Lecture 3 Intro GIS UNM-1920 PDF

Introduction to GIS Map Principles Dr Lawal Billa Introduction to Geographical Information Systems [email protected] School of Geography, University of Nottingham.MC Presentation outline Coordinate system Parallels and Meridians Spatial Re...

Introduction to GIS Map Principles Dr Lawal Billa Introduction to Geographical Information Systems [email protected] School of Geography, University of Nottingham.MC Presentation outline Coordinate system Parallels and Meridians Spatial Referencing Rectangular Coordinate Systems Non Coordinate Systems  Coordinate Systems The Earth is a three-dimensional object, and appears to be spherical. Its surface has a constant radius and so any point on it is uniquely identified using a polar two- coordinate system A point's latitude and longitude (usually measured in degrees) define the crossing of a parallel and a meridian - A Latitude means north-to-south angles from a reference parallel - A Longitude means west-to-east angles from a reference meridian. Origins of Latitude and Longitude The arrows in the diagram defines the origins of the coordinate system's. One arrow lies over the north- south geographical axis, while the other is defined by east – west convention. Any point on the surface is located using two angles or coordinates, called the longitude (λ) and latitude (φ). Longitude and Latitude: 3D Schematic Longitude and Latitude in 2D Parallels and Meridians A graticule is a spherical grid of coordinate lines over the planetary surface, It comprises circles on planes normal (perpendicular) to the north- south axis, called parallels (red) and semicircular arcs with that axis as chord, called meridians (blue). Parallels do not cross one another, while all meridians meet at each geographic pole. Every parallel crosses every meridian at an angle of 90°.  Properties of Parallels - Parallels are natural reference - The longest parallel divides the Earth in two equal hemispheres, north and south (Equator) - The geographical north-south axis is actually tilted slightly less than 23.5° from the plane of the Earth's orbit around the sun. - It accounts for the different seasons and different lengths of day and night periods throughout the year  Properties of Meridians All points on a meridian have the same solar or local time. The world is divided in 24 time zones, each 15° wide. Every point inside a time zone is considered having the same standard time Time-change boundaries seldom follow the meridians, bending at national or regional borderlines) to keep related places conveniently in the same time The Prime meridian was fixed in 1884 at Greenwich, near London The opposite prime meridian is the international date line, which separates world in to day and night  SPATIAL REFERENCING A system used to locate features on the surface of the Earth, or a 2D representation of this surface The system should be: Consistent and stable Able to show points, lines and polygons Able to allow measurement of length, area and shape GIS uses three types of spatial referencing: Geographic coordinate systems Rectangular coordinate systems Non coordinate systems  Geographic Coordinate Systems Earth assumed to be spherical Sphere divided into lines of latitude (north- south) and longitude (east-west) Equator = 0º N/S Greenwich Meridian = 0º E/W Angles drawn from point at centre of sphere to intersect with point on surface provide angular reference: Kuala Lumpur: 3º 08’ 19.67”N, 101º 41’ 08.71” E UNMC: 2o 56’ 41.16”N, 101o 52’ 26.48”E Introduction to Geographic Information Systems School of Geography, University of Nottingham Issues of: Geographic Spatial Referencing Assumes the Earth to be spherical Distances between lines of latitude and longitude get less at poles – issues of precision Lines of latitude and longitude on projected maps are distorted – may be highly irregular: Distortion of longitude/ latitude in Bonne’s projection Introduction to Geographic Information Systems School of Geography, University of Nottingham  Rectangular Coordinate Systems (Cartesian Grid) Regular, Cartesian grid laid over a projection of Earth’s surface Grid lines will be consistent and regular across map surface, unlike Lat /Long However, if grid is applied over too big an area the difference between grid space and geographic space varies greatly across area So, rectangular coordinate systems are only applied to small areas (e.g. British National Grid). Dr Nick Mount Introduction to Geographic Information Systems [email protected] School of Geography, University of Nottingham  British National Grid (Cartesian Grid) Ordnance Survey map references of Great Britain , usually : [2 Grid letters] [3 figures(easting)] [3 figures(northing)] eg NA, 123:456 or [6 figures(easting)] , [6 figures(northing)] eg. NA123456 O S map reference is based division of series of 500km squares from Southwest corner of the country. Each 500km squares is allocated a single reference letter (S, T, N, H or O). Each 500km square is again subdivided into 25 squares. Each of the 100km squares is allocated a reference letter (A to Z, omitting I, starting with A in the north-west corner of the parent 500km square). Introduction to Geographic Information Systems Dr Nick Mount School of Geography, University of Nottingham [email protected]  Advantages of : Rectangular Coordinates - Cartesian system makes it easy to undertake spatial computations (distance, directions etc.) Location data can be given in metres x,y coordinates match well with the x,y coordinates on a computer screen It is especially useful in Geographic Information Systems software Dr Nick Mount Introduction to Geographic Information Systems [email protected] School of Geography, University of Nottingham  Non Coordinate Systems Provide spatial reference by a given descriptive code This Code can be translated to a coordinate system In UK, the most commonly used Code is postcode: Dr Nick Mount Introduction to Geographic Information Systems [email protected] School of Geography, University of Nottingham  Non-coordinate systems Postcodes are hierarchical Postcode region: e.g. NG Kuala Lumpur: 50xxx-60xxx Postcode district: e.g. NG7 Putrajaya : 62xxx Postcode sector: e.g. NG7 2 Sarawak : 93xxx- 98xxx Postcode units: e.g. NG7 2RD Sabah : 88xxx- 91xxx Postcodes identify locations of increasingly small areas but NOT points Only useful for entities assigned a postcode So, limited to certain types of analysis / study Conversion of postcode to centroid point is often required Dr Nick Mount Introduction to Geographic Information Systems [email protected] School of Geography, University of Nottingham  Spatial Referencing - Summary There are a number of methods for referencing space Geographical systems provide a global coverage Rectangular coordinates easier to work with and are used for smaller regions (e.g. national grids) Non coordinate systems (e.g. postcodes) useful in certain, limited situations and still need translating to a national or global referencing system Dr Nick Mount Introduction to Geographic Information Systems [email protected] School of Geography, University of Nottingham