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3D analysis

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. 3D- surface Raster: image and grid

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3D analysis

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  1. 3D analysis Lecture 11 April 7, 2008

  2. 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, • ……

  3. Flat surface view 3D surface view

  4. 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

  5. 3D-surface Raster TIN

  6. 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

  7. 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

  8. 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.

  9. 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

  10. 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.

  11. 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

  12. 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.

  13. 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.

  14. 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

  15. 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.

  16. 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).

  17. Other conversions • raster to feature (vector) • raster to TIN • TIN to feature (3D) • TIN to raster

  18. 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.

  19. 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 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).

  20. 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

  21. 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.

  22. 3A analyst in ArcScene and ArcGlobe • Provides tools in ArcScene and ArcGlobe for visualizing 3D data, creating surfaces and analyzing surfaces

  23. 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->Programs->ArcGIS ->ArcScene.

  24. 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) 5 2

  25. 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 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

  26. ArcGlobe • Is part of ArcGIS, and is part of 3D analyst • launch the ArcBlobe from Start->Programs->ArcGIS ->ArcGlobe.

  27. Main references • ESRI book: Using ArcGIS 3D Analyst • ESRI visual campus: campus.ersi.com

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