SEC201_Introduction to GIS.pdf

SEC 201 Introduction to Geographic Information Systems Ayushi Vijhani, Ph.D. Assistant Professor Department of Natural and Applied Sciences TERI School of Advanced Studies Geographical Concepts Basic geographic concepts are...

SEC 201 Introduction to Geographic Information Systems Ayushi Vijhani, Ph.D. Assistant Professor Department of Natural and Applied Sciences TERI School of Advanced Studies Geographical Concepts Basic geographic concepts are: Location Region Place (physical and cultural attributes) Density, Dispersion, Pattern Spatial Interaction Size and Scale Geographical Concepts Location Location can be described in two ways: absolute and relative. Answers the question of “Where is it?” Absolute describes the position of a feature or event in space, using some form of geographic coordinates. Relative uses descriptive text to describe the position of the feature or event in relationship to another object or event. What is the distance and direction of a place from another? For example, floods hit Sangli, Kolhapur districts of Maharashtra. Understanding of the location, zones affected and those that are at high risk of getting flooded is vital for GIS analysis. Geographical Concepts Region Regions are groupings of geographic information. A region is a geographic area defined by one or more distinctive characteristics. Regions can be based on physical features (such as a watershed), political boundaries (a district, country, or continent) or other categorized geographies. Regions can be formal, functional or perceptual Formal regions are also known as homogenous or uniform region. Entities within a formal region share one or more common traits. Example: residents of a country. Functional region is a region anchored by a focal point. Examples are a customer service area for a restaurant delivery service or the school district for an elementary school. Perceptual region is a geographic area that exists as part of a cultural or ethnic identity and therefore don’t adhere to political or formal regional boundaries. Geographical Concepts Place Place looks at the physical and/or cultural attributes/characteristics of a location Physical characteristics include: weather and temperature, land and soil, and plant and animal life Cultural attributes include: languages, religions and ethnicities, where and how people settle, transportation, economics, and politics Density, Dispersion, Pattern Understanding spatial pattern is an important aspect of geographic inquiry Spatial pattern looks at commonality in geography across regions o How are things arranged? o Is the arrangement regular? o Is there a pattern to the distribution? Geographical Concepts Spatial Interaction Spatial interaction is the cause and effect of an event in one region or area that affects another area and takes a look at the connectivity and relationships of features. For example, High deforestation in the upstream areas of Chamoli district, would possibly result in increasing number of landslides during the rainy season. This will affect downstream communities due to poor road connectivity. Size and scale Geographic features are visualized using a map which is a representation of reality. The size and scale affects the degree of generalization of the features being mapped. A small scale shows a larger geographic area (e.g. a map of the world or of a continent) but shows more generalized features and less detail (e.g. only major highways and major rivers). A large scale map shows a smaller geographic area (e.g. a map of a city or a neighbourhood) but shows a greater amount of detail (e.g. the entire street network and all branches of a river). Projection and Transformation A map projection is the transformation of Earth's curved surface (or a portion of) onto a two-dimensional flat surface by means of mathematical equations. A map projection is the manner in which the spherical surface (three – dimensional) of the Earth is represented on a flat (two-dimensional) surface. This can be accomplished by direct geometric projection or by a mathematically derived transformation. There are many kinds of projections, but all involve transfer of the distinctive global patterns of parallels of latitude and meridians of longitude onto an easily flattened surface, or developable surface. A systematic arrangement of intersecting lines on a plane that represent and has a one to one correspondence to the meridians and parallels on the datum surface. Projection and Transformation Coordinate System Map projections require a point of reference on the Earth’s surface. Most often this is the center, or origin, of the projection. Two coordinate systems: 1. Projected Coordinate System (PCS): Also known as Planar/Cartesian/Grid reference system 2. Geographic Coordinate System (GCS): Projected Coordinate System Planar, or Cartesian, coordinates are defined by a column and row position on a planar grid (X,Y). The