3.5. GIS Data Models - Hill Agric
3.5.GIS Data Models:3.5.1.Spatial Data ModelsTraditionally spatial data has been stored and presented in the form of a map. Three basic types ofspatial data models have evolved for storing geographic data digitally. These are referred to as : Vector Raster ImageThe following diagram reflects the two primary spatial data encoding techniques. These are vector andraster. Image data utilizes techniques very similar to raster data, however typically lacks theinternal formats required for analysis and modeling of the data. Images reflects pictures or photographsof the landscape.3.5.2.Vector Data FormatsAll spatial data models are approaches for storing the spatial location of geographic features in adatabase. Vector storage implies the use of vectors (directional lines) to represent a geographic feature.Vector data is characterized by the use of sequential points or vertices to define a linear segment. Eachvertex consists of an X coordinate and a Y coordinate.Vector lines are often referred to as arcs and consist of a string of vertices terminated by a node. Anode is defined as a vertex that starts or ends an arc segment. Point features are defined by onecoordinate pair, a vertex. Polygonal features are defined by a set of closed coordinate pairs. In vectorrepresentation, the storage of the vertices for each feature is important, as well as the connectivitybetween features, e.g. the sharing of common vertices where features connect.Point representations21
Points are defined as single coordinate pairs (x, y) when we work in 2D or coordinate triplets (x, y, z)when we work in 3D. Points are used to represent objects that are best described as shape- and sizeless,single-locality features. Whether this is the case really depends on the purposes of the spatialapplication and also on the spatial extent of the objects compared to the scale applied in theapplication. For a tourist city map, parks will not usually be considered as point features, but perhapsmuseums will be, and certainly public phone booths could be represented as point features.Besides the georeference, usually extra data is stored for each point object. This so-called administrativeor thematic data, can capture anything that is considered relevant about the object. For phone boothobjects, this may include the owning telephone company, the phone number, the data last serviced etcetera.Line representationsLine data are used to represent one-dimensional objects such as roads, railroads, canals, rivers andpower lines. Again, there is an issue of relevance for the application and the scale that the applicationrequires. For the example application of mapping tourist information, bus, subway and streetcar routesare likely to be relevant line features. Some cadastral systems, on the other hand, may consider roads tobe two-dimensional features, i.e., having a width as well.The two end nodes and zero or more internal nodes define a line. Another word for internal node isvertex (plural: vertices); another phrase for line that is used in some GISs is polyline, arc or edge. A nodeor vertex is like a point (as discussed above) but it only serves to define the line; it has no specialmeaning to the application other than that.The vertices of a line help to shape it, and to obtain a better approximation of the actual feature. Thestraight parts of a line between two consecutive vertices or end nodes are called line segments. ManyGISs store a line as a simple sequence of coordinates of its end nodes and vertices, assuming that all itssegments are straight. This is usually good enough, as cases in which a single straight line segment isconsidered an unsatisfactory representation can be dealt with by using multiple (smaller) line segmentsinstead of only one.Still, there are cases in which we would like to have the opportunity to use arbitrary curvilinear featuresas representation of real-world phenomena. Think of garden design with perfect circular or ellipticallawns, or of detailed topographic maps representing roundabouts and the annex sidewalks. All of thiscan be had in GIS in principle, but many systems do not at present accommodate such shapes. If a GISsupports some of these curvilinear features, it does so using parameterized mathematical descriptions.22
Collections of (connected) lines may represent phenomena that are best viewed as networks. Withnetworks, specific type of interesting questions arise, that have to do with connectivity and networkcapacity. Such issues come up in traffic monitoring, watershed management and other applicationdomains. With network elements—i.e., the lines that make up the network—extra values are commonlyassociated like distance, quality of the link, or carrying capacity.Area representationsWhen area objects are stored using a vector approach, the usual technique is to apply a boundarymodel. This means that each area feature is represented by some arc/node structure that determines apolygon as the area's boundary. Common sense dictates that area features of the same kind are beststored in a single data layer, represented by mutually non-overlapping polygons. In essence, what wethen get is an application-determined (i.e., adaptive) partition of space, similar to, but not quite like anirregular tessellation of the raster approach.Observe that a polygon representation for an area object is yet another example of a finiteapproximation of a phenomenon that inherently may have a curvilinear boundary. In the case that theobject can be perceived as having a fuzzy boundary, a polygon is an even worse approximation, thoughpotentially the only one possible.A simple but naive representation of area features would be to list for each polygon simply the list oflines that describes its boundary. Each line in the list would, as before, be a sequence that starts with anode and ends with one, possibly with vertices in between. But this is far from optimal.3.5.3.Raster Data FormatsRaster data models incorporate the use of a grid-cell data structure where the geographic area isdivided into cells identified by row and column. This data structure is commonly called raster. While the23
