3D analysis
Lecture 11
April 7, 2008
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,
……
Flat surface view
3D surface view
Types of 3D data
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3D- surface
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Raster: image and grid
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TIN
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With a regular grid of locations (values stored in each grid cell)
Image from remote sensors
Grid created by geostatistic analyst
Irregular network (values stored at nodes)
TIN created from vector data (mass points, breaklines, polygons)
3D- feature
- shapefile
- geodatabase feature class
3D-surface
TIN surface created from
vector data
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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
Some concepts in a TIN
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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
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.
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
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.
erase polygons
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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
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.
3D-feature
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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.
Create 3D features
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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
Converting 2D to 3D
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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 zvalue. If it is a line or polygon layer, each feature
vertex gets a z-value.
Creating and digitizing 3D
features
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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).
Other conversions
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raster to feature (vector)
raster to TIN
TIN to feature (3D)
TIN to raster
Raster to TIN
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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 ztolerance 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.
TIN to raster conversion
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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 TIN’s 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 raster’s extent must be rectangular, areas that are not
interpolated are assigned the NoData value (symbolized in gray).
TIN to Feature
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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.
3A analyst in ArcScene and
ArcGlobe
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Provides tools in ArcScene and ArcGlobe
for visualizing 3D data, creating surfaces and
analyzing surfaces
ArcScene
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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->Programs->ArcGIS
->ArcScene.
ArcScene: scene property
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Scene property:
- vertical exaggeration
vertical exaggeration is a purely visual effect and does not influence analysis
- illumination
azimuth and sun altitude (elevation)
5
2
ArcScene: Layer property
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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 information—2D feature
layers and image rasters—borrow 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
ArcGlobe
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Is part of ArcGIS, and is part of 3D
analyst
launch the ArcBlobe from Start>Programs->ArcGIS
->ArcGlobe.
Main references
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ESRI book: Using ArcGIS 3D Analyst
ESRI visual campus: campus.ersi.com