origin of a planar coordinate system is typically located south and west of the origin of the projection (natural origin) - at which the ellipsoid and flat map surfaces coincide, at which point the projection formulas generate a coordinate of (0,0). Coordinates increase from (0,0) going east and north. The origin of the projection, being a false origin, is defined by values of false easting and false northing In practice, this eliminates negative coordinate values and allows locations on a map projection to be defined by positive coordinate pairs. Values of false easting are read first and may be in meters or feet. Locations of geographic objects are defined relative to the origin, using the notation (x,y), where x refers to the distance along the horizontal axis, and y refers to the distance along the vertical axis. The origin is defined as (0,0). Projected Coordinate System A value relating to distance north of a standard latitude but with a constant added to make the numbers convenient. False easting: A value applied to the origin of a coordinate system to change the x-coordinate readings. False northing: A value applied to the origin of a coordinate system to change the y-coordinate readings. Projected Coordinate System Example: Universal Transverse Mercator (UTM) Geographic Coordinate System Geographic, or spherical, coordinates are based on the network of latitude and longitude that make up the graticule of the Earth. Within the graticule, lines of longitude are called meridians, which run north/south, with the prime meridian at 0° (Greenwich, England). Meridians are designated as 0° to 180°, east or west of the prime meridian. Lines of latitude are called parallels, which run east/west. Parallels are designated as 0° at the equator to 90° at the poles. The equator is the largest parallel. Unit: The angular measures of longitude and latitude may be expressed in degrees-minutes-seconds (DMS), decimal degrees (DD), or radians (rad). Given that 1 degree equals 60 minutes and 1 minute equals 60 seconds, we can convert between DMS and DD. For example, a latitude value of 45°52´30´´ would be equal to 45.875° (45+52/60+30/3600). One radian equals 57.2958°, and one degree equals 0.01745 rad. Geographic Coordinate System The North Pole has latitude of 900 north and the South Pole has latitude of 900 south. Latitude: The angular distance in degrees, minutes, and seconds of a point north or south of the Equator. Lines of latitude are often referred to as parallels. Longitude: The angular distance in degrees, minutes, and seconds, of a point east or west of the Prime (Greenwich) Meridian. Lines of longitude are often referred to as meridians. Geographic Coordinate System Spherical Coordinate System Spherical coordinates are a set of three numbers that form an ordered triplet and are used to describe a point in the spherical coordinate system. Spherical coordinates use the radial distance, the polar angle, and the azimuthal angle of the orthogonal projection to locate a point in three-dimensional space. Spherical coordinates usually use radians rather than degrees to depict angles related to the position of a point. Here, ρ represents the radial distance between point P and the origin. The spherical coordinates of the origin, O, are (0, 0, 0). θ represents the polar angle and is used to describe the location of P. Let Q be the projection of point P on the xy plane. Then the angle between the line segment drawn to point Q from the origin and the positive x-axis is represented by θ. φ is the angle between the line segment from the origin to P and the positive z-axis. This is also known as the azimuthal angle. Thus, the coordinates of P are given as (ρ,θ,φ). Projection The Earth’s surface (3-D) can be projected onto a paper/computer screen (2-D) in two ways : o Perspective Geometrical Projection : The centre of projection is at a finite distance away from the projection plane (2-D surface). o Non-Perspective Geometrical Projection/Parallel Projection : The centre of projection is at an infinite distance away from the projection plane. Perspective Geometrical Projection Non-Perspective Geometrical Projection Projection S.NO. PERSPECTIVE PROJECTION PARALLEL PROJECTION 1. If Centre Of Projection is located at a finite point If Centre Of Projection is located at infinity, all the in 3 space , the result is a perspective projection. projectors are parallel and the result is a parallel projection. 2. Perspective projection represents objects in a Parallel projection is much like seeing objects through three-dimensional way. a telescope, letting parallel light rays into the eyes which produce visual representations without depth 3. In perspective projection, objects that are far away Parallel projection does not create this effect. appear smaller, and objects that are near appear bigger 4. Perspective projections require a distance between In parallel projection the center of projection is at the viewer and the target point. infinity, while in prospective projection, the center of projection is at a point. 