term raster implies a regularly spaced grid other tessellated data structures do exist in grid basedGIS systems.The size of cells in a tessellated data structure is selected on the basis of the data accuracy and theresolution needed by the user. There is no explicit coding of geographic coordinates required sincethat is implicit in the layout of the cells. A raster data structure is in fact a matrix where any coordinatecan be quickly calculated if the origin point is known, and the size of the grid cells is known. Since gridcells can be handled as two-dimensional arrays in computer encoding many analytical operations areeasy to program. This makes tessellated data structures a popular choice for many GIS software.Topology is not a relevant concept with tessellated structures since adjacency and connectivity areimplicit in the location of a particular cell in the data matrix.Most raster based GIS software requires that the raster cell contain only a single discrete value.Accordingly, a data layer, e.g. forest inventory stands, may be broken down into a series of raster maps,each representing an attribute type, e.g. a species map, a height map, a density map, etc. These areoften referred to as one attribute maps. This is in contrast to most conventional vector data models thatmaintain data as multiple attribute maps, e.g. forest inventory polygons linked to a database tablecontaining all attributes as columns. This basic distinction of raster data storage provides thefoundation for quantitative analysis techniques. This is often referred to as raster or map algebra. Theuse of raster data structures allow for sophisticated mathematical modelling processes while vectorbased systems are often constrained by the capabilities and language of a relational DBMS.Real world road3.5.4.Vector Representation as LineRaster RepresentationAttribute Data ModelsA separate data model is used to store and maintain attribute data for GIS software. These datamodels may exist internally within the GIS software, or may be reflected in external commercial24
Database Management Software (DBMS). A variety of different data models exist for the storage andmanagement of attribute data. The Relational Model is the most commonly used data model.The relational database organizes data in tables. Each table, is identified by a unique table name, and isorganized by rows and columns. Each column within a table also has a unique name. Columns store thevalues for a specific attribute, e.g. cover group, tree height. Rows represent one record in the table. In aGIS each row is usually linked to a separate spatial feature, e.g. a forestry stand. Accordingly, eachrow would be comprised of several columns, each column containing a specific value for thatgeographic feature. The following figure presents a sample table for forest inventory features. This tablehas 4 rows and 5 columns. The forest stand number would be the label for the spatial feature as well asthe primary key for the database table. This serves as the linkage between the spatial definition of thefeature and the attribute data for the feature.Data is often stored in several tables. Tables can be joined or referenced to each other by commoncolumns (relational fields). Usually the common column is an identification number for a selectedgeographic feature, e.g. a forestry stand polygon number. This identification number acts as theprimary key for the table. The ability to join tables through use of a common column is the essence ofthe relational model. Such relational joins are usually ad hoc in nature and form the basis of for queryingin a relational GIS product. Unlike the other previously discussed database types, relationships areimplicit in the character of the data as opposed to explicit characteristics of the database set up.3.6.General spatial topologyTopology deals with spatial properties that do not change under certain transformations. Theserelationships are invariant under a continuous transformation. Such properties are called topologicalproperties. There are a number of advantages when our computer representations of geographicphenomena have built-in sensitivity of topological issues. Questions related to the 'neighbourhood' ofan area are a point in case. To obtain some 'topological sensitivity' simple building blocks have beenproposed with which more complicated representations can be constructed.We can use the topological properties of interior and boundary to define relationships between spatialfeatures. Since the properties of interior and boundary do not change under topological mappings, wecan investigate their possible relations between spatial features.Suppose we consider a spatial region A. It has a boundary and an interior, both seen as (infinite) sets ofpoints, and which are denoted by boundary(A) and interior (A), respectively. We consider all possiblecombinations of intersections (n) between the boundary and the interior of A with those of another25
