Introduction to Geographic Information Systems in Forest Resources
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Introduction to Geographic Information Systems in Forest Resources |
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Discussion:
One of the most powerful sets of functions in GIS is the ability to manage, display, and analyze surfaces. Raster analysis contains methods for analyzing surface data, as we have seen in previous lectures and exercises. However, although raster analysis contains methods for analyzing and displaying 3-dimensional surfaces, the graphical output is usually limited to planimetric display. A large amount of information is communicated with the use of 3D images.
ArcView's 3D Analyst Extension provides some powerful and impressive tools for analysis and display of 3D surfaces, as well as integration with traditional 2D raster and vector data sources. The 3D Analyst is an extension that adds support for 3D shapes, surface modeling, and real-time perspective viewing to ArcView. With it, you can create and visualize spatial data using a third dimension to provide insight, reveal trends, and solve problems.
Here is a typical 3D perspective view of Pack Forest elevation with streams and roads. The view has a 3x vertical exaggeration applied, which makes topographic features more distinct. For skilled map readers, it is easy to visualize the relationship of topography, roads, and streams as a landscape system. However, reading a topographic map requires certain knowledge and experience that not everyone possesses. Even for experienced map readers, an image like this conveys far more intuitive information than a flat paper map.
Frequently, numerical attributes are mapped with graduated color or size symbols to emphasize features with high values. Even these types of maps need some type of intellectual interpretation. The 3D scene is usually used to display elevation surfaces, but it can be used to display any kind of data which can be viewed as a response surface. Raster or TIN surfaces establish the 3D framework for the display of raster or vector themes. Vector data raster data sources can be draped over surfaces. Shapes can be extruded up or down, to imply volume height, or depth.
These two displays show essentially the same data in nearly the same classification style. However, the use of the third dimension adds more visual impact and communicative power.
Limitations exist for displaying choropleth (graduated color) maps. After about 5 classes are used, the map becomes unreadable, because the reader cannot discern among subtle changes in shade. With the 3D map, more subtle changes in feature extrusion can be displayed, which can convey more information than simple color classification. With feature extrusion, every different polygon can be compared, whereas in simple color classification, only the classes can be compared.
Some numerical attribute data are impossible to visualize without 3D display. Here is a view displaying US cities in a graduated color symbol based on the number of mobile homes within each city. The problem here is that the number of cities displays obscures the view, and some low value points draw atop high value points.
The same data have been interpolated to a grid with the Inverse Distance Weighting function. This view, looking from the north, shows places within the continental USA with large numbers of mobile homes are displayed with peaks, and those areas with few mobile homes are relatively flat.
A new ArcView document type: the 3D scene document
The 3D scene document The 3D scene GUI
Data sources for 3D display and analysis
Grid data sources TIN data sources 2D shapefiles and raster data 3D shapefiles
3D visualization
Legend editing Seeing 2D features in 3D by draping and extruding Navigation and moving around in real-time Real-time perspective viewing
Surface modeling
Contouring Profiling Color hillshade mapping Steepest descent Viewshed and visibility Slope and aspect calculation
A new ArcView document type: the 3D scene document
Along with the 3D Analyst comes a new ArcView document type: the 3D Scene. The 3D scene document is used to display data themes in 3D perspective. This document is similar in ways to the normal view document type, but rather than using a simple planimetric display, it uses a perspective rendering, in which themes can be draped over surfaces, tipped, turned, and tilted.
3D scenes are available at the bottom of the Project window after the 3D Analyst Extension is enabled. 3D scenes are different from normal map views, in that the 3D scene is contained in two different windows. The 3D scene Table of Contents is contained within the ArcView application window, but the scene viewer(s) are separate window(s) from the ArcView application window. A single 3D scene can be linked to several views of the same scene.
Note in this example the new document type listed at the bottom of the project window. In this project, there are 5 3D scenes. 3D Scene Mobile Homes is open, and it contains 2 viewers, which are both controlled by the same table of contents. The only difference possible between the different viewers is the zoom factor and position of the map data; the legend, and hence the symbology, for both viewers is controlled by the table of contents.
Several new tools and buttons are added to the 3D viewer GUI. Those tools and buttons also have matching menu controls.