5. Types: Types: 1. One point perspective, 1.Orthographic 2. Two point perspective, 2.Oblique 3. Three point perspective, Types of Projection Projections can be categorized as follows according to the surface used for transferring the graticule: Azimuthal Projections: In polar aspect, an azimuthal projection maps to a plane tangent to the Earth at one of the poles, with meridians projected as straight lines radiating from the pole, and parallels shown as complete circles centered at the pole Conical Projections: A conic projection is derived from the projection of the globe onto a cone placed over it. For the normal aspect, the apex of the cone lies on the polar axis of the Earth Cylindrical Projections: The map projection is the image of the globe projected onto the cylindrical surface, which is then unwrapped into a flat surface. When the cylinder aligns with the polar axis, parallels appear as horizontal lines and meridians as vertical lines Types of Projection Azimuthal Projections Conical Projections Cylindrical Projections Projection Challenges Regardless of what type of projection is used, it is inevitable that some error or distortion occurs in transforming a spherical surface into a flat surface. Ideally, a distortion-free map has four valuable properties: Conformality/ Orthomorphic/ true shape Equivalence/ Homolography/ equal area Equidistance is the characteristic of true distance Azimuthal/true direction/true bearing The preserved property of a map projection is often included in its name. Example: Lambert conformal conic projection (LCC), or the Albers equal-area conic projection. Azimuthal Projections Azimuthal projections may be centered: On the poles (polar aspect): When the plane surface is a tangent at one of the poles. At a point on the equator (equatorial aspect): When the plane surface tangent at any point on the equator. At any other orientation (oblique aspect): When the plane surface is a tangent at any other point. Azimuthal Projections The light can be placed at the centre of the globe, or at any point of the equator, or at any point outside the globe. The origin of the projection lines—that is, the perspective point—may also assume various positions. For example, it may be: o When the light at center of the Earth (gnomonic) o When the light at an infinite distance away (orthographic) o When the light is at any point on the Earth’s surface, opposite the projection plane (stereographic) Conical Projections Conical projections are accomplished by intersecting, or touching, a cone with the global surface and mathematically projecting lines onto this developable surface. A tangent cone intersects the global surface to form a circle. Along this line of intersection, the map is error-free and possess equidistance. Usually, this line is a parallel, termed the standard parallel (SP). Cones may also be secant, and intersect the global surface, forming two circles that possess equidistance. In this case, the cone slices underneath the global surface, between the standard parallels. Cylindrical Projections Cylindrical projections are accomplished by intersecting, or touching, a cylinder with the global surface. The surface is mathematically projected onto the cylinder, which is then cut and unrolled. A tangent cylinder intersects the global surface on only one line to form a circle. This central line of the projection is commonly the equator and possesses equidistance. A secant cylinder, one slightly less in diameter than the globe, has two lines possessing equidistance. If the cylinder is rotated 900 from the vertical (i.e., the long axis becomes horizontal), then the aspect becomes transverse, wherein the central line of the projection becomes a chosen standard meridian as opposed to a standard parallel. Choosing a Map Projection Selecting a map projection for the GIS database enables you to (Maling, 1992): o Decide how to best display the area of interest or illustrate the results of analysis o Register all imagery to a single coordinate system for easier comparisons o Test the accuracy of the information and perform measurements on the data Deciding Factors depending on your applications and the uses for the maps created, one or several map projections may be used. Many factors must be weighed when selecting a projection, including: o Type of map : depending on the type of surface o Special properties that must be preserved : area, distance, direction o Map accuracy o Scale Choosing a Map Projection Since the sixteenth century, there have been three fundamental rules regarding map projection use (Maling, 1992): o If the country to be mapped lies in the tropics, use a cylindrical projection o If the country to be mapped lies in the temperate latitudes, use a conical projection o If the map is required to show one of the polar regions, use an azimuthal projection Map