Coordinate Systems and Map James Frew ESM 263 Winter

Transcription

1 Coordinate Systems and Map 1

2 Outline and longitude and coordinate systems Placenames Postal addresses and postal codes Linear referencing systems Cadasters georeferences 2

3 Georeferencing Linking to specific Unique = f (georeference) Shared means the same thing to everybody Persistent means the same thing tomorrow 3

4 Types of Georeferences Nominal Placenames Ordinal street addresses (in some of the world, anyway ) Interval or Cyclic linear or angular distance from fixed places e.g. from Equator or Greenwich Meridian 4

5 Geographic Coordinates Spherical coordinates Longitude defined by Center of mass Equator = f(rota@on) Zero meridian = f(poli@cs) Spherical Earth R km A km 2 5

6 Prime Meridian at Greenwich, UK Why the Greenwich meridian moved 6

7 Longitude Angle between planes of Zero meridian and axis 7

8 Requires model of Earth s shape Closer to an ellipsoid than a sphere an ellipse rotated about its minor axis N-S diameter ~ 1/300 less than E-W WGS 84 ellipsoid (current best model) Radius at Equator: km Flagening: 1/

9 and the Ellipsoid is angle between perpendicular to surface plane of the Equator Same pt, different ellipsoids: Same (almost) longitude Different 9

10 Why Different Ellipsoids? Before satellites: many Each country used local best fit ellipsoid US: North American Datum of 1927 (NAD 27) Clarke 1866 ellipsoid tangent to surface at Meades Ranch, KS US: North Amercian Datum of 1983 (NAD 83) GRS 1980 ellipsoid Earth-centered <=200 m offset from NAD 27 (in US) Aner satellites: WGS 84 Earth-centered ellipsoid <1 cm offset from NAD 83 (in US) 10

11 NAD27 NAD83 datum shin 11

12 Datum Connects Ellipsoid to Earth 12

13 What Happens When You Mix Datums USS LA MOURE COUNTY aground in Caleta Cifuncho Bay, Chile after navigating with GPS (WGS-84 datum) on a local chart on local datum. 13

14 Why project Earth s surface onto plane? See whole Earth surface at once GIS maps/displays scan, digi@ze print, plot Much easier to measure distance Represent Earth surface as a rectangular grid we ll talk more about this in the raster sec@on... 14

15 All distort the Earth in some way Which is most important? (Pick one) Shape? Use conformal same in all Area? Use equal area Distorts shapes to preserve area (No has both of these (And maybe add) Distance? Use equidistant Only from 1 or 2 points, or along 1 line. 15

16 Cylindrical Wrap a cylinder around the Earth 16

17 Cylindrical Example: Mercator 17

18 Conic Wrap a cone around the Earth Standard Parallel(s):where cone intersects Earth surface 18

19 Conic Example: Lambert Conformal Conic 19

20 The Unprojected (aka Plate Carrée) X = longitude, Y = la@tude Cylindrical projec@on Neither conformal nor equal area 20

21 Comparing U.S. Mercator Plate Carrée Lambert Conformal Conic 21

22 Comparing North America Albers Equal Area Lambert Conformal Conic 22

23 Universal Transverse Mercator (UTM) standard Originally military System of 60 zones w/in zone <= 0.04% Transverse Cylindrical wrapped around Poles, not Equator 23

24 UTM Zones Zones are each six degrees of longitude, numbered from W to E 24

25 UTM Coordinates X: (meters east) Y: northing (meters north) X = 500 central meridian false eas@ng Y = Equator Eas@ngs and northings are both in meters allowing easy es@ma@on of distance on the projecaon UTM georeference zone number six-digit eas@ng seven-digit northing E.g., 14, E, N 25

26 UTM Zones: Each zone is a different projec@on Adjacent zones won t fit along border What about areas that span zones? Either: pick 1 zone accept >normal distor@ons in other zone Or: use other projec@on that spans area E.g. CA (zones 10 & 11) UTM zone 10½ Teale Albers (equal area conic) 26

27 State Plane Coordinates (SPC83) Defined in US by each state Many different Some states use zones 27

28 Which should I use? For data generally: equal-area so calcula@ons and comparisons make sense specifically: whatever your project/client already uses so you don t conver@ng stuff For maps generally: whatever s most familiar to the map s users specifically: conformal: if it s most important to recognize shapes equal-area: if it s most important to compare sizes (The smaller the area, the less important this dis@nc@on is.) 28

29 Placenames Earliest form of georeferencing & most commonly used in everyday Work at many different scales villages neighborhoods Evolve Peking Peip ing Beijing Taprobane Ceylon Sri Lanka Poland 29

30 Uniqueness: Which One? Domain-specific many instances of Springfield in the U.S. but only one per state Context-specific Paris, France vs Paris, Texas 30

31 Postal Address as Georeference Works iff mail (dwelling, office, ) is unique along: Street name is unique within: Local area (city, county, ) is unique within: Region (state, province, ) but not for... Rural areas e.g. Star Route 1 Box 198 changing to West 210th St. Natural features (lakes, mountains, rivers, ) Non-sequen@al street addresses e.g. Japan 31

32 Postcodes as Georeferences Defined in many countries US ZIP codes UK postcodes Hierarchically structured Leading characters large areas Trailing characters smaller areas vs. postal address? Ubiquitous Less precise 32

33 Linear Referencing on a network road, rail, river,... Expressed as name of link distance along link from fixed point e.g. intersec@on Used for Maintenance Public safety 33

34 Linear Referencing Problems Distance measurement inaccuracy e.g. car odometer Fixed points may be far apart Exacerbates measurement inaccuracy "street1 x street2" not always unique may intersect more than once 34

35 Cadasters Land ownership (property boundary) maps Parcel fabrics in ArcGIS US Public Land Survey System (PLSS) Georeferences local cadasters esp. in western US Natural resources Similar systems in other countries 35

36 Township and Range System (PLSS) land ownership in most of western US townships laid out in six mile squares on either side of accurately surveyed Principal Meridian. Square mile within township numbered as shown east of PM, reversed west the Principal Meridian. 36

37 Georeferences Transform coordinates to new ArcGIS projec,on and transforma,on tools Geocoding Convert street addresses to coordinates ArcGIS geocoding tools Google geocoding web service Gazegeer database of: place = (name, loca@on, type) U.S. Board on Geographic Names GeoNames 37