1. Map Basics Lecture #3, Intro to GIS spring 2006

2. Topics Map definitions Types of maps Map projections Geodetic Datums Coordinate Systems

3. What are maps? Maps are models of reality Attempts to represent the two- dimensional curved surface of the Earth or other body on a plane (flat surface) main data source for GIS

4. The beginnings of Cartography Ancient Greece: Ptolemy (90 - 168 BC)’s Geographia –bases of coordinate systems –maps –list of places

5. Ptolemy’s world map Sri Lanka oversized

6. Muhammed Al’Idrisi’s view of the world (1154) What’s strange about this map?

7. Kepler’s map of the world

8. Types of maps 1) Topographic map: "Topography”: pertaining to the shape of the surface, represented by contours and/or shading, but topographic maps also show roads and other prominent features

9. Topographic maps

10. Thematic map (choropleth): show geographical concepts such as the distribution of  population  climate  land use etc.

11. Thematic maps

12. Map elements Map features: points, lines, areas, … Scale Accuracy Extent

13. Scale the amount of reduction in the representation of a real world geographic phenomenon on a map. or, the ratio of map distance to earth distance 1:24,000 1 inch = 2000 ft 1 2000 ft verbal scale representative fraction bar scale

14. Accuracy: what is it? Quality of the source data Map scale mapping skills distortions from flattening Earth’s surface to fit on a sheet of paper

15. Construction of a map: summary Steps: 1. Select projection 2. Select a model for the shape of the earth (sphere or ellipsoid)sphereellipsoid 3. Transform geographic coordinates (longitude and latitude) to plane coordinates (eastings and northings or x,y)longitudelatitude 4. Reduction of scale

16. Map Projections Used to display locations on the curved surface of the earth on a flat sheet or surface according to some set of rules Involve distortions in: shape (of a region) distance (between two points) direction (bearing from one point to another) area (of a region)

17. Properties of map projections 1. Equal-area: correctly represents areas sizes of the sphere on the map e.g. Lambert cylindrical equal-area projection 2. Equidistant: correctly represents distances e.g. Plate Carrée projection 3. Conformal: represents angles and shapes correctly at infinitely small locations. e.g. Mercator projection

18. The classification of Map Projections

19. A. Planar (azimuthal) flat sheet is placed in contact with a globe, and points are projected from the globe to the sheet

20. Ex: Lambert's azimuthal equal- area projection Preserves area Distorts shapes & distances

21. B. Conical projections

22. distorts scale and distance except along standard parallels Ex: Alber’s equal area conical projection

23. C. Cylindrical projections project the sphere onto a cylinder tangent to a central meridian meridians and parallels intersect at right angles

24. E.g.Mercator projection (conformal) Preserves angles Distorts scale, distance, direction and area distortions increase away from the central meridian used in sailing (direction more important than distance) Note an obvious distortion?

25. Plate carrée (geographic projection) latitude and longitude coordinates many distortions; mostly used in thematic mapping Compare the size of Greenland

26. D. “Compromise” projections Distortions are balanced Make things "look right” shape distorted more in the polar regions than at the equator

27. E.g. Robinson projection Mainly used for geographic maps, e.g National Geographic

28. Earth’s gravitational field Gravity = force of attraction between two bodies with a mass Earth is not a perfect sphere density not uniform local topography (e.g presence of mountains) and geology (the density of rocks) also influence the gravitational field.

29. Geoid and Ellipsoid Geoid = Earth’s shape (minus topographic relief) Ellipsoid = approximation to the shape of the geoid, defined mathematically Ellipsoid parameters

30. Geoid-ellipsoid relationships Mean Sea Level (MSL) = an approximation to the geoid, used as reference surfaces for height measurements (orthometric heights).

31. Common datums: Why the need for a common Reference System? For historical reasons each country has its own geodetic network and national geodetic reference frame. World Geodetic System (WGS84): GLOBAL NORTH AMERICAN DATUM OF 1927 NORTH AMERICAN DATUM of 1983

32. ProjectionsProjections demo: ArcIMS

33. Projections Demo: ArcGIS Latitude, Longitude, Map Projections and Great CirclesLatitude, Longitude, Map Projections and Great Circles