Projection When transferring the image of the earth and its irregularities on the plane surface of the map, three factors are involved. They are: Geoid: It is an imaginary sea level surface that undulates (has a wavy surface) over all of the earth; it isn't just for the oceanic areas, it also extends through the land masses. Ellipsoid: Geoids are then transferred to a regular geometric reference surface Projection: Geographical relationship of the ellipsoid (3D) transformed into 2D plane of a map Datum Mathematical Model that describes the shape of the ellipsoid, position, direction, and scale relationships of a reference surface to positions on the surface of Earth. o Horizontal datum consists of the longitude and latitude on the surface of the Earth o Vertical datum consists of land elevations/water heights/depths. Datums are the basis for coordinate systems. It can be described as a reference mapping surface. Defines the size and shape of the Earth and the origin and orientation of the coordinate system used. There are datums for different parts of the earth based on different measurements. Large diversity of datums due to high precision of GPS. Assigning the wrong datums to a coordinate system may result in errors of hundreds of meters. Examples of Local datums : European Datum, the Australian Geodetic Datum, the Tokyo Datum, and the Indian Datum (for India and several adjacent countries- Everest 1830), North American Datum of 1983 (NAD83) Example of Global datum: WGS84 Spheroid/ Ellipsoid Calculation of a map projection requires definition of the spheroid (or ellipsoid) in terms of the length of axes and eccentricity squared (or radius of the reference sphere). Several principal spheroids are in use by one or more countries. Differences are primarily due to calculation of the spheroid for a particular region of the Earth’s surface. Only recently satellite tracking data provided spheroid determinations for the entire Earth. However, these spheroids may not give the best fit for a particular region. In North America, the spheroid in use is the Clarke 1866 for NAD27 and GRS 1980 for NAD83 (State Plane). Spheroid/ Ellipsoid The direct geometric map projections assumes that the Earth is a sphere, and for many maps this is satisfactory. However, due to rotation of the Earth around its axis, the planet bulges slightly at the equator. This flattening of the sphere makes it an oblate spheroid, which is an ellipse rotated around its shorter axis. An ellipse is defined by its semi-major (long) and semi-minor (short) axes. The amount of flattening (f) of the Earth is expressed as the ratio: f = (a – b ) / a. Where: a = the equatorial radius (semi-major axis) b = the polar radius (semi-minor axis) Most map projections use eccentricity (e2) rather than flattening. The relationship is: e2 = 2 f – f 2 The more flattened the ellipse is, the greater the value of its eccentricity. The more circular, the smaller the value or closer to zero is the eccentricity. The eccentricity ranges between one and zero. If the eccentricity is one, it will be a straight line and if it is zero, it will be a perfect circle. Types of Ellipsoids Best-Fit Ellipsoid/Local Ellipsoid (Local datum): based on the measurements within a region, so it best fits that region only. The centre of such reference ellipsoid does not coincide with the centre of gravity of the Earth. Example : Everest Ellipsoid Geocentric Ellipsoid (Global datum): The centre of geocentric ellipsoid coincides with the centre of the Earth. This type of the ellipsoid can be used worldwide. Example : WGS 84 ellipsoid Introduction to Maps A map is a representation of all or part of the Earth drawn on a flat surface at a specific scale. Cartography is the study and practice of making maps and one who make maps is called a cartographer. It is a visual representation of an area – a symbolic depiction highlighting relationships between elements of that space such as objects, regions, and themes. A generalized view of an area, usually some portion of Earth’s surface, as seen from above at a greatly reduced size Any geographical image of the environment A two-dimensional representation of the spatial distribution of selected phenomena Why make a map? To represent a larger area than we can see To show a phenomenon or process we To show spatial relationships can’t see with our eyes To present information concisely What do cartographers think about? How will the flat map represent a curved surface? (projection) Maps are selective views of reality (simplification) Size of the map relative to reality (scale) What’s on the map (symbolization) What size of unit will be measured by the map (aggregation) What type of map is being used? (reference or thematic) Elements of a map Most maps contain the same common elements: 1. data (or map) frame 2. map legend 3. map title 4. north arrow 5. map scale bar 6. metadata (or map citation) 7. border (or neatline) 8. inset (or locator) map