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					Identifying Potential Areas Of
  Relief Displacement In Soil
  Survey Mapsheets Using A
       GIS-based Model
         Eero M. Kilkson
                   Outline
• Soil Surveys
  – Base photograph
• Photogrammetry
  – Scale datum
  – Relief displacement
• PAL (Potential Area of displacement Locator)
• Applications
    Mapsheet On The DEM




Note: Stream flows uphill
     The Ubiquitous Question
“If soil surveys are not accurate
  representations of the soils within a single
  map unit, why should we care if the
  mapsheet is accurate?”
 The accuracy is appropriate for
   the scale of the mapsheet
• Soil surveys are cartographic products
   – compromising the accuracy of the boundary
     lines, compromises the integrity of the survey
• Including every identifiable soil type is not possible
  at the scale of the mapsheets
   – Minimum mapping unit is 2-3 acres
• County soil surveys are the product of many years
  of field mapping
• The soil survey is a legal document
   – used for tax assessment of agricultural lands
   Production Of A Soil Survey
• An understanding of the soil-landscape relationship
  is gained through field observations and core
  sampling
• Soil scientists use a rectified aerial photograph in
  the field to draw the boundary lines in association
  to the land features and their understanding of the
  soil-landscape relationships
• The boundary lines are transferred from the field
  sheet to a photobase
        U.S.D.A. Soil Surveys
• Soil polygons
  drawn on a
  photobase
• Base
  photograph
  can be:
  – Unrectified
  – Rectified
  – Orthophoto
                           USDA Soil Conservation Service
          Base Photographs
• Orthorectified
  – Perspective errors removed
  – Photogrammetrically best quality photographs
• Rectified
  – Tilt and tip corrected
  – Focus of PAL
• Unrectified
  – Uncorrected, unreliable
Status of MN County Soil Surveys
    Types Of Displacement in
      Rectified Photographs
• Radial displacement
  – Directly proportional to distance from the
    principle point (plum-line center of the
    photograph)
  – Less significant
• Relief displacement
  – Major source of error
  – Directly proportional to elevation difference from
    scale datum
               Scale Datum
• The horizontal plane at the elevation that the
  scale of the photograph is calculated
• Points with elevations above and below the
  scale will be displaced
• Set at the mean elevation of the landscape
Perspective View




                   By Raphael
      Relief Displacement
                                             • The top of the
                                               tower is farther
                                               from the scale
                                               datum than the
                                               base
                                             • On the
                                               photograph the
                                               top is at
                                               different
                                               geographic
                                               location than the
                                               base

From Photogrammetry by Moffitt and Mikhail
• Towers
  appear to
  lean
• The top of
  one tower is
  over the
  base of
  another
  tower




  MN DNR
Relief Displacement In A
        Landscape
              • Displacement can occur
                due to elevation
                differences in a landscape
              • The summit and toeslope
                will appear farther apart
              • Plateaus will be displaced
                even if they appear flat




             From Moffitt and Mikhail
 Displacement In A Soil Survey
• Both the soil polygons and the base
  photograph contain relief displacement
• In areas that contain relief, digitizing the
  mapsheet will transfer the relief
  displacement into the digital form
  – The lines will not be accurate
    Mapsheet On The DEM




Note: Stream flows uphill
Soil Survey Mapsheet
                    •The lines
                    correspond
                    with the land
                    features in
                    the
                    photograph
                    •The
                    photograph
                    contains
                    relief
                    displacement


                  USDA Soil Conservation Service
USDA
USDA
•Displacement
appears in the
valleys and the
flat field
•There is little
or no
displacement on
the valley walls,
the elevation of
the scale datum.




USDA
    How Can The Areas Of
  Displacement Be Predicted?
• Slope
  – Are steeper areas displaced?
• Opposing aspect
  – Can you predict stream valleys by finding
    opposite banks?
• Elevation difference from scale datum
  – Is the flat top of a butte displaced?
        Scale Datum Is Most
            Appropriate
• Not slope
   – The river bottoms and plateaus are flat, yet
     displaced
• Not opposing aspect
   – Stream bottoms are not the only land feature
     that are displaced
• Elevation difference from scale datum works
   – Identifies displacement potential independent
     from the feature type
                       PAL
• The Potential Areas of Displacement Locator
• Requires only a 7.5 minute quad Digital Elevation
  Model (30-m DEM)
• Provides a conservative estimate of areas that
  may be displaced
• Flexible
   – Mapsheet represent a cropped portion of a
     larger photo
   – Wide variety of photo sources (different
     flying heights…)
              PAL Features
• Uses 7.5 minute quad DEMs
  – Since flying programs plan flying height, focal
    length, and scale datum on a whole quad basis
• Provides the user default setting if details
  of flying program are not known
• Allows the user to enter settings if details
  of the flying program are known
                 PAL Procedure
Select appropriate DEM
       PAL Procedure
          Input DEM, select
          default values or
          enter preferred
          calculated values.
          Name the output file
          and run the program.