The identify tool and zoom buttons act exactly as they do in a normal planimetric map view. The new tools are:
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selection of 3D shapes or other vector features on 3D scene viewer |
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selection of 3D graphical primitives on 3D scene viewer |
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navigation (zoom, pan, rotate, tilt) |
The new buttons are:
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animate 3D viewer scene with rotation |
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save 3D viewer as bmp or jpg |
The ArcView application window also contains a new button while a 3D Scene
Viewer is active.
The New Viewer button 
opens a new view of
the current 3D scene.
In order to display themes draped over surfaces, the first requirement is the existence of a surface model. A surface model is a data theme that represents a continuous surface. At any point in the surface model, there is a Z-coordinate value. That value may represent any numeric attribute. Most commonly, elevation is represented, but any numeric value may be used to represent a surface, such as median income, number of children per household, or number of cases of HIV.
Normal vector data are not good at representing surfaces. Neither lines nor points can be considered spatially continuous, because spatial gaps always exist among points and lines. Polygon data, although frequently spatially continuous, are generally not continuous with respect to attribute. The two best sources of surface model data are the grid and the Triangulated Irregular Network (TIN). The grid data model is covered in more detail in previous sections (Spatial Data Model; Raster Analysis I; Raster Analysis II).
The TIN data model has not been discussed up to now, as its strongest place is in surface display and analysis. What exactly is a TIN? As we know from geometry, a plane is defined by three points. The spatial orientation (slope and aspect) of a plane is determined by the elevation of each vertex of the triangle defining the plane's existence. Because there is a linear relationship between any two points on a plane, it is easy to determine the elevation of any point on a given triangular planar section. A complex surface can then be modeled by the use of a series of interconnected and non-overlapping triangles. Where surface features are more complex, there are generally a larger number of smaller triangles.
This diagram shows the basic arrangement of two triangles in a TIN. Each triangle represents a planar section with a constant slope and aspect. The X and Y coordinates of the vertices are not regularly spaced. The surface Z value of each vertex controls the absolute orientation of each triangle. Each triangle has a constant slope and aspect, regardless of the location on the triangle. However, unless a triangle is flat and level, elevation changes continuously across the triangle.
Where surfaces are less complex, there are fewer and larger triangles. In this way, a TIN is potentially a more economical and accurate surface model than a grid, since a TIN only needs to contain more data where the surface is more complex. A grid may under-sample complex parts of a surface and over-sample simple parts of a surface.
Here is a planimetric view of a TIN developed for Pack Forest, with elevation shading. The lines are triangle edges.
Here is the same area, in a planimetric view, with streams and 100-ft elevation contour lines displayed. If you look closely, you will see where some of the triangle edges are.
Here is the same area, draped in a 3D view over the TIN, looking from the north.
TINs are added to a view exactly as other theme types are added to views. When the 3D Analyst is enabled, TIN Data Source is an additional choice in the Data Source Types dropdown list.
Any supported vector or raster data sources can be used in 3D display and analysis, as long as they either (1) contain numeric values that can be interpreted and displayed as Z-coordinates, or (2) are displayed in conjunction with a surface model. Those feature themes that contain elevational attributes can be loaded into 3D scenes and placed in 3D space according to their Z-value. Shapefiles, grids, and images without Z-value attributes can be draped over existing surface models by obtaining a base elevation from the surface model. These can also be extruded or offset by a constant or by the value of an attribute.
Here is an orthophoto draped over a TIN, along with elevation contours and streams. All three themes take their elevations from the underlying TIN.
All of the data sources we have used up to now can be characterized by the combination of Cartesian (X and Y) coordinate data and relational-tabular attribute data. In some cases, the tabular data represent Z-coordinate values. For example, all of the grid data we have used can be thought of as the representation of a surface, with point samples taken in regular intervals of X and Y. For gridded DEM data, the value is indeed a true Z-coordinate. For other data, such as gridded line, point, or polygon data, the cell values are usually not representations of explicit spatial coordinates. However, any numeric value for grid data can be interpreted as a surface value.