Evaluation criteria for maps Southworth and Southworth (1982), for example, list these design characteristics of successful maps: A map should be suited to the needs of its users. A map should be easy to use. Maps should be accurate, presenting information without error, distortions, or misrepresentation. The language of the map should relate to the elements or qualities represented. A map should be clear and attractive. Many maps would ideally permit interaction with the user, allowing change, updating, or personalization. Types of maps According to the ICSM (Intergovernmental Committee on Surveying and Mapping), there are five different types of maps: 1. General Reference, 2. Topographical, 3. Thematic, 4. Navigation Charts and 5. Cadastral Maps and Plans Topographical Cadastral Maps and Plans General Reference Types of maps General Purpose Maps (Basemaps or Reference maps): They display natural and man-made features of general interest, and are intended for widespread public use. General Reference maps often enlarge some features to aid users. For example, road maps show roads boldly and may use road widths and colour to distinguish between major and minor roads. As a general rule, General Reference Maps would only show relief (the difference in height between features on the map) in a stylized manner. Basemap Street and tourist maps are good examples of general reference maps. Types of maps Thematic Maps are sometimes also called special purpose, single topic, or statistical maps. They highlight features, data, or concepts, and these data may be qualitative, quantitative, or both. Thematic maps can be further divided into two main categories: qualitative and quantitative. Qualitative thematic maps show the spatial extent of categorical, or nominal, data (e.g., soil type, land cover, political districts). Quantitative thematic maps, conversely, demonstrate the spatial patterns of numerical data (e.g., income, age, population). Types of maps Cartometric Maps are a more specialized type of map and are designed for making accurate measurements. Cartometrics, or cartometric analysis, refers to mathematical operations such as counting, measuring, and estimating—thus, cartometric maps are maps which are optimized for these purposes. Examples include aeronautical and nautical navigational charts—used for routing over land or sea—and USGS topographic maps, which are often used for tasks requiring accurate distance calculations, such as surveying, hiking, and resource management. Types of maps Topographical maps They show elevation using contour lines (a line which joins points of equal elevation above sea level). They have an emphasis on showing human settlement (roads, cities, buildings etc), but may include some thematic information such as vegetation or the boundaries of national parks. They are typically produced by government agencies for civilian or defense purpose. They have well defined standards (called Specifications) which are strictly adhered to – these vary between mapping agencies and the scale of the map. They have very good location reference systems – including latitude and longitude, but may also have grid lines. Types of maps Navigational Charts Navigational charts are another invaluable tool when it comes to actually getting around, whether you’re at sea or in the air. The charts tend to include information that’s important to avoiding accidents – such as features in and around the water, like submerged rocks – along with any specific navigational aids. Map Scale Ratio of the distance on a map to that on the ground between the same two points, in the same units. Map has fixed scale but GIS is scale-less, i.e. data may be enlarged or reduced to any size. However, if we go too far from the scale at which map was made before it was captured into the GIS, problems of scale appear. 𝑴𝒂𝒑 𝑫𝒊𝒔𝒕𝒂𝒏𝒄𝒆 Scale = 𝑮𝒓𝒐𝒖𝒏𝒅 𝑫𝒊𝒔𝒕𝒂𝒏𝒄𝒆 o Fraction-Representative fraction (RF) is the ratio of distances on the map to the same distance on the ground. E.g., 1 : 50000 or 1/50,000 o Verbal-Statement scale expresses the ratio in words. for 1 : 1000000 : “One millimeter to one kilometer” o Graphic-Bar scale is a linear graphical scale drawn on the map Map Scale The map scale is printed in the map legend. It is given as a ratio of inches on the map corresponding to inches, feet, or miles on the ground. For example, a map scale indicating a ratio of 1:24,000 (in/in), means that for every 1 inch on the map, 24,000 inches have been covered on the ground. Ground distances on maps are usually given in feet or miles. Map Scale Examples: 1. Convert the map scale of 1:24,000 (in/in) to (in/ft). On the map 1 inch is equal to 2,000 feet on the ground, 1:2,000 (in/ft). 2. Convert the 1:2,000 (in/ft) to (in/mile). On the map 1 inch is equal to 0.4 miles. 3. The map distance between two points is 6 inches. The map scale is 1:24,000 (in/in). What is the ground distance in feet? The ground distance is 12,000 feet.