Coordinate Systems, Datums and Map Projections

1. Coordinate Systems, Datums and Map Projections

2. Geodesy The study of the size and shape of the Earth.

3. The Earth is… • 3000 BP Bablonians An oyster • 600 BP Greeks Flat • 500 BP a perfect shape A sphere • 300 BP Circumference ~ 25,000 miles, • Dark ages Flat again • 1492 Columbus A pear • 1753 French Oblate ellipsoid • Spheriod • Geoid to describe the deviations from a spheriod

4. The Earth as a Geoid

5. Geographic Coordinate System • Parallels • Meridians • Great and Small Circles

6. Geographic Coordinate System GCS uses a 3-D spherical surface to define locations on Earth. GCS includes an angular unit of measure, a prime meridian and a datum.

7. Geographic Coordinate System • Longitude and Latitude • Degrees, minutes, seconds • 1o latitude = ~110.5 km (equator) • 1o longitude = cosine of the latitude • 1 minute of latitude = ~1852 meters

8. How to convert form DMS to DD • Example: 37°36’30” • Divide each value by the number of minutes or seconds in a degree • 36 minutes = .60 degrees (36/60) • 30 seconds = .00833 degree (30/3600) • Add it all up • 37° + .60 + .00833 = 37.60833 DD

9. Sphere vs. Speroid A sphere is okay for small scale maps (<1:5,000,000). For larger scale maps a spheroid is necessary, the spheroid used will depend upon the purpose, location, and accuracy of the data.

10. Datums • Reference frame for locating points on Earth’s surface • Defines origin & orientation of latitude/longitude lines • Defined by spheroid and spheroid’s position relative to Earth’s center.

11. Position of the Capital of Texas

12. N. American Datums • NAD27 • Clarke 1866 spheroid • Meades Ranch, KS • Local datum • NAD83 • GRS80 spheroid • Earth-centered datum • GPS-compatible • NAD27 NAD83 up to 500’ shift

13. Datums and Elevation • Horizontal and Vertical Datums • Sea - level? • Panama Canal • Height Above Ellipsoid (HAE) • Height Above Geoid (HAG)

14. A spherical planet in plane terms Projected coordinate systems are any coordinate system designed for a flat surface.

15. Map Projection • Distortions are inherent in maps • Earth is round, map is flat • Projectionis the term used to describe the process of mapping a round surface to flat paper 13

16. Projection & Datum Overview • Earth is three-dimensional • Map (screen) is 2-D • Projections convert 3-D to 2-D • 3-D to 2-D causes distortions • Datums locate in 3-D

17. Map Projections • Map projections always introduce distortion of some kind • Distance • Direction • Shape • Area

18. Map Projections, cont. • There are many kinds of projections for different parts of the world • Different projections produce different distortions • Key is choosing appropriate projection for a particular situation

19. Map Projections Types 14

20. Map Projections and GIS, cont. • Should be considered before starting to automate GIS data or starting a GIS project • Must also be considered if you obtain data from other sources (who doesn’t?) • Map projections are central to insuring different GIS databases properly register to one another (really important in GIS!) • Important for valid spatial analysis

21. The State Plane Coordinate System (SPCS) • Not a true map projection but rather a set of coordinate systems for every state. • Designed in the 1930s to provide a local reference system tied to a national datum. • Most USGS 7.5 minute maps indicate state plane coordinates (as well as UTM, and lat-long grid) • Some states (depending on size and latitudinal extent) have multiple zones

22. State Plane Zones for NAD 1983 1 2 3 4 5 6

23. The State Plane Coordinate System (SPCS), cont. • East-West oriented States use the Lambert Conformal Conic projection (ex. Tennesse) • North-South oriented States use the Transverse Mercator projection (ex. Illinois) • One zone in Alaska uses Oblique Mercator • Based on the North American Datums: NAD27 and NAD83 • Reference Eastings and Northings

24. UTMUniversal Transverse Mercator • Transverse Cylindrical (Mercator) Projection • 60 zones • 6o wide • Central meridian is 500,000 m • Eastings • 0 is 500,000m west of central meridian • Northing • depends on hemisphere and is measured in meters from the equator • We are in Zone 10 N

25. UTM Zones

26. A Generic UTM Zone We are in UTM zone 10 north

27. Basic Feature of Maps • Scale can never be totally accurate all over the map, unless the map covers a very small area. • Features: Title, Date, Legend (key), North Arrow (magnetic vs. true north)

28. Map Characteristics • Scale • Ratio between distance on map to distance on Earth • Small scale - cover large area • e.g. 1 : 1,000,000 • Large scale - shows more detail • e.g. the 1 : 2,400 • Types: Representative fraction, linear, verbal • Resolution • accuracy that location and shape can be depicted • Diminishes with scale 10

29. Topographical vs. Thematic Maps • Topographical • Maps whose primary purpose is to indicate the general lay of the land • outline natural features • topographical maps often show other features such as roads, boundaries, rail links 5

30. Topographical vs Thematic Maps • Thematic Maps • displays geographic concepts • population density • climate • land use • etc. 6

31. Types of Thematic Maps • Chloropleth Maps • maps that display information using man-made reporting areas. Areas shaped according to their value. • census tracts, ZIP codes, counties, etc. • Area Class Maps • maps that display information by constant attribute • e.g. coniferous Vs deciduous forest 7

32. Types of Thematic Maps • Isopleth (Isoline) Maps • maps showing imaginary surfaces -connects all points with equal value. • contours • pressure • temperature • pollution levels • Isolines are: • drawn at regular intervals • never cross • they are closed lines • values inside are higher than outside 8