Geodetics: Implications for GIS Professionals May 10, 2018

Transcription

1 Experts in Geomatics, Surveying, Positioning, Geospatial Data, and Mapping Sciences Geodetics: Implications for GIS Professionals May 10, 2018 Michael Barnes APSG Education Foundation Chair APSG Chair

2 30 minutes Content Geodetic Datums and Map Projections Ten geodetic awareness topics for you A Random Thought: geodetics on the world stage Finally References:

3 Hierarchy of Mapping Ellipsoid λp Equator Yp Φp hp P Zp Xp Projected CRS is a derivative of the Datum (Geog CRS) Easting, Northing, Elevation (above MSL) stable Unstable w/o Datum Datum (ellipsoid & origin) is the Foundation X,Y,Z Cartesian and Lat, Long, Ht Source: APSG 3

4 Beware... Knowing the map projection and all its parameters is insufficient (even if the ellipsoid is known) Unless associated with a Geographic Coordinate Reference System / Datum The Map Projection coupled with a GeogCRS / Datum makes a Projected Coordinate Reference System (or ProjCRS)

5 Geodesy Reference Surfaces h H N Normal to Ellipsoid Normal to Geoid N=h-H Ellipsoid Geoid Terrain b a Geoid Equipotential surface which approximates to MSL Ellipsoid Surface where Computations are made 1/f = a/(a-b) Source: APSG 5

6 Half of the story Reference Ellipsoids Fundamental to hundreds of Geodetic Datums 77 in the epsg-registry Most commonly used in petroleum are: EPSG Code Reference Ellipsoid Name Used % of total 7022 International % 7019 GRS % 7012 Clarke 1880 (RGS) 47 9% 7004 Bessel % 7030 WGS % 7008 Clarke % 7011 Clarke 1880 (IGN) 19 4% 7024 Krassowsky % Others 70 15% 6

7 Two part story Examples of Geodetic Datums 7

8 Wrong Datum, Wrong Location ( meters ) 0 Selected 8 Projects Datums in Red ( meters ) Source: Phil Summerfield 8

9 Consequences of Poor Datum Handling 450 NAD 27 ( meters ) 225 WGS 84 NAD 83 (CSRS) SAD 69 PSAD 56 0 Cape Pulkova Minna Aratu Abidjan ( meters ) 9

10 Map Projections characteristics Area Azimuth & Angle General Length Shape Distortion is unavoidable in any projection Shear

11 Typical Map Projection Methods Universal Transverse Mercator Transverse Mercator Lambert Conformal Conic Albers Mercator Others less frequently: Stereographic, Oblique Stereographic, Oblique Mercator

12 Map projections: Area Calculations for CONUS Areas: Ellipsoidal 8,067, km 2 Albers Equal Area Conic 8,072, km 2 [+0.06%] Lambert Conformal Conic 7,193, km 2 [-10.83%] Mercator Cylindrical 13,702, km 2 [+69.86%] Source: Phil Summerfield 12

13 Experts in Geomatics, Surveying, Positioning, Geospatial Data, and Mapping Sciences 10 Self Assessment Questions: 10 seconds response Easy! Don t know, I need more awareness What is Barnes talking about? 13

14 Experts in Geomatics, Surveying, Positioning, Geospatial Data, and Mapping Sciences Q1. How many Horizontal Geodetic Datums are databased in the EPSG registry? 14

15 Review Horizontal Geodetic Datums in the Database 15

16 Experts in Geomatics, Surveying, Positioning, Geospatial Data, and Mapping Sciences Q2. What is the definition difference between a Coordinate System and a Coordinate Reference System? 16

17 CS is a subset of CRS Coordinate Systems are: Cartesian, Ellipsoidal, Spherical, Vertical Coordinate System Code - CS CS Name CS Type Dimension Remarks 4400 Cartesian 2D CS. Axes: easting, northing (E,N). Orientations: east, north. UoM: m. Cartesian 2 Used in projected and engineering coordinate reference systems. Coordinate Reference System: Geographic 2D, Geographic 3D, Geocentric, Projected, Compound, Engineering, Vertical Coordinate Reference System Code - CRS CRS Name Area code CRS Type Coord Sys code Datum code Base CRS Code Projn Conv Code Cmpd Hor CRS Code Cmpd Vert CRS Code CRS Scope WGS 84 / UTM zone 15N 2028 projected Large and medium scale topographic mapping and engineering survey. 17

