Map Projection. Overview. Projection: An Introduction. LA561 Computer Applications in LA (GIS)

Transcription

1 LA561 Computer Applications in LA (GIS) Map Projection Dr. Ragab Khalil Department of Landscape Architecture Faculty of Environmental Design King AbdulAziz University Room 103 Overview 1. Geographic coordinate system 2. What is a map projection 3. When should you use a projection 4. Spatial distortion and types of map projections 5. How to Choose a map projection 6. Map projections used commonly in KSA Dr. Ragab Khalil KAAU - FED LA561: GIS 2/31 Projection: An Introduction We use maps to model the earth. A map is a representation of the spatial organization of the physical universe at any scale. A map contains a wide variety of information, both static and dynamic. Maps are usually created on a flat, two-dimensional surface. An important step in modeling geographic phenomena on a map is the conversion of the 3D world into a 2D model. Dr. Ragab Khalil KAAU - FED LA561: GIS 3/31 1

2 Coordinate systems We have two types of coordinate systems Geographic coordinate system Projected coordinate system An ellipsoid, datum, a projection and units make up a coordinate system Dr. Ragab Khalil KAAU - FED LA561: GIS 4/31 Geographic Coordinates The Geographic Reference System consists of latitude and longitude. This system treats the globe as if it were a sphere. The sphere is divided into equal parts called degrees. The standard origin is where the Greenwich Prime Meridian meets the Equator. All points north of the Equator or east of the Prime Meridian are positive. Dr. Ragab Khalil KAAU - FED LA561: GIS 5/31 Geographic Coordinates Lines of longitude (meridians) run north and south and measure the number of degrees east or west of the Prime Meridian. Values range from -180 to +180 degrees. Lines of latitude (parallels) run from east to west and measure the number of degrees north or south of the Equator. Values range from the North Pole, at +90 degrees, to the South Pole at -90 degrees. Dr. Ragab Khalil KAAU - FED LA561: GIS 6/31 2

3 What is a map projection? Due to the fact that the Earth is a sphere (3D) and a map is flat (2D), we need to convert locations from a curved surface to a flat surface. The mathematical formula used to do this is called a map projection. A map projection is a result of the process of systematically transforming positions on the Earth s spherical surface to a flat map, while maintaining spatial relationships. Dr. Ragab Khalil KAAU - FED LA561: GIS 7/31 What is a map projection? A light positioned in the center of the sphere shines ("projects") outward, casting shadows from the lines Shadows are cast upon a surface outside the sphere The surface is opened flat Dr. Ragab Khalil KAAU - FED LA561: GIS 8/31 When should you use a projection? When using measurements to make important decisions. When measuring distance or direction. When comparing geographic features/phenomena. When you are analyzing spatial relationships of data. Aligning image and feature themes. Dr. Ragab Khalil KAAU - FED LA561: GIS 9/31 3

4 Distortion due to projection Map projections are not perfect! It is impossible to convert a 3D set of features to a 2D set of features without some errors. The errors caused when applying a map projection are called distortion. Distortion can affect one or more of these spatial properties: Shape (the geographic shape of a feature) Area (the amount of area of a feature) Distance (how far away features are from each other) Direction (the orientation NESW traveling from one feature to another) Dr. Ragab Khalil KAAU - FED LA561: GIS 10/31 Distortion due to projection Depending on the use of the map certain spatial properties will be more important than others. The amount of distortion caused by a map projection increases as you travel away from the reference origin. By choosing a projection centered on the location being mapped, the effects of distortion will be minimal for the area of interest. Dr. Ragab Khalil KAAU - FED LA561: GIS 11/31 Categories Map projections can be categorized according to the spatial property they preserve. Property Area Shape Distance Direction Category Equal-area Conformal Equidistant Azimuthal NOTE: It is impossible to construct a map projection that is both equal area and conformal. Dr. Ragab Khalil KAAU - FED LA561: GIS 12/31 4

