3D analysis

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3D analysis

• Lecture 11
• April 7, 2008

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3D data and Z-value

• 3D data has a specified z-value, while 2D data
  does not
• Z-value can be
• elevation,
• rainfall,
• temperature,
• population,

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Flat surface view
3D surface view
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Types of 3D data

• 3D- surface
• Raster image and grid
• With a regular grid of locations (values stored
  in each grid cell)
• Image from remote sensors
• Grid created by geostatistic analyst
• TIN
• Irregular network (values stored at nodes)
• TIN created from vector data (mass points,
  breaklines, polygons)
• 3D- feature
• - shapefile
• - geodatabase feature class

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3D-surface
Raster
TIN
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TIN surface created from vector data

• To create a TIN surface, you start with a set of
  input points (point features, vertices of line or
  polygon features) and connect the dots.
• Once you have a TIN surface, you can always
  refine it to get a better model of natural or
  manmade features such as lakes, ridgelines,
  graded slopes, and other distinct formations. You
  can also tag triangle faces with attribute
  values, which allows you to symbolize a TIN not
  only by elevation, slope, or aspect, but by any
  other characteristic you like (vegetation, land
  use, and so on).

Rules of Delaunay triangulation methods 1. The
triangles are as equi-angular as possible, thus
reducing potential numerical precision problems
created by long skinny triangles 2. A circle
drawn through the three nodes of any triangle
contains no other input point 3. The
triangulation is independent of the order the
points are processed
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Some concepts in a TIN

• mass points
• - are the nodes from which triangles are
  constructed
• breaklines
• - are lines, telling there is a distinct change
  in slope on either side of line. They are used to
  represent surface formations like ridges,
  streams, dams, shorelines, and building
  footprints. Hard breaklines capture abrupt
  changes in a surface soft breaklines do not
  affect the shape of the surface (such as study
  area boundaries)
• replace polygons
• - create a flat area (a single elevation value)
  on a TIN surface. They are used to model
  formations like building foundations, terraces,
  water body, and other graded areas.
• clip polygons
• - Define a boundary for interpolation. Input
  data falls outside of the clip polygon is
  excluded from the interpolation and analysis
  operations.
• erase polygons
• - Define a boundary for interpolation. Input
  data falls within the erase polygon is excluded
  from the interpolation and analysis operations
• fill polygons
• - Fill polygons assign an integer attribute
  value to all triangles that fall within the fill
  polygon. The surface height is unaffected, and no
  clipping or erasing takes place. Fill polygons
  are used to represent continuous surface features
  like land cover and land use or discrete features
  like flood zones or endangered species habitats

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Breakline
Top Without breaklines, the triangles cross the
ridge of the dam. Bottom With breaklines (red)
along both sides of the ridge, the TIN is
retriangulated. No triangles cross a breakline
Top Simple mass point triangulation does not
adequately model the dam. Bottom The dam is
successfully modeled with breaklines.
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replace polygons
Top The blue polygon (a creek) will be added to
the TIN as a replace polygon. This is necessary
because the default triangulation wrongly
represents the area as sloped. Bottom The
replace polygon sides become triangle edges. The
area within the replace polygon has a constant
elevation (no slope).
Top Simple mass point triangulation does not
adequately model the creek. Bottom The creek is
modeled with a replace polygon
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clip polygons
Top The light blue polygon will be added to the
TIN as a clip polygon. Middle The TIN is
clipped to the polygon extent. Bottom Clipping
does not actually change the extent of the
triangulated area, only the zone of
interpolation. By default, triangles outside the
zone are not displayed, but they can be turned
on, as they are here.
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erase polygons

• Top The light blue polygon will be added as an
  erase polygon. Middle The polygon area is cut
  out of the TIN (excluded from the zone of
  interpolation). Bottom As with a clip polygon,
  the uninterpolated area is still triangulated.
  The erase polygon sides become triangle edges

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fill polygons
Top The polygon layer will be added to the
TIN as fill polygons. Middle The TIN is
retriangulated. The blue lines, indicating
polygon boundaries, become triangle edges. (They
look wavy, but they are straight line segments.)
Bottom The TIN is symbolized by the polygon
attribute values.
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3D-feature

• 3D feature is used to display discrete geographic
  features (like buildings, rivers, and wells) on
  or beneath surfaces. 3D features can be stored in
  shapefiles or geodatabase feature classes
• In ArcScene, you can also render 2D features in
  3D by manipulating their layer properties

3D feature classes can be identified by the ZM
values in the Shape field of their attribute
tables.
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Create 3D features

• 3D features differ from 2D features in that they
  store a z-value as part of their spatial
  definition. 3D features
• - can be converted from existing 2D features,
  or
• - can be created from defining a new feature
  class to be 3D when you create it

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Converting 2D to 3D

• To convert a 2D layer to 3D, you need z-values.
  There are three ways to get z-values
• - From a raster or TIN layer that shares a
  common spatial extent with the 2D features
• - From an attribute in the 2D layer attribute
  table
• - By typing a value (which is then applied to
  all features in the 2D layer)
• If the 2D layer is a point layer, each feature
  gets a z-value. If it is a line or polygon layer,
  each feature vertex gets a z-value.

