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Geodesy: measuring the Earth. Geodesy: a branch of earth sciences, is the scientific discipline that deals with the measurement and representation of the Earth, including its gravity field, in a three-dimensional time varying space.Geodesy deals with the determination of the earth's shape and size, as well as its gravity field, i.e. the measurement of gravitational acceleration. The knowledge of the gravity field is important, because almost every geodetic measurement is closely related to gr15
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1. You will want to pay attention…
there will be a test on this material!!! The Exhilarating World of Geodesy and Map Projections
2. Geodesy: measuring the Earth Geodesy: a branch of earth sciences, is the scientific discipline that deals with the measurement and representation of the Earth, including its gravity field, in a three-dimensional time varying space.
Geodesy deals with the determination of the earth's shape and size, as well as its gravity field, i.e. the measurement of gravitational acceleration.
The knowledge of the gravity field is important, because almost every geodetic measurement is closely related to gravity, e.g. instruments have to be leveled along the a horizontal surface.
3. Calculating Routes on the Earth’s Surface Calculations however (including calculations of a route for high-speed trains) do not use the actual shape of the earth, but mathematically defined surfaces and bodies such as planes, spheres or ellipsoids of rotation.
The difference between reality and these simplified mathematical models is shown in the figure below.
4. Geodesy and GPS It is critical to know about the difference between heights above sea level (which are related to the gravitational field and thus the only way to determine the slope) and the ellipsoidal heights, which are a result of GPS measurements.
Geodesy provides information about the reference surface of the heights above sea level - the so-called geoid. The geoid is represented as the surface of an imaginary, calm sea, continued below the continents.
It is the connection between the heights measured with GPS and heights above sea level, used in national vertical control networks.
5. Earth as a Geoid Earth has a dynamic surface and complex structure.
A Geoid is too complex a shape for daily needs so easier to think of Earth as an ellipsoid
6. Geoid-Spheroid-Ellipsoid A GEOID is the earth’s actual shape (including elevation above/below sea level)
A SPHEROID is an ellipsoid that approximates the shape of a sphere
An ELLIPSOID is created by rotating an ellipse about either major axis or minor axis
7. Earth as Reference Ellipsoid Ellipsoid defined by an equatorial radius, polar radius and the reciprocal of the flattening
Ellipsoid approximates the geoid in size and position
Different ellipsoids in use. They differ in equatorial radius and flattening as well as positioning of their center.
8. Graticule: parallels and meridians System of imaginary intersecting lines was created based on 360 degree Babylonian system for dividing a circle or sphere
Map projections are based on:
Central Meridians
Standard Parallels
9. The Graticule …Projected
10. What is a Map Projection? A map is a flat representation of a globe
A map projection is a systematic transfer of points on a curved surface to a flat projection surface
11. SSADD: 5 Key Properties of Maps Scale: relationship of distance and area on Earth to distance and area on map
Shape: shape is preserved when the scale of any point on the map is the same in any direction
Area: proportional relationship between study region and area of the Earth that it represents
Distance: length from center of the projection to any other place on the map
Direction: angles from a point on a line to another point are portrayed correctly in all directions
12. Why use Map Projections? GIS and paper maps are flat and more convenient than 3D models for most applications
Scanning and digitizing maps is a big source of spatial data used in GIS
Most common spatial data models are flat and cannot be created on curved surface
Earth has to be projected to see all of it
Much easier to measure distance on a plane
13. Problems with Map Projections Maps can preserve some of the properties but not all
No projection can retain more than one of these properties over large portion of globe
Every map projection distorts the earth is several ways
Decisions of which projection to use depends on reducing distortion
14. Projection Parameters Projection Center: point of projection
Projection Families: Developable Surfaces
Projection Aspect: orientation of developable surfaces
Location of developable surface
15. Projection Centers Gnomonic: projection point from Earth’s center
Stereographic: at antipodal surface
Orthographic: at infinity
16. 3 Families of Projections Cylindrical
Conical
Planar
17. Cylindrical Projections Used by mariners for navigation
Meridians run north - south
Parallels run
east- west
True at equator and distortion increases towards the poles
18. Conical Projections Used for mid latitude maps
True at areas between standard parallels
19. Planar Projections Used to navigate flight routes
Preserves Great Circle Lines
In polar aspect, these maps project meridians as straight lines radiating from the poles and parallels as complete circles centered at the pole
True at poles and distortions increases outwards
20. 3 basic rules for choosing projection family
21. 4 Types of Projection Aspect Normal: oriented with polar axis
Transverse: perpendicular to polar axis
Polar: centered on N/S Poles
Oblique: all others
22. Location of Developable Surface Tangent: DS touches the globe
Secant: DS cuts into the globe
23. Map Property Preservation If a projection preserves…
SHAPE it is called CONFORMAL .
