Remote Sensing _GEOREFERENCING

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GEOREFERENCING
By JWAN M. ALDOSKI Geospatial Information
Science Research Center (GISRC), Faculty of
Engineering, Universiti Putra Malaysia, 43400
UPM Serdang, Selangor Darul Ehsan. Malaysia.
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GEO-REFERENCINGReferencing features to the Earth

• Features on earth surfaces are not easy to refer
  and measure
• Features on planes are easier to measure and
  calculate
• distance
• angle
• area
• Coordinate systems provide a measurement framework

• Arbitrary coordinate system
• Global Cartesian coordinates (x,y,z) for the
  whole earth.
• Geographic coordinates (f,?, h) for the whole
  earth.
• Projected coordinates (x, y) or (N,E) on a local
  area of the earths surface.

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Understanding coordinates is vey important in GIS
Coordinate Systems
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Datums

• Models of the shape of the earths surface

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Arbitrary coordinate systems

• Typical of CAD systems
• Pick any origin
• Map units may be feet, meters, inches

Row-column coordinates

• Typical for image processing packages
• Start at upper left
• Count rows down, columns across
• Typical for image processing packages
• Start at upper left
• Count rows down, columns across

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Global Cartesian Coordinates
Geographic Coordinates
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Geographic Coordinates
(0ºN, 0ºE) Equator, Prime Meridian
Longitude line (Meridian)
Latitude line (Parallel)
N
N
W
E
W
E
S
S
Range 90ºS - 0º - 90ºN
Range 180ºW - 0º - 180ºE
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Geographic Coordinates
Z
Meridian of longitude
Greenwich meridian l 0
N
Parallel of latitude
P
? - Geographic longitude

?0-90N
? - Geographic latitude
?
E
W
O

Y
R - Earth radius
R
?


O - Geocenter
?0-180E
X
Equator f 0
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Latitude f
Longitude l

• Take a point S on the surface of the ellipsoid
  and define there the tangent plane mn.
• Define the line pq through S and normal to the
  tangent plane.
• Angle pqr is the latitude f, of point S

l the angle between a cutting plane on the
prime meridian and the cutting plane on the
meridian through the point, P
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Cartesian Coordinates
A planar cartesian coordinate system is defined
by a pair of orthogonal (x,y) axes drawn through
an origin.
Y
X
Origin
(xo,yo)
(fo, lo)
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Datums and Defining Parameters

• To translate one datum to another we must
  know the relationship between the chosen
  ellipsoids in terms of position and orientation.
  The relationship is is defined by 7 constants.
• i. 3 - Distance of the ellipsoid center from the
  center of the earth (?X, ?Y, ?Z)
• ii. 3 - Rotations around the X, Y, and Z of the
  Cartesian Coordinate System Axes (?, ?, ?)
• iii. 1 - Scale change (S) of the survey control
  network

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Translations (3 Parameters)
Movement of points along an Axis
?Z
?Y
?X
Rotations (3 Parameters)
Movement of points around an Axis
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Scale (1 Parameter)
Changing the distance between points
S
Transformation Effect
7 Parameters
B
A
A
B
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Transformations

• Different coordinate systems represent the same
  point in different ways
• Transformations convert points between coordinate
  systems

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2D Transformation

• Given a 2D object, transformation is to change
  the objects
• Position (translation)
• Size (scaling)
• Orientation (rotation)
• Shapes (shear)

x x ?x y y ?y
x x . Sx y y . Sy

• A 2D rotation is three shears
• Shearing will not change the area of the object
• Any 2D shearing can be done by a rotation,
  followed by a scaling, and followed by a rotation

x r cos (f) y r sin (f)
x r cos (f q) y r sin (f q)
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Projections

• Maps are flat
• easy to carry
• good for measurement
• scaleable
• Map projections are used to project data from
  a sphere onto a planar surface_

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Map projections
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Cylindrical Projections

• cylinder as developable surface
• an imaginary light is projectedonto a
  developable surface
• coordinate space becomes implicit

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Azimuthal Projections

• plane as developable surface

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Military Grid Coordinates
UTM Coordinates
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Projections

• Map projections always introduce error and
  distortion
• Distortion may be minimized in one or more of the
  following properties
• Shape gt conformal
• Distance gt equidistant
• True Direction gt true direction
• Area gt equal area

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Land-ownership boundaries
Plane rectangular grids

• Provides an easily used locational system that
  doesnt require corrections for the curvature of
  the earth
• Measurements are specified in terms of of
  distance from the origin where typically measured
  north and east of a point
• If the origin lies outside the area being mapped,
  it is called a false origin

• Generally referred to as cadastral surveys
• When the country was first settled,
  land-ownership patterns could develop in 2 ways
• Unsystematic subdivision (mapped with
  metes-and-bounds)
• Systematic subdivision (mapped with the Land
  Survey system)