18 Experts in Geomatics, Surveying, Positioning, Geospatial Data, and Mapping Sciences Q3. What accuracy dilution can I expect from a horizontal geodetic datum transformation? 18

19 Estimated accuracies of 2,114 Datum Transformations m 4% 5-10 m 7% m 6% m 0.3% absentia 10% Less than 1 m 56% 1-5 m 17% Source: EPSG Registry v9.2 19

20 Experts in Geomatics, Surveying, Positioning, Geospatial Data, and Mapping Sciences Q4. Name 3 common map projections that preserve area ratios? 20

21 Equal Area Map Projections Albers Equal Area Conic Lambert Azimuthal Equal Area Lambert Cylindrical Equal Area 21

22 Experts in Geomatics, Surveying, Positioning, Geospatial Data, and Mapping Sciences Q5. How many Reference Norths are used? 22

23 5 kinds of Reference North True North = Direction of the meridian through a point Magnetic North = Differs from True North by declination and varies with time Grid North = Differs from True North by convergence Gyro North = a measurement of True North by instrument Plant or Project or Rig North = strictly a local non-georeferenced orientation 23

24 Experts in Geomatics, Surveying, Positioning, Geospatial Data, and Mapping Sciences Q6. What is the meaning of EPSG code 32615? 24

25 EPSG CRS WGS 84 UTM zone 15N Definition codes are really useful, they assist data handling (ensuring; consistency, completeness, correctness and verified) 25

26 Experts in Geomatics, Surveying, Positioning, Geospatial Data, and Mapping Sciences Q7. How many Units of Measure do we encounter? 26

27 There is no standard foot 13 kinds of feet and 2 kinds of meters in use Reference Units: Biggest Petroleum Industry Problem US Survey Foot vs. International Foot 1 ftus = ft (Intl), but this is still important for UTM, generating foot errors, depending on latitude Other Feet: Clarke s Foot, Sears 1922, 2 for Benoit (both 1895), Gold Coast Ft., 4 Indian Ft. (1865, 1937, 1962 & 1975) and 2 British Feet (1865 & 1935) 2 meters: International Meter and German Legal Meter Other linear units: Chains, Links, and Yards 27

28 Experts in Geomatics, Surveying, Positioning, Geospatial Data, and Mapping Sciences Q8. Where is a good source of guidance for understanding of Vertical Data in GIS? 28

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30 Experts in Geomatics, Surveying, Positioning, Geospatial Data, and Mapping Sciences Q9. What types of lines are used for delineation of Block Licenses? 30

31 Basic geodetic considerations Block License Delineation Joining Turn Points grid (plane) parallel, meridian, geodesic loxodrome (rhumb line) But also note: Calculation of Area Map Grid (with and without corrected area scale factor) Ellipsoidal Ground or other surface level Ref: OGP GN s and Prior APSG presentations for technical explanations 31

32 Experts in Geomatics, Surveying, Positioning, Geospatial Data, and Mapping Sciences Q10. Decimal degrees truncated at degrees implies what realistic metric accuracy? 32

33 Latitude dependent degrees 6 to 60 cm Original source: APSG 33

34 If you remember nothing else.. not unique Latitudes and Longitudes are not unique unless qualified with datum name Projection Coordinates are not unique unless qualified with projection name, zone and datum Heights are not unique unless qualified with Surface Reference (Vertical Datum) Reference Norths are not unique unless qualified with Heading Reference Units of Measure are not unique unless qualified with Unit Reference 34

35 Experts in Geomatics, Surveying, Positioning, Geospatial Data, and Mapping Sciences A random thought: Geodetics matters to International Peace 35

36 Core to UNCLOS is maritime states rights to resources Exclusive Economic Zones (EEZ) at 200 nm 200 nautical miles = 230 miles = 370 km 36

37 Boundaries are politically sensitive, legally and technically complex, and need good geodesy Disputes over boundaries and transboundary resources 37

38 20 nations with license blocks beyond 200 nm Subject to an additional ISA Royalty Payments? 38

39 Complex Multiple Overlapping Claims Geopolitical tangle in South China Sea 39

40 Before GIS, we had 200 years of geodetic surveying and mapping.. the science is still the same.. National Geodetic Survey of Nepal

41 Experts in Geomatics, Surveying, Positioning, Geospatial Data, and Mapping Sciences Final thoughts on Geodetics: Implications for GIS Professionals 41

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