region B, and test whether they are the empty set (Ø) or not. From these intersection patterns, we canderive eight (mutually exclusive) spatial relationships between two regions. If, for instance, the interiorsof A and B do not intersect, but their boundaries do, yet a boundary of one does not intersect theinterior of the other, we say that A and B meet. These can be graphically represented as:3.7.Map ProjectionsMaps of the world or large areas are often either 'political' or 'physical'. The most important purpose ofthe political map is to show territorial borders; the purpose of the physical is to show features ofgeography such as mountains, soil type or land use including infrastructures such as roads, railroads andbuildings. Topographic maps show elevations and relief with contour lines or shading. Geological mapsshow not only the physical surface, but characteristics of the underlying rock, fault lines, and subsurfacestructures.Maps that depict the surface of the Earth also use a projection, a way of translating the threedimensional real surface of the geoid to a two-dimensional picture. Perhaps the best-known world-mapprojection is the Mercator projection, originally designed as a form of nautical chart.Aeroplane pilots use aeronautical charts based on a Lambert conformal conic projection, in which a coneis laid over the section of the earth to be mapped. The cone intersects the sphere (the earth) at one ortwo parallels which are chosen as standard lines. This allows the pilots to plot a great-circle routeapproximation on a flat, two-dimensional chart. Azimuthal or Gnomonic map projections are often usedin planning air routes due to their ability to represent great circles as straight lines.3.7.1.Map projectionA map projection is any method of representing the surface of a sphere or other three-dimensional bodyon a plane. Map projections are necessary for creating maps. All map projections distort the surface insome fashion. Depending on the purpose of the map, some distortions are acceptable and others are26
not; therefore different map projections exist in order to preserve some properties of the sphere-likebody at the expense of other properties. There is no limit to the number of possible map projections.3.7.2.Geographical Coordinate System (GCS)A geographic coordinate system is a coordinate system that enables every location on the Earth to bespecified by a set of numbers. The coordinates are often chosen such that one of the numbers representvertical position, and two or three of the numbers represent horizontal position. A common choice ofcoordinates is latitude, longitude and elevation.A geographical coordinate system (GCS) uses a three dimensional spherical surface to define locationson the earth. A point is referenced by its longitude and latitude values. Longitude and latitude are anglesmeasured from the earth’s center to a point on the earth’s surface. The angles often are measured indegreesThe geographic latitude (abbreviation: Lat., φ, or phi) of a point on the Earth's surface is the anglebetween the equatorial plane and a line that passes through that point and is normal to the surface of areference ellipsoid which approximates the shape of the Earth. This line passes a few kilometers awayfrom the center of the Earth except at the poles and the equator where it passes through Earth's center.Lines joining points of the same latitude trace circles on the surface of the Earth called parallels, as theyare parallel to the equator and to each other. The north pole is 90 N; the south pole is 90 S. The 0 parallel of latitude is designated the equator, the fundamental plane of all geographic coordinatesystems. The equator divides the globe into Northern and Southern Hemispheres.The Longitude (abbreviation: Long., λ, or lambda) of a point on the Earth's surface is the angle east orwest from a reference meridian to another meridian that passes through that point. All meridians arehalves of great ellipses (often improperly called great circles), which converge at the north and southpoles.A line passing near the Royal Observatory, Greenwich (near London in the UK) has been chosen as theinternational zero-longitude reference line, the Prime Meridian. Places to the east are in the easternhemisphere, and places to the west are in the western hemisphere. The antipodal meridian ofGreenwich is both 180 W and 180 E. The zero/zero point is located in the Gulf of Guinea about 625 kmsouth of Tema, Ghana.27
Representation of Earth3.7.3.Latitude and LongitudeSpheroids, Spheres & DatumWhile a spheroid approximates the shape of the earth, a datum defines the position of the spheroidrelative to the center of the earth. A datum provides a frame of reference for measuring locations onthe surface of the earth. It defines the origin and orientation of latitude and longitude lines.Sphere and spherioid3.7.4.DatumGeodetic datumA geodetic datum (plural datums, not data) is a reference from which measurements are made. Insurveying, a datum is a set of reference points on the Earth's surface against which positionmeasurements are made, and (often) an associated model of the shape of the earth (reference ellipsoid)to define a geographic coordinate system. Horizontal datums are used for describing a point on theearth's surface, in latitude and longitude or another coordinate system. Vertical datums measureelevations or depths.A reference datum is a known and constant surface which is used to describe the location of unknownpoints on the earth. Since reference datums can have different radii and different center points, aspecific point on the earth can have substantially different coordinates depending on the datum used tomake the measurement. There are hundreds of locally-developed reference datums around the world,28
usually referenced to some convenient local reference point. Contemporary datums, based onincreasingly accurate measurements of the shape of the earth, are intended to cover larger areas.Geodetic datum defines the size and shape of the earth, and the origin and orientation of the coordinatesystems used to map the earth. Reference surface used for map projections. Several models of geodeticdatums in use: eg. Everest ellipsoid, WGS84Indian Standard is EVEREST ELLIPSOID. World Standard is WGS84 ELLIPSOID3.7.5.The Universal Transverse Mercator (UTM)The Universal Transverse Mercator (UTM) geographic coordinate system uses a 2-dimensional Cartesiancoordinate system to give locations on the surface of the Earth. It is a horizontal position representation,i.e. it is used to identify locations on the earth independently of vertical position, but differs from thetraditional method of latitude and longitude in several respects.29