AREA is called EQUAL-AREA or Equivalent.
DISTANCE it is called EQUIDISTANT
DIRECTION it is called AZIMUTHAL
24. Tissot’s Indicatrix: Graphical Tool to Analyze Projection Properties
25. Example: Mercator Projection Cylindrical
Conformal
Areas not preserved and increase towards the top and bottom of the map
Preserves true-direction along graticule lines
Used for ocean navigation
26. Mercator Projection
27. Example: Lambert Conformal Conical Conical
Conformal
Preserves shape of geographic features
Useful in mapping mid latitudes
28. Lambert Conformal Conical
29. Example: Orthographic Azimuthal Planar
Azimuthal
Equatorial or Oblique Aspects
Azimuthal not suitable for displaying entire Earth in one view
30. Orthographic Azimuthal
31. Robinson Projection
32. Projection Systems A Projected Coordinate System is a projection based on two different projections depending on the shape of the region it describes
State Plane Coordinate System (SPCS)
States extending east to west, uses Lambert conformal conical projection
States extending north to south, uses tranverse cylindrical Mercator projection
Measured in feet
Minimizes distortions within 1 foot
33. Projection Systems Universal Tranverse Mercator Coordinate System (UTM)
Uses 60 zones, each 6 degrees of longitude wide
Measured in meters
Uses a secant variation
Minimizes distortion < 1meter within each zone
34. Unprojected vs. Undefined Data Unprojected geographic coordinate system allows more flexibility in setting ArcMap’s data frame coordinate system to suit analysis needs
Undefined coordinate system is missing files to enable ArcMap to read and reference them with other data layers. You will need to define the coordinate system to fully use the file in an ArcMap project.
35. Final Thoughts Map projections are critical for transferring 3D globe onto 2D flat surface
Many types of projections used for different purposes
Need to understand map projections to work with multiple layer files in GIS to make each layer ‘fit’ with the other.
Go to JH Labs website and explore more unusual map projections. Bring a copy of your favorite unusual projection with a description of which properties are preserved and which are distorted.
36. Assignment Go to JH Labs website (JH Labs: Java Map Projection Library http:www.jhlabs.com/java/maps/proj)
and explore more unusual map projections.
Bring a copy of your favorite unusual projection with a description of which properties are preserved and which are distorted.
Go online to search how geodesy is used in making maps, refining measurements of earth’s surface, navigation systems, or another interesting aspect of how it is used by scientists and engineers. Write a one page (double spaced 12 font) summary of the information you found. Provide the website URLs and any images that will help explain your findings.
Be prepared to share your projection selection and your geodesy research summary with a small group.
37. Quiz Material Know definitions for geoid, spheroid, ellipsoid
Know definition of geodesy and ‘map projection’
Know definitions of the 5 Key Properties of Maps (SSADD)
Know at least one reason for using map projections and at least one problem with map projections
Know 3 types of projection centers
Know 3 families of projection
Name 3 specific map projections and the characteristics of one of them (pros and cons)
Name one example of a Projected Coordinate System
38. Resources used for presentation JH Labs: Java Map Projection Library http:www.jhlabs.com/java/maps/proj
Information on Geodesy and Geoinfomatics Engineering http://www.uni-stuttgart.de/studieren/angebot/geodaesie/index.en.html#diploma
Geodesy and Geoinformaiton http://www.gug.bv.tum.de/seiten-e/technik/physik.html
Hydrology and Water Management Ecology Centre: http://www.hydrology.uni-kiel.de/lehre/vorlesung/ws04-05/vl_msc0104_02.pdf
GEOG101: http://faculty.winthrop.edu/storiec/files/GEOG101/lectures/05-GEOG101-MapsGIS.pdf
Kate Beard’s SIE 509 Principles of GIS: Map Projections
higheredbcs.wiley.com/legacy/college/strahler/0471480533/student_pres/ch03.ppt –
http://www.biology.ualberta.ca/facilities/gis/uploads/instructions/5_GCP_Fall2004.pdf