Map Scale Map Projections RESM 440 Wed Sept 1, 2010

1. Map Scale & Map ProjectionsRESM 440Wed Sept 1, 2010

2. 2 Today Announcements Topic: Map scale, map projections, coordinates This week�s lab: Using spatial datasets, projections Required readings: USGS Fact Sheet: Map Scales USGS Poster PDF: Projections (skim it!)

3. 3 Review from Monday Spatial data formats: Vector features (and attributes) Raster data Many different file formats Properties of spatial data files: Scale Projection

4. 4 Map scale defined Scale: �The ratio or relationship between a distance or area on a map and the corresponding distance or area on the ground� Why is scale important to know? Key property of a spatial dataset Determines amount of detail shown in a map or contained in a data layer Allows map users to measure distance accurately

5. 5 How is scale reported? Ratio scale (no units) A distance conversion Map units to real-world units Bar scale Usually found on printed maps Valid if map is copied, reduced, enlarged Provides a reference for user Equivalent scale Usually expressed with distance units

6. 6 How is scale determined? Aerial photographs have inherent scale Depends on: Focal length of camera lens ( f ) Height above ground ( H ) Scale = Example: Camera with 150mm lens Flying height 3000m above ground Scale = 0.150 / 3000 = 20,000 (1:20,000)

7. 7 How can you determine the scale? For maps: Compare known distances in same units and compute ratio:

8. 8 Determining scale (example 2) Scale is independent of measurement units Distance of 1 cm on map Corresponds to 2 km on ground Find the map scale?

9. 9 Map scale comparison: Large vs. small

10. 10 Map scale comparison

11. 11 Map scale comparison � Common USGS maps

12. 12 Map generalization Maps are only approximations of reality � generalization is required! Map features may be generalized or detailed depending on scale Map features may appear different at different scales �Best� scale for your map depends on your particular needs � no real right/wrong

13. 13 Map projections Projection: a mathematical transformation required to display the round earth on a flat surface (map, computer screen, etc.) Georeferencing: correctly associating your GIS data with real-world locations

14. 14 Why are projections useful? Projections allow us to make measurements on flat maps: Distance Area X, Y coordinates Correct georeferencing is important in GIS for: Making measurements and area calculations Accurately representing shapes Overlaying layers

15. 15 Projections make geographers S-A-D-D All projections produce distortion in mapped features: Choose projection best suited for your study area or map purpose

16. 16 Projection types

17. 17 Types of projections Generally classified and named according to the type of projection and the map property maintained by the projection:

18. 18 Projections and coordinate systems The coordinate system is a method used to reference your map to locations on the earth�s surface Two types of coordinate systems in GIS: Geographic coordinate system (lat/long) Projected coordinate system (all others)

19. 19 Two basic types of projections in GIS

20. 20 Projections in GIS Geographic Coordinate Systems Coordinates measured in longitude, latitude Not as good for measuring distances Projected Coordinate Systems Better for measuring distances 100�s of different projections Includes UTM, Lambert, Albers, etc. mentioned in readings and this lecture

21. 21 Longitude and latitude (X and Y)

22. 22 Longitude and latitude coordinates

23. 23 Projected map coordinates (X and Y) Projected coordinate systems do not use longitude and latitude to measure locations Instead, they use map units of the projection itself: Meters (UTM, Albers, most others) Feet (State Plane, few others)

24. 24 Commonly seen map projections in the U.S. Projection used is related to type of map and area covered by map (extent) Continental-scale data (North America) Geographic projection (decimal degrees) Lambert Conformal Conic Albers Equal Area State-level data (West Virginia) UTM Projection, zone 17 State Plane

25. 25 Map projection examples

26. 26 Map projection examples Albers Equal Area � Good for continental scale data

27. 27 Map projection examples UTM � Good for smaller study areas (states?) Universal Transverse Mercator UTM grid divides world into 60 north-south zones Grids are evenly spaced, XY coordinates measured in meters relative to origin points X = easting Y = northing

28. 28 Properties to know when using a projection in GIS Name of projection Type of coordinate system Geographic or projected Projection map units (how are XY coordinates measured?) Decimal degrees, meters, feet, etc. Other projection parameters Zone number or name Standard reference lines Datum

29. 29 Datums What is a datum? Datum is a reference system for measuring true locations on the earth (earth is an irregular sphere) Different datums used: North American Datum of 1927 (NAD27) North American Datum of 1983 (NAD83) Others

30. 30 Influence of datum Locations on the earth are referenced to a datum Different datums have different coordinate values for the same location

31. 31 Projections in GIS Projection is a property of each individual dataset (e.g. a shapefile, a GRID, an image) ArcGIS stores projections in *.prj file Projection must be correctly specified by the user Projection name, type, datum, and other properties are stored by the GIS

32. 32 Projections and GIS GIS tasks related to projections: Define a map projection for a dataset Project a dataset from one projection to another Overlay map layers with different map projections (project on the fly) Change projection of data frame Measure distances in different units

33. 33 Summary and key points