospatial Data 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.
5 The History of Ellipsoids 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 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
6 Projections and Coordinates There are many reasons for wanting to project the Earth s surface onto a plane, rather than deal with the curved surface 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
7 Need a link between: Geodetic Datums geoid -- ellipsoid -- sphere 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 a geodetic datum is a reference for mapping
8 Geographic Coordinates spherical coordinate system unprojected! expressed in terms of two angles (latitude & longitude) 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 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
9 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.
10 Geographic Coordinates latitude positive in n. hemisphere negative in s. hemisphere longitude positive east of Prime Meridian negative west of Prime Meridian
11 Cartesian Coordinates Computationally, it is much simpler to work with Cartesian coordinates than spherical coordinates x,y coordinates referred to as eastings & northings defined units, e.g. meters, feet common examples: Universal Transverse Mercator: Cartesian coordinate system applicable nearly world-wide Many countries also have Cartesian systems U.S. - State Plane U.K. - Ordnance Survey National Grid
12 Distortions Any projection must distort the Earth in some way Two types of projections are important in GIS 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
13 Cylindrical Projections Conceptualized as the result of wrapping a cylinder of paper around the Earth The Mercator projection is the best-known cylindrical projection The cylinder is wrapped around the Equator The projection is conformal At any point scale is the same in both directions Shape of small features is preserved Features in high latitudes are significantly enlarged
14 Conic Projections Conceptualized as the result of wrapping a cone of paper around the Earth 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
15 The Universal Transverse Mercator (UTM) Projection A type of cylindrical projection Implemented as an internationally standard coordinate system Initially devised as a military standard Uses a system of 60 zones Maximum distortion is 0.04% Transverse Mercator because the cylinder is wrapped around the Poles, not the Equator
16 Universal Transverse Mercator (UTM) 60 zones, each 6 longitude wide zones run from 80 S to 84 N poles covered by Universal Polar System (UPS)
17 Universal Transverse Mercator (UTM) Units: meters Each 6 zone subdivided into North and South zones N and S zones have separate coordinate systems x-origin set 500,000m east of central meridian N zone y-origin: Equator S zone y-origin: 10,000,000m south of Equator
18 State Plane Coordinates Defined in the US by each state Some states use multiple zones Several different types of projections are used by the system Provides less distortion than UTM Preferred for applications needing very high accuracy, such as surveying
19 U.S. State Plane Coordinate System Each U.S. state composed of one or more zones Zones trend predominantly N-S or E-W Each zone has separate coordinate system and appropriate projection
20 Data Collection One of most expensive GIS activities Many diverse sources Two broad types of collection Data capture (direct collection) Data transfer Two broad capture methods Primary (direct measurement) Secondary (indirect derivation)
21 Primary Data Capture Capture specifically for GIS use Raster remote sensing e.g. SPOT and IKONOS satellites and aerial photography Passive and active sensors Resolution is key consideration Spatial Spectral Temporal
22 Typical Reflectance Signatures
23 Vector Primary Data Capture Surveying GPS 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 Collection of satellites used to fix locations on Earth s surface Differential GPS used to improve accuracy
24 Total Station
26 Secondary Geographic Data Capture Data collected for other purposes can be converted for use in GIS Raster conversion Scanning of maps, aerial photographs, documents, etc Important scanning parameters are spatial and spectral (bit depth) resolution
27 Vector Secondary Data Capture Collection of vector objects from maps, photographs, plans, etc. Digitizing Manual (table) Heads-up and vectorization Photogrammetry the science and technology of making measurements from photographs, etc.
29 Tablet Digitizing & Scanning Developing data from analog (paper) maps -- convert information from the analog map into digital form process called digitzing, accomplished using: tablet digitizer -or- scanner Both approaches require good quality source maps free of physical distortion (wrinkling, shrinkage) coordinate information visible on map statement of projection, coordinate units, datum, etc. Tablet digitizing trace map from tablet assign attributes Scanning scan map to create digital picture trace picture on-screen or using vectorization software assign attributes
30 contains fine ( ) mesh of electromagnetically charged wire common grid resolutions & pucks lead to accuracies ranging from.05mm to.25mm. 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). Tablet Digitizer & Software
31 Digitizing Geographic Features Generally digitize one "layer" (set of related features) from the map at a time e.g digitize roads separately from hydrography, etc. each digitized set of features becomes a separate vector data layer in GIS Trace the features from the map using the digitizing puck digitize a single x-y location for point feature digitize a series of points to form a line feature endpoints of lines are nodes points defining shape along lines are vertices digitize a series of lines to form a polygon Feature digitizing issues: coordinate entry mode: point mode -vs. vs.- stream mode common polygon borders treat arcs/lines forming common boundaries as separate entities? or enter common arcs/lines only once? major topological consequences...
32 Automation During Tablet Digitizing 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
33 Scan Digitizing Alternative method for digitizing... sometimes called automatic digitizing.but it isn t necessarily very automatic Equipment scanner "large-format" scanners available as flat-bed or roller scanners scanner "takes picture" of map -- creates a raster image software capabilities to read scanned image display image on screen for "heads-up" digitizing or to do automatic vectorization
34 Factors Affecting Accuracy Source map inherent spatial resolution of source map (dependent on map scale) positional & attribute coding errors present in source map physical condition of map Digitizing or scanning process care with which map is affixed to digitizing tablet (digitizing) 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 effects of generalization, edge matching, rubber sheeting, etc.
35 Vector to Raster Raster spatial resolution finer resolution = better representation of the converted vector data coarser resolution = more information loss! Method used to determine cell values How do we know what is in each cell? We choose: cell center (centroid) majority weighting weighted values based on priority/importance
36 Raster to Vector (Vectorization) Points & polys - relatively simple points: if cell=value, then a vector point is created at cell centroid with attribute=value polygons: polygon with attribute=value is created for all adjoining cells=value; poly boundary follows exterior of cells Lines - more complex must somehow determine: start/end/intersection points (nodes) for lines shape points along lines (vertices) topological relationships
37 Managing Data Capture Projects Key principles Clear plan, adequate resources, appropriate funding, and sufficient time Fundamental tradeoff between Quality, speed and price Two strategies Incremental Blitzkrieg (all at once) Alternative resource options In house Specialist external agency
38 TIGER/Line Files 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
39 Definitions 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
40 Distributed GIS: Outline Introduction Distributing the data The mobile user Distributing the software: GIServices
41 Distributing a GIS The component parts can be at different locations The user The data The software The network links all of the parts together
42 Geolibraries and Geoportals A Geolibrary is a digital library containing georeferenced information Its search mechanism uses geographic location as the primary key A Geoportal is a digital library of geographic data and GIServices A one-stop shop for information relevant to GIS
43 The Mobile User It is increasingly possible to obtain the services of a GIS through hand-held and wearable devices Some cell phones can be used to generate maps Such maps can be centered on the user's current location