The UTM system is not a single map projection. The system instead divides the Earth into sixty zones,each a six-degree band of longitude, and uses a secant transverse Mercator projection in each zone.3.7.6.World Geodetic SystemThe World Geodetic System is a standard for use in cartography, geodesy, and navigation. It comprises astandard coordinate frame for the Earth, a standard spheroidal reference surface (the datum orreference ellipsoid) for raw altitude data, and a gravitational equipotential surface (the geoid) thatdefines the nominal sea level. WGS 84 is the reference coordinate system used by the Global PositioningSystem.3.8.Projected Coordinate SystemsA Projected Coordinate System is defined on a flat, two-dimensional surface. It has constant lengths,angles, and areas across the two dimensions. It is always based on a geographic coordinate system thatis based on a sphere or spheroid.Map projections can lead to distortions.Conic Projection The most simple conic projection is tangent to the globe along a line of latitude. This line iscalled the standard parallel. The meridians are projected onto the conical surface, meeting atthe apex, or point, of the cone. Parallel lines of latitude are projected onto the cone as rings.30
The cone is then ‘cut’ along any meridian to produce the final conic projection, which hasstraight converging lines for meridians and concentric circular arcs for parallels. The meridianopposite the cut line becomes the central meridian.Cylindrical projections Meridians are geometrically projected onto the cylindrical surface, and parallels aremathematically projected. This produces graticular angles of 90 degrees. The cylinder is ‘cut’along any meridian to produce the final cylindrical projection. The meridians are equally spaced,while the spacing between parallel lines of latitude increases toward the polesPlanar projectionsPlanar projections project map data onto a flat surface touching the globe. A planar projection is alsoknown as an azimuthal projection or a zenithal projection.31
The Gnomonic projection views the surface data from the center of the earth, The Stereographic projection views it from pole to pole. The Orthographic projection views the earth from an infinite point, as if from deep spaceBEHRMANN EQUAL AREA CYLINDRICAL projectionShape:Shape distortion is minimized near the standard parallels (30N and 30S).Area:Area is maintained.Direction:Directions are generally distorted.Distance:Directions are generally distorted except along the equator.USES AND APPLICATIONS: Only useful for world maps32
GIS Data Models: 3.5.1. Spatial Data Models Traditionally spatial data has been stored and presented in the form of a map. Three basic types of spatial data models have evolved for storing geographic data digitally. These are referred to as : Vector Raster Image The following diagram reflects the two primary spatial data encoding .
1 CHAPTER 1 INTRODUCTION 1.1 GIS? 1.1.1 Components of a GIS 1.1.2 A Brief History of GIS 1.1.3 GIS Software Products Box 1.1 A List of GIS Software Producers and Their Main Products 1.2 GIS Applications Box 1.2 Google Maps, Microsoft Virtual Earth, and
GIS Data Models. GIS Data Models. Spatial Information is usually modeled in one of two ways: . GIS Data Conceptual Model. GIS Data Model: Vector vs. Raster. Raster model Data is represented as a surface modeled by a matrix of values (pixels) Useful for "continuous" data (ie
Background –Chris Owen . 2004 - MACECOM 911 hires GIS to provide them road and addressing data 2005 / 2006 - new GIS Technicians and Analysts hired 2007 - GIS was moved from Public Works Road Fund and made an "Enterprise Fund" 2008 / 2009 - GIS Manager quits. GIS Manager position is not rehired.
tarikh tarikh . penghargaan . 2.4 kriteria penentuan lokasi rumah kos rendah bab 3.0 aplikasi gis dalam perancangan 3.1 pengenalan 3.2 gis dalam perancangan 3.3 gis untuk perumahan 3.4 peranan sistem maklumat gis 3.5 sejarah pembangunan gis 3.6 definisi gis 3.7 pangkalan data ii ill vi vi vi 1-1 1-1 1.2 1-3 1-4
MIT 11.188/11.520 Web Service Notes 1 Internet GIS and Geospatial Web Services Introduction Section 1 -- What is Internet GIS? Section 2 -- Internet GIS: state of practice Section 3 -- Future development of Internet GIS Section 4 -- Function comparisons of current Internet GIS programs Section 5 -- Internet GIS applications Section 6 – I
desktop GIS, remote sensing software and 3D visualization tools). Only summarized descriptions for the rest of open source GIS software have been provided due to the white paper page limits. 2.1 Basic desktop GIS Basic desktop GIS software can provide basic GIS functions, such as data input, map display
2G1/3G4 GIS TUTORIAL General informaion. What is GIS? The acronym GIS stands for Geographic Information Systems. GIS refers to one of the several software platforms for the capturing, storage, retrieval, analysis and display of geographic spatial data. GIS assigns abstract statistical information to physical geographic elements.
What is a GIS? A GIS is a tool for making and using spatial information. Among the many defini-tions of GIS, we choose: A GIS is a computer-based system to aid in the collection, maintenance, storage, analysis, output, and distribution of spa-tial data and information. When used wisely, GIS can help us live healthier, wealthier, and safer lives.
