Implicit vs' Explicit Representation

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Implicit vs. Explicit Representation
Geographic Information Systems for Resource
Management FW 5550 Lecture 4
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A triangulated irregular network (TIN) is a
vector format data structure used for the
representation of a continuous surface Composed
of irregularly distributed nodes and arcs with
3-D coordinates (x,y, and z) that are arranged in
a network of nonoverlapping triangles TINs are
often derived from a digital elevation model
(DEM)
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Using a TIN to model elevation Each triangle
within the TIN assumes a constant value
(elevation). Areas of little change (flat) will
have fewer and larger triangles. Areas of rapid
change (steep slopes) will have smaller, more
frequent triangles.
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Delaunay triangulation for a set of points in the
plane is a triangulation such that no point is
inside the circumcircle of any triangle. Delaunay
triangulations maximize the minimum angle of all
the angles of the triangles in the triangulation
they tend to avoid "sliver" triangles.
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Delaunay triangulation is a proximal method that
satisfies the requirement that a circle drawn
through the three nodes of a triangle will
contain no other node
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The triangles are as equi-angular as possible,
reducing numerical precision problems created by
long skinny triangles Ensures that any point on
the surface is as close as possible to a node
The triangulation is independent of the order
the points are processed
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Raster Data Models

• Most GIS data is 8-bit (switches) or 16 -bit.
• - 8-bit data allows for 256 different pieces of
  information
• - 16-bit data allows for 65,536 different pieces
  of information
• Commercial software packages can handle 32-bit
  data as well.
• Data may be signed or unsigned. Signed data
  indicates that the negative sign is meaningful.

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Land Use/Cover Files

• Land Use/Cover files are usually stored as 8-bit
  unsigned files. WHY?

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Digital Elevation Models

• Digital Elevation Models (DEM) use signed 16-bit
  data.
• WHY???

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Digital Elevation Model Data

• Sampled array at regularly spaced intervals of
  elevations for a number of ground positions.
  Produced as part of National Mapping Program
• - 1250,000 DEM (small scale)
• - 1100,000 DEM (intermediate scale)
• - 163,360 DEM (intermediate scale)
• - 124,000 DEM (large scale)
• Raster Data Structure
• SDTS format
• 124,000 DEMs
• Correspond to the USGS 124,000 and 125,000
  topographic quadrangle map series for the US and
  its territories. Pixel size - 30 meters or 10
  meters. UTM coordinate system.

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Level 1 DEM

• Elevations are created from scanned National High
  Altitude Photography (NHAPP or NAPP). The photos
  are registered to a geographic coordinate system
  and scanned with a terrain mapper which
  automatically notes changes in terrain.

• This procedure introduces striping and in some
  instances the striping may be so severe as to
  render the data nearly useless.
• RSME is /-15 meters

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Level 2 DEM

• Elevations have been processed or smoothed for
  consistency and edited to remove identifiable
  systematic errors.

• RMSE is /-½ of contour interval

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Level 1 vs Level 2
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Level 3 DEM

• Derived from DLG data by incorporating selected
  elements from both hypsography (contours and spot
  elevations) and hydrography (lakes, shorelines,
  drainage).
• Elevations for each pixel are then interpolated
  and checked for obvious problems.
• RMSE is /- 1/3 of a contour interval.