1 / 52

Map projections and datums

This article explains the different types of map projections and coordinate systems used to represent the curved Earth on flat maps. It discusses the concept of distortion, true directions, angles, distances, and areas, as well as the use of geodetic datums. It also provides an overview of various projection families and their characteristics.

boltons
Download Presentation

Map projections and datums

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Map projections and datums

  2. flat Earth is curved Maps are flat

  3. Map Distortion • No map is as good as a globe. • A map can show some of these features • True direction or azimuth • True angle • True distance • True area • True shape But not all of them!

  4. Coordinate System common frame of reference for all data on a map

  5. GIS needs Coordinate Systems to: • perform calculations • relate one feature to another • specify position in terms of distances and directions from fixed points, lines, and surfaces

  6. Coordinate Systems Cartesian coordinate systems: perpendicular distances and directions from fixed axes define positions Polar coordinate systems:distance from a point of origin and an angle define positions

  7. Each coordinate system uses a different model to map the Earth’s surface to a plane

  8. GCSGeographic Coordinate Systems • Degrees of latitude and longitude • Spherical polar coordinate system • “Real” distance varies

  9. Spherical Coordinates • Any point uniquely defined by angles passing through the center of the sphere Meridian  Equator 

  10. The Graticule • Map grid (lines of latitude and longitude) • A transformation of Earth’s surface to a plane, cylinder or cone that is unfolded to a flat surface

  11. Decimal Degrees 30º 30' 0" = 30.5º 42º 49' 50" = 42.83º 35º 45' 15" = ? 35.7541

  12. Standard Geographic Features

  13. Parallels of Latitude Equator Slicing the Earth into pieces

  14. Measuring Parallels Give the slices values

  15. Lines of Longitude Antimeridian A Meridian A Establish a way of slicing the Earth from pole to pole

  16. Prime Meridian Establishes an orthogonal way of slicing the earth

  17. Longitude North America Values of pole-to-pole slices

  18. Earth Grid Comparing the parallels and the lines

  19. Latitude and Longitude Combining the parallels and the lines

  20. Grid for US What is wrong with this map? Parallels and Lines for US

  21. Sphere vs. Ellipsoid Globes versus Earth

  22. Shape of the Earth • Approximated by an ellipsoid • Rotate an ellipse about its minor axis = earth’s axis of rotation • Semi-major axis a = 6378 km • Semi-minor axis b = 6356 km NP b a SP

  23. Ellipsoids and Geoids • The rotation of the earth generates a centrifugal force that causes the surface of the oceans to protrude more at the equator than at the poles. • This causes the shape of the earth to be an ellipsoid or a spheroid, and not a sphere. • The nonuniformity of the earth’s shape is described by the term geoid. The geoid is essentially an ellipsoid with a highly irregular surface; a geoid resembles a potato or pear.

  24. The Ellipsoid The ellipsoid is an approximation of the Earth’sshape that does not account for variations caused by non-uniform density of the Earth. Examples

  25. Satellite measurements have led to the use of geodetic datums WGS-84 (World Geodetic System) and GRS-1980 (Geodetic Reference System) as the best ellipsoids for the entire geoid.

  26. The Geoid • The maximum discrepancy between the geoid and the WGS-84 ellipsoid is 60 meters above and 100 meters below. • Because the Earth’s radius is about 6,000,000 meters (~6350 km), the maximum error is one part in 100,000.

  27. Geodetic Datums

  28. Geodetic Datum • Defined by the reference ellipsoid to which the geographic coordinate system is linked • The degree of flattening f (or ellipticity, ablateness, or compression, or squashedness) • f = (a - b)/a • f = 1/294 to 1/300

  29. Geodetic Datums • A datum is a mathematical model • Provide a smooth approximation of the Earth’s surface. • Some Geodetic Datums

  30. Common U S Datums • NAD27 North American Datum 1927 • NAD83 North American Datum 1983 • WGS84 World Geodetic System 1984 (based on NAD83)

  31. Map Projections

  32. Making a Map Concept of the Light Source

  33. Projection Families

  34. Types of Projection Families

  35. Standard Point/Line for Projection

  36. Regular Azimuthal

  37. Azimuthal Projections

  38. Azimuthal Projections • Shapes are distorted everywhere except at the center • Distortion increases from center • True directions can be plotted from the center outward • Distances are accurate from the center point

  39. Polyconic Projections • A series of conic projections stacked together • Have curved rather than straight meridians • Not good choice for tiles across large areas

  40. Albers Conic Equal AreaProjections • Good choice for mid-latitude regions of greater east-west than north-south extent • Scale factor along two standard parallels is 1.0000 • Scale is reduced between the two standard parallels and increased north or south of the two standard parallels

  41. Equal Area Projections • Projections that preserve area are called equivalent or equal area. • Equal area projections are good for small scale maps (large areas) • Examples: Mollweide and Goode • Equal-area projections distort the shape of objects

  42. Conformal Map Projections • Projections that maintain local angles are called conformal. • Conformal maps preserve angles • Conformal maps show small features accurately but distort the shapes and areas of large regions • Examples: Mercator, Lambert Conformal Conic

  43. Conformal Map Projections • The area of Greenland is approximately 1/8 that of South America. However on a Mercator map, Greenland and South America appear to have the same area. • Greenland’s shape is distorted.

  44. Map Projections • For a tall area, extended in north-south direction, such as Idaho, you want longitude lines to show the least distortion. • You may want to use a coordinate system based on the Transverse Mercator projection.

  45. Map Projections • For wide areas, extending in the east-west direction, such as Nebraska, you want latitude lines to show the least distortion. • Use a coordinate system based on the Lambert Conformal Conic projection.

  46. Map Projections • For a large area that includes both hemispheres, such as North and South America, choose a projection like Mercator. • For an area that is circular, use a normal planar (azimuthal) projection

  47. The UTM System

  48. Universal Transverse Mercator • 1940s, US Army • 120 zones (coordinate systems) to cover the whole world • Based on the Transverse Mercator Projection • Sixty zones, each six degrees wide

  49. UTM Zones • Zone 1 Longitude Start and End 180 W to 174 W Linear Units Meter False Easting 500,000 False Northing 0 Central Meridian 177 W Latitude of Origin Equator Scale of Central Meridian 0.9996

More Related