L3 - Coordinate Systems PDF

Coordinate Systems A diagram of a globe Description automatically generated 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. = latitude and longitude. Latitude...

Coordinate Systems A diagram of a globe Description automatically generated 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. = latitude and longitude. Latitude and longitude form a geographic coordinate system. **Latitude** specifies the north-south position of a point on Earth, while **longitude** defines its east-west position. Lines of latitude are parallel to each other and are horizontal. Lines of longitude are vertical. These lines are not parallel, so they are furthest apart at the equator and closest near the poles. Lines of latitude and longitude form a model called a **graticule**. A point\'s latitude and longitude (usually measured in degrees) define the crossing of a parallel and a meridian = where the two crosses is the coordinate. \- A Latitude means north-to-south angles from a reference parallel. - A Longitude means west-to-east angles from a reference meridian. A diagram of a compass Description automatically generated The arrows in the diagram defines the origins of the 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). Latitude Latitude (ϕ or phi) specifies the north-south position of a point on the Earth's surface. Lines of latitude encircle the Earth, running parallel to the equator, so they are also called *parallels*. Lines of latitude run from 0° to 90° (North or South). Parallels are natural reference - The longest parallel divides the Earth in two equal hemispheres, north and south (Equator). - The geographical north-south axis is 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 Lines of latitude  The lines of latitude line up with the ecliptic at the equinox, but are 23.5° off at the solstice. (Peter Mercator, CC 3.0) Technically, there are many lines of latitude or parallels that divide the Earth. There is a total of 180° of latitude, but not all of the lines neatly fall onto degrees. There are 7 main lines of latitude: - Equator (0°) - [Tropic of Cancer](https://sciencenotes.org/tropic-of-cancer-and-tropic-of-capricorn/) (23.5° north) - Tropic of Capricorn (23.5° south) - Arctic circle (66.5° north) - Antarctic circle (66.5° south) - North Pole (90° north) - South Pole (90° south) There are 5 main lines of latitude if you omit the North Pole and South Pole, which are really points rather than lines. The lines of latitude line up with the ecliptic (the plane containing the Sun and Earth) at the vernal and autumnal [equinox](https://sciencenotes.org/sun-rises-sets-fastest-equinox/). But, the Earth is tilted on its axis. The lines are tilted 23.5° with respect to the ecliptic on the summer and winter [solstice](https://sciencenotes.org/solstice-fast-facts/). Longitude Longitude (λ or lambda) specifies the east-west position of a point on the Earth's surface. Each line of longitude runs from pole to pole and is called a meridian. The Prime Meridian passes near the Royal Observatory in Greenwich, England and has a defined value of 0°. Longitude runs 180° east and 180° west. In addition to location, longitude also defines Greenwich Mean Time. - 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 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 cross every meridian at an angle of 90°. Spatial Referencing A spatial reference system is a system used to define the locations of objects on the Earth\'s surface, including the coordinate system (such as latitude and longitude) and the datum (a mathematical model of the Earth). Spatial referencing is a method used to define the position of objects in space, often in relation to a specific coordinate system. Different spatial reference systems can be used to represent the same location on the Earth\'s surface, but they may use different coordinate systems or datums, which can affect the accuracy of the location. Example: WGS 84: This is a widely used spatial reference system that is based on the World Geodetic System 1984 (WGS 84) datum. It is used by GPS devices and many online mapping services, such as Google Maps and Bing Maps. 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 1. Geographic coordinate systems - Earth assumed to be spherical - Sphere divided into lines of latitude (northsouth) 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 to provide angular reference: Kuala Lumpur: 3º 08' 19.67"N, 101º 41' 08.71" E (N = latitude or parallel and E = longitude or meridian) 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:  2. Rectangular coordinate systems A map of the united kingdom Description automatically generated - A two-dimensional (or three-dimensional) system for locating points in space using perpendicular axes. - Regular, Cartesian grid laid over a projection of Earth's surface. - In two dimensions, there are two axes: the x-axis (horizontal) and the y-axis (vertical). (x,y) - In three dimensions, a third axis, the z-axis, is added, which typically represents depth. (x,y,z). - 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) Example: Ordnance Survey map references of Great Britain - Usually: \[2 Grid letters\] \[3 figures(easting)\] \[3 figures(northing)\] e.g. NA, 123:456 or \[6 figures(easting)\], \[6 figures(northing)\] e.g. 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). - 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 3. Non- coordinates systems - Provide spatial reference by a given descriptive code - This Code can be translated to a coordinate system - In UK, the most used Code is postcode Example: Postcodes are hierarchical British Postcode Malaysia Postcode ------------------- ------------------ --------------------------- Postcode region NG Kuala Lumpur: 50xxx-60xxx Postcode district NG7 Putrajaya : 62xxx Postcode sector NG7 2 Sarawak : 93xxx- 98xxx Postcode units NG7 2RD Sabah : 88xxx- 91xxx Advantages and disadvantages - 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 Summary - There are several methods for referencing space  - Geographical systems provide a global coverage - Rectangular coordinates are 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