USGS
                 PAL User Interface




•   Background values default = 0
•   Default threshold = 12
        Threshold Calculation
• Threshold value represents the smallest elevation
  difference that would cause displacement of a point
  at the extreme corner of a 9”X9” photo
• Can be calculated by: d=rh/H
   –     d = relief displacement
   –     r = radial distance from center of the
     photograph
   –     h = ground elevation above datum
   –     H = Altitude of camera above datum
• Displacement should be calculated to be greater
  than National Map Accuracy Standard
       National Map Accuracy
             Standard
• “For published maps of scales larger than
  1:20,000, not more than 10 % of the points tested
  shall be in error by more than 1/30 inch, measured
  on the publication scale, for 1:20,000 or smaller,
  1/50inch” (U.S. Bureau of Budget)
• Not ideal for digital information but the standard
  to which the surveys adhere
• Most soil surveys are either 1:15,840 or 1:20,000
       PAL Procedure




                       Overlay
                                  USGS


                       the
USGS                   product
                       on the
                       DEM or a
                       DOQ
                  PAL On A DOQ




                                                       USGS



• Displacement can be expected in the valleys and the flat fields of
  the higher elevation
 PAL On A DEM




• Relief displacement can be found
  in areas that do not seem to be of
  high relief
PAL Will Flag Areas That May Be
            Displaced




                                  USDA
             Test Quads



• Welch Quad in Goodhue County, MN
• Dovray Quad in Sheboygan County, WI
Test Quads
              Dovray Quad

• In Sheboygan Co., WI
• Low relief topography with a gentle slope
  from the southwest to the northeast
• Elevation ranges from 430m to 492m
               Welch Quad

• In Goodhue Co., MN
• High relief landscape ranging from level to
  very steep
• Elevation ranging from 205m to 348m
        Method Of Evaluation
• Transform two copies of a scanned mapsheet using
  two different methods
   – Orthorectification (removes displacement errors)
   – Affine-transformation (does not remove
     displacement errors)
• Create a union of the soil boundary lines from the
  two transformations
• Identify sliver polygons {5*(perimeter)>=(area)}
• Evaluate PAL’s ability to predict displacement
Sliver Polygons

   Represent
   areas of
   displacement
                       Results
Welch Quad
Sliver Potentially Displaced Total Area Polygons (#) % of Slivers
Yes Yes                       119212.391       2385           100
Yes No                                 0           0            0



Dovray Quad
Sliver   Potentially Displaced Total Area   Polygons (#) % of Slivers
Yes      No                     26849.28771         422       22.086
No       No                     897982.4402         482 N/A
Yes      Yes                    94717.07982        1333       77.914
No       Yes                    3139663.669        1419 N/A
  Comparison of Quad Results
• Greater amount of displacement in Welch than in
  Dovray
• Due to the lesser degree of displacement, the sliver
  polygon in Dovray were much smaller and more
  numerous due to a braiding of the lines
• None of the sliver polygons that where not in the
  PAL potentially displaced area, had a width greater
  than 8m
   – All of the unflagged areas of displacement are
     acceptable under NMAS
                 Conclusion
• Relief displacement should be considered when
  using soil surveys or when digitizing soil surveys
• Use of the Scale datum is the most consistent
  method of predicting areas of displacement
• PAL is an easy to use and flexible method of
  accurately identifying areas of displacement in a
  mapsheet regardless of what is known of the
  original photography
          Applications Of PAL
• To assist soil survey users in warning of possibly
  displaced areas on map sheets
• To aid in the evaluation of whether in the process
  of digitizing, a county’s soil survey should be
  orthorectified or if an affine transformation would
  be adequate
• Since counties are updated at a rate of one per
  year, existing surveys must be used in the interim
    Future Areas Of Research

• Integration of PAL with an Soil Survey
  Orthorectification model
• Development of an easily distributed PC
  version
• Put PAL coverages on the WWW
                 Thank you
This project was funded by the LCMR with
assistance from the Board of Water and Soil
Resources

Thanks to Greg Larson

NRCS for information and assistance

				
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