5 Types of Projections Conic (Albers Equal Area, Lambert Conformal Conic), good for East- West land areas, parallels are concentric circles Cylindrical (UTM), good for North-South land areas, parallels are straight Planer (Lambert Azimuthal Equal Area), good for global views, parallels are eccentric circles Dr. Ragab Khalil KAAU - FED LA561: GIS 13/31 How to choose a map projection Questions to ask when choosing a map projection: 1. Is my other data already projected? If the other data you are using is already projected then you must project your data to the same projection as this data. 2. Which spatial properties are most important to me? you must choose which spatial properties are most important for what you are doing: shape, area, distance or direction. Choose a map projection that protects these important spatial properties. 3. What is the location and scale of my data? Map projections have been specially designed for different areas of the earth and also for different map scales. Dr. Ragab Khalil KAAU - FED LA561: GIS 14/31 Example of differences due to different projections Dr. Ragab Khalil KAAU - FED LA561: GIS 15/31 5

6 Example of differences due to different projections Small scale Dr. Ragab Khalil KAAU - FED LA561: GIS 16/31 Map Projections Used in KSA Coordinate system: Universal Transverse Mercator (UTM) Spheroid: International [semi-major= m, 1/f= Datum: Ain Alabd 1970 Zone number: 37, 38, 39, 40 with central meridians 39, 45, 51, 57 East respectively Scale factor: Lambert Conformal Conic projection is also used The first and second standard parallels are 17 and 33 N Dr. Ragab Khalil KAAU - FED LA561: GIS 17/31 Map Projections Used in KSA The Mercator projection, however, is conformal; therefore, it is excellent for navigational purposes but only preserves local shapes. Countries in the mid-latitudes are best represented by a Conic projection The result is less overall distortion of land and water areas. The Lambert Conformal Conic projection is a commonly used Conic version. Dr. Ragab Khalil KAAU - FED LA561: GIS 18/31 6

7 Modeling the Earth s s shape Spherical models are useful for small-scale maps (features are small) such as world atlases since the error at that scale is not usually noticeable or important enough. The ellipsoidal model is commonly used to construct topographic maps and for other large and medium scale maps that need to accurately depict the land surface. Dr. Ragab Khalil KAAU - FED LA561: GIS 19/31 Reference Ellipsoids A spheroid ( ellipsoid) is a mathematical description of the earth. It is usually defined by semi-major (equatorial radius) and flattening (the relationship between equatorial and polar radii). Semi-minor axis Semi-major axis Dr. Ragab Khalil KAAU - FED LA561: GIS 20/31 Reference Ellipsoids Many reference ellipsoids are in use by different nations and agencies. Dr. Ragab Khalil KAAU - FED LA561: GIS 21/31 7

8 Datums A datum is a set of control points whose geometric relationships are known, either through measurement or calculation, and is used to define a coordinate system. Datums are always based on ellipsoids that best represent the geoid within the region the datum is going to be used for. Geoid Dr. Ragab Khalil KAAU - FED LA561: GIS 22/31 Datums Commonly used datums in Saudi Arabia Ain Al Abd 1970 World Geodetic System of 1984 (WGS84) Dr. Ragab Khalil KAAU - FED LA561: GIS 23/31 UTM Universal Transverse Mercator (UTM) Every place on earth falls in a particular zone Dr. Ragab Khalil KAAU - FED LA561: GIS 24/31 8

9 UTM Universal Transverse Mercator (UTM) Every place on earth falls in a particular zone Dr. Ragab Khalil KAAU - FED LA561: GIS 25/31 Summary Dr. Ragab Khalil KAAU - FED LA561: GIS 26/31 Projecting data frames start with unprojected data Dr. Ragab Khalil KAAU - FED LA561: GIS 27/31 9

10 Projecting data frames then apply a projection to the data frame Dr. Ragab Khalil KAAU - FED LA561: GIS 28/31 saving data sets in projected units Export from a projected data frame Dr. Ragab Khalil KAAU - FED LA561: GIS 29/31 Projecting spatial data sets Utility for converting data sets from one system to another Used for going between projections Dr. Ragab Khalil KAAU - FED LA561: GIS 30/31 10

11 Next week 1 st Quiz 5 points Time 15 minutes Covers all lectures Dr. Ragab Khalil KAAU - FED LA561: GIS 31/31 11