Reference Systems (Projections, Datums, Coordinates) and Surveys

1. Reference Systems (Projections, Datums, Coordinates) and Surveys Source: Peter H. Dana, The Geographer's Craft Project, Department of Geography, The University of Colorado at Boulder, http://www.ncgia.ucsb.edu/education/curricula/gisccused with permission

2. Projections • World’s not flat (despite what you have heard from Dr. K!) • We want to tie our plane surveys to global systems • Submeter accuracy • GPS • Satellite imagery

3. Projections • Conformality • Distance • Direction • Scale • Area

6. Mercatur Lambert Lat-Long (unprojected)

11. Scale true on 2 parallels

12. Secant at 45° (minimizes shape distortion)

13. Note shift of latitude lines(minimizes area exaggeration)

14. Constant azimuth for lines

15. Preserve distance along great circle

16. Pseudocylindrical

18. North American Projections Equal Area Equal Dist.

19. Iowa State Plane USGS topos

20. Similar to Gall, no secant

21. State Systems (hybrids) • Origin • 31:10 North • 100:00 West • standard parallels • 27:25 North • 34:55 North

22. Iowa DOT Lambert Hybrid

24. Datums • Define the shape of the earth • Range from flat-earth to complex • Wrong datum may produce 100s of meters in error cartography, surveying, navigation, and astronomy, geodesy

25. Geometric Earth Models • Early ideas of the figure of the earth resulted in descriptions of the earth as an oyster (The Babylonians before 3000 B.C.), a rectangular box, a circular disk, a cylindrical column, a spherical ball, and a very round pear (Columbus in the last years of his life). You are here!

26. Geometric Earth Models • Flat earth models are still used for plane surveying, over distances short enough so that earth curvature is insignificant (less than 10 kms).

27. Looks like a sphere, but flat here, and here

28. The best ellipsoidal models can represent the shape of the earth over the smoothed, averaged sea-surface to within about one-hundred meters.

30. Sea level: average surface of the oceans ( is far more complex) • Tidal forces and gravity cause surface to vary by hundreds of meters! • Gravity models and geoids are used to represent local variations in gravity that change the local definition of a level surface

34. Geodetic Height

38. Coordinate Systems Based on … • Datums • Units • Projections • Reference systems

40. UTM • Note false easting • False northing in southern hemisphere

45. Local Adjustments • May need a scaling factor to make total station measurements match regional coordinate systems • e.g., Iowa DOT develops a scaling factor for each project • Based on an accurately measured point in the center of the project • Not using a scaling factor can produce a 12’ error 30 miles from project center

46. Public Land Rectangular Surveys (USPLS) • Townships, square with six miles on each side, are numbered with reference to a baseline and principal meridian. • actually, few townships are truly square due to convergence of the meridians. • Ranges are the distances and directions from baseline and meridian expressed in numbers of townships. • Every four townships, a new baseline is established so that orthogonal meridians can remain north oriented.

48. Metes and Bounds • Metes and Bounds identify the boundaries of land parcels by describing lengths and directions of a sequence of lines forming the property boundary.

50. Linear Referencing Systems • Methods • Milepost • Milepoint • Cogo (project coordinates) • Lat-Long • Projected coordinates • Address • Literal description