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Creating and digitizing 3D features

• create a feature layer and specify it store
  z-values in ArcCatalog
• digitize the feature layer in an ArcMap edit
  session if your map document contains a raster or
  TIN layer
• The 3D digitizing tools (one for points, one for
  lines, and one for polygons) are located on the
  ArcMap 3D Analyst toolbar.
• Digitized 3D features have a z-value for every
  vertex you digitize. Just like features that are
  converted from 2D to 3D, they also have vertices
  at cell-size intervals (if you are digitizing on
  a raster) or where features cross triangle edges
  (if you are digitizing on a TIN).

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Other conversions

• raster to feature (vector)
• raster to TIN
• TIN to feature (3D)
• TIN to raster

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Raster to TIN

• When a raster is converted to a TIN, a certain
  number of raster mesh points become nodes in the
  TIN. (A mesh point is a location where four cell
  corners meet.)
• The number of mesh points used to create the TIN
  is the smallest number that satisfies two
  conditions. First, the output TIN must cover the
  entire surface area of the input raster. Second,
  a user-specified z-tolerance must be met. The
  z-tolerance is a number that limits z-value
  differences between the input and output
  surfaces.
• A large z-tolerance allows the TIN surface to
  conform less closely to the raster. The output
  TIN has fewer nodes and triangles and the
  conversion process is faster. A small z-tolerance
  makes the TIN conform more closely to the raster.
  The TIN has more nodes and triangles and takes
  longer to process.

Left The background raster has been converted to
the foreground TIN using a z-tolerance of 50
units. The output TIN has 169 nodes and 269
triangles. (Only nodes and edges are symbolized.)
Right The same raster is converted using a
z-tolerance of 25 units. The output TIN has 328
nodes and 563 triangles.
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TIN to raster conversion

• To convert a TIN to a raster, all you need to do
  is choose a cell size, values of the TIN surface
  can then be interpolated at regularly-spaced
  intervals across the surface.
• As you make the cell size smaller, more points
  are interpolated and the output raster resembles
  the input TIN more closely.
• A TINs slope and aspect values can also be
  converted to rasters.

Left A 2D view of a TIN layer. Right A raster
converted from the TIN. Since a rasters extent
must be rectangular, areas that are not
interpolated are assigned the NoData value
(symbolized in gray).
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TIN to Feature

• TIN layers can be converted to two different
  kinds of point layers and three different kinds
  of polygon layers. Converting TIN data to
  features allows you to use it in ArcMap for
  feature analysis operations like buffer,
  intersect, clip, spatial join, and select by
  location.
• Nodes to points (data nodes only)
• - Triangle nodes are converted to 3D point
  features. The point features correspond to nodes
  within the TIN zone of interpolation
• Nodes to points (all nodes)
• - Triangle nodes are converted to 3D point
  features. The point features correspond to nodes
  inside and outside the TIN zone of interpolation.
  (For instance, if you clip a TIN and then convert
  all nodes to points, you will get points that
  were nodes in the original unclipped TIN.)
• Interpolation zone to polygon
• - The boundary of the TIN zone of interpolation
  is converted to a single polygon feature

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• Triangles classified by slope to polygons
• - Triangles are converted to polygons with
  attributes that represent a slope classification.
  By default, the TIN slope renderer groups
  triangles into nine classes. The conversion
  process creates a polygon layer with attributes
  ranging from 1 to 9.
• Triangles classified by aspect to polygons
• - Triangles are converted to polygons with
  attributes that represent an aspect
  classification. By default, the TIN aspect
  renderer groups triangles into ten classes (N,
  NE, E, SE, S, SW, W, NW, and N again, plus a
  class for flat slopes). The conversion process
  creates a polygon layer with attributes ranging
  from 1 to 9 for the directions, plus 1 (flat).

Left A TIN symbolized in ArcMap with the aspect
renderer (hillshade illumination is turned
off). Middle The TIN converted to polygons
classified by aspect. Right The polygon layer
symbolized with the aspect color ramp.
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3A analyst in ArcScene and ArcGlobe

• Provides tools in ArcScene and ArcGlobe for
  visualizing 3D data, creating surfaces and
  analyzing surfaces

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ArcScene

• Is part of ArcGIS, and is part of 3D analyst
• launch the ArcScene from 3D View Tools of
  ArcCatalog, 3D Analyst of ArcMap, or
  Start-gtPrograms-gtArcGIS
• -gtArcScene.

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ArcScene scene property

• Scene property
• - vertical exaggeration
• vertical exaggeration is a purely visual effect
  and does not influence analysis
• - illumination
• azimuth and sun altitude (elevation)

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ArcScene Layer property

• Layer property
• - base heights
• the elevation values that are used to display a
  layer in 3D. 3D feature layers use their
  Z-values, TIN use their node values, raster use
  cell values. Layers which do not store elevation
  information2D feature layers and image
  rastersborrow their base heights from a TIN or
  elevation raster
• - extrusion
• extrusion is three-dimensional extension for
  features. An extruded point becomes a line an
  extruded line becomes a wall an extruded polygon
  becomes a block

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ArcGlobe

• Is part of ArcGIS, and is part of 3D analyst
• launch the ArcBlobe from Start-gtPrograms-gtArcGIS
• -gtArcGlobe.

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Main references

• ESRI book Using ArcGIS 3D Analyst
• ESRI visual campus campus.ersi.com