Georeferencing and Spatial Adjustment

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Georeferencing and Spatial Adjustment

• Georeference
• The process of defining how raster is situated
  in map coordinates. Georeferencing raster data
  allows it to be viewed, queried, and analyzed
  with other geographic data.
• Spatial adjustment lets you transform,
  rubbersheet, and edgematch features in your map

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Geometric Transformation

• Geometric transformation is the process of
  converting a digital map from one coordinate
  system to another by using a set of control
  points and transformation equations.
• There are several types of transformations

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Control Points

• Links that define common points in each
  coordinate system.
• The corners of a map are very often used
• A USGS 7.5 minute quad has 16 tic marks that can
  be used to georeference a map
• Control points should be distributed over the
  area of interest .

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Equiarea Transformation

• Equiarea (congruence) Transformation
• The method allows rotation of the rectangle and
  preserves its shape and size.

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Similarity Transformation

• Similarity Transformation
• The method allows rotation of the rectangle and
  preserves its shape but not size.

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Affine Transformation

• Affine Transformation
• The method allows angular distortion but
  preserves the parallelism of lines.
• While preserving line parallelism, the affine
  transformation allows rotation, translation,
  skew, and differential scaling on the rectangular
  object.

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Projective Transformation

• Projective Transformation
• The method allows angular and length distortion,
  thus allowing the rectangle to be transformed
  into an irregular quadrilateral.

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Topological Transformation

• Topological Transformation
• The method preserves the topological properties
  of an object but not shape, thus allowing the
  rectangle to be transformed into a circle.
• Also called rubber sheeting

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First Order Polynomial Transformation

• Affine transformation
• x Ax By C y Dx Ey F
• A Sxcos(t)
• B Syk cos(t) sin(t)
• D Sxsin(t)
• E Syk sin(t) cos(t)
• C Translation in x direction
• F Translation in y direction
• k sheer factor Skew angle arctan(k)
• Sx Scale factor in x direction
• Sy Scale factor in y direction

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Matrix Estimation of Transform Coefficients
C F A D B E
N Sx Sy Sx Sx2 Sxy Sy Sxy Sy2
-1
SX SY SxX SxY Sy SyY

.
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RMS Root Mean Square error

• RMS for a tic sqrt((xact xest)2 (yact
  yest)2)
• Averaage RMS sqrt((S(xact,i-xext,i)2
  S(yact,i yest,i)2 /n)

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Higher order polynomial transformation

• Named for the largest exponent in the
  transformation equation.
• ArcMap supports first to third order
  transformation.
• Command line supports first to twelfth order
  transformation.
• Use the lowest order transformation that provides
  acceptable results
• Number of control points required is
• N (p 1)(p 2) / 2

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Orthorectification

• Process for correcting aerial an satellite
  imagery
• Correct for viewing angle, elevation distortion,
  and camera geometry
• Requires ground control points, camera report,
  and DEM.
• ArcGIS transformation methods do not correct for
  elevation distortion
• ArcGIS does not support Orthorectification. If
  you need to Ortho correct an image use another
  product like Erdas Imagine, GRASS

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Georeferencing Process

• Create Links
• Look at the link table for acceptable RMS on each
  Point.
• Remove links with error that is to high
• It is better to keep a link that you are sure is
  correct. If a link has a high RMS but is
  correctly located on the image removing it will
  improve the reported RMS but add error to the
  map.
• Select the order of transformation
• Rectify the map selecting the resample method

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Resampling Process
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Spatial Adjustment
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Address Geocoding

• More than 80 of all data stored in the world has
  a spatial attribute
• The vast majority of that data uses an address as
  its spatial attribute
• Address Geocoding is the process of converting an
  address to a location in map space.

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Requirements for Address Geocoding

• A street coverage with address ranges
• A database with addresses in a form the geocoding
  engine can parse.

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Geocoding Process

• The Geocoding engine searches the data for
  candidate address
• The candidate address are scored
• The candidate with the best score is added to the
  map if that score is greater than the minimum
  match score