Sep. 2122, 2006

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v
Sep. 21-22, 2006
FME Worldwide User Conference Vancouver
Reprojection Problems Art or Error? Dean Hintz,
Safe Software
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ASRP Mixes Cartesian and Polar LL84
All ASRP LL84 coords, But zone 1,8 are polar
projections
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Our Fix
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Need for Degrees to Meters conversion Based on
Latitude

• The Problem
• 1 degree can represent radically different
  distances on the earths surface depending on
  where you are.
• This can become a problem when defining a spatial
  reference. Image source resolution usually based
  on ground distance.
• SDE rastermap first tile in sets the resolution.
  
• For example, at the equator
• 1 degree lat 110574m
• 1 degree long 111319m
• At 50N
• 1 degree lat 111229 m
• 1 degree long 71696 m

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Source ASRP UKH601
Sample source ASRP UKH601
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SDE mosaic Northern tile is loaded first
UKH601 in SDE mosaic when EUH805 is loaded first
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SDE Mosaic Southern tile is loaded first
UKH601 in SDE mosaic when UKH601 is loaded first
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Approaches Considered

• Solution 1 force most southern tile to load
  first, or always use equitorial resolution.
• Solution 2 Calculate resolution based on minimum
  latitude

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Step 1 Calculate meters per degree for minimum
latitude

• rlat reference latitude in radians reference
  latitude pi/180
• At 50 degrees N
• rlat 50 pi/180 0.872665 rads
• Meters per degree Longitude
• 111132.92 - 559.82 cos(2 rlat)
  1.175cos(4rlat)
• 111132.92 - 559.82 cos(2 0.872665)1.175cos(
  40.872665)
• 111132.92 97.21 - 1.10 111229.0 m/degree
  latitude
• Meters per degree Latitude
• 111412.84 cos(rlat) - 93.5 cos(3rlat)
• 111412.84 cos(0.872665) - 93.5
  cos(30.872665)
• 71614.79 80.97 71695.8 m/degree longitude

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Step 2 Calculate degrees per meter for desired
resolution

• Meters Degrees (meters per degree)
• Degrees Meters / (meters per degree)
• DegreesX 100m / 71696 m per degree
• 0.00139 degrees / 100 m per degree
• DegreesY 100m/111229 m per degree
• 0.000899 degrees / 100 m per degree
• Desired resolution at 50N for 100x100m pixel is
• X 0.00139, Y 0.000899

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Resampler Workspace
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SDE mosaic tiles resampled to southern most
resolution
UKH601 in SDE mosaic when all tiles are resampled
to UKH601 resolution.
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Theory References

• Converting UTM to Latitude and Longitude (Or Vice
  Versa) Steven Dutch, Natural and Applied
  Sciences, University of Wisconsin - Green Bay
• http//www.uwgb.edu/dutchs/UsefulData/UTMFormulas.
  htm
• An on-line calculator implemented with Java
  Script can be seen on the Federation of American
  Scientists website at
• http//www.fas.org/news/reference/calc/degree.html
  
• An example function implementation from Physical
  Oceanography, U of California, San Diego
• http//www-pord.ucsd.edu/matlab/coord.htm
• .

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Global Projections

• Mercator Demo
• Canada as the centre of the Universe

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Mercator - default
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Mercator - Clipped