Along with the 3D Extension comes a new shapefile standard; the 3D shapefile. Unlike other feature data, in which the features are stored in planimetric coordinates (X and Y), the 3D shapefile actually contains Z-values as part of the coordinate data (not simply as numeric values in the theme attribute table). 3D shapefiles can be displayed in three dimensions without the necessity for a supporting surface model.
Any 2D shapefile can be converted to a 3D shapefile. The Z-coordinate for the output 3D shapefile can come from a constant value, from a numeric attribute, or from an existing surface model data source (grid or TIN). Once converted to a 3D shapefile, this theme no longer needs a surface model to enable 3D display.
Legend editing for 3D scenes is similar to legend editing for 2D views. The table of contents is used to make certain themes active. Legends can be altered for active themes in the same way they are altered for other themes.
The exception to this is the legend for TIN themes. TIN themes can displayed as multi-feature themes. It is possible to display simultaneously triangle vertices (points), triangle edges (lines), and triangle faces (polygons). The TIN Legend Editor is used as an interface to the legend editors for each individual feature type within the TIN.
Clicking the Edit buttons for each feature type opens the Legend Editor for that feature type. These are exactly like the general Legend Editor for 2D feature themes, including symbol editing and classification.
Any features, whether they are raster or vector, 2D or 3D, can be viewed in a 3D scene. For display of landscape surfaces (elevations), draping is the most common 3D effect. Features are draped on a surface model, much like a cloth is draped over a wire frame.
In order to drape features, even if those features are part of the surface model defining the 3D view, it is necessary to specify the 3D Theme Properties for each draped theme.
3D theme properties control the base height of a theme, either as a constant, a numeric attribute or arithmetic expression, from a surface model, or from the Z-coordinates inherent in 3D shapefiles. Z-exaggeration can be applied, which multiplies every Z-coordinate by a constant value.
Features can be offset from the base elevation by a constant, by a numeric attribute or arithmetic expression. This can be used to float features over a surface (such as utility lines). Sometimes features do not display well right at surface level, so a slight positive Z-offset can make them stand out visually.
Features can also be extruded from the surface to create poles, wells, walls, and other geometric solids.
Features can be shaded to give them a more realistic 3D look. Finally, themes can be made not to draw during interaction. If 3D scenes are being altered by changing perspective, zoom, and tilt, they can be made not to draw until the scene is held in the same position for a given length of time.
Most of the images in this section up to now are surface drapes. In this 3D scene, the CFI plot outlines are not draped over a surface, so the background appears flat. CFI plot centers are shown twice, once as black points offset at a value proportionate to wood volume, and again as extruded lines at the height proportionate to the wood volume attribute.
It is easy to move around a 3D scene. Fast computers with good video cards will be able to draw and respond more rapidly to navigational control.
The Navigation tool is the controller for rotation,
panning, zooming, and altering the orientation of the surface. When this tool
is active, the pointer turns into a sailing ship (for navigation), a magnifying
glass (for zooming in and out), and a hand (for panning)
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click and hold the left button & move left or right |
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click and hold the right button & move the mouse
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click and hold both buttons & move up, down, left, or right. |
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(zoom in)
(zoom out) |
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move the cursor to a feature, hold down the <CTRL> key, then click the left mouse button | |
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move the cursor to the feature, hold down the <CTRL> key, then click the right mouse button | |
When the cursor is in the display area, during rotation, panning, or zooming, stopping the motion of the mouse stops the motion of the surface in the scene. To have the motion continue, move the cursor outside the viewer while keeping the mouse button pressed. Small movements of the mouse outside the viewer will initiate movement and allow it to continue (feature supported under Microsoft Windows platforms only).
Contouring is covered in Raster Analysis II.
The Spatial Analyst Extension includes a few functions for line of sight and visibility analysis.
The Surface Tools pulldown 
, whose Contour
tool was used before, also
includes the Line of Sight tool 
, which analyzes the
visibility of a line drawn between two points. Places of the landscape that
are visible along the line between the observer and target are shown in green,
and places that are not visible are shown in red. The blue dot is the automatically
placed at the first obstruction along the line of sight.
The Surface > Calculate Viewshed menu choice is used to determine what parts of the landscape are visible from a given point. This generates a grid which shows all areas that are visible, rather than simply what places are visible along a line between a target and observer. Here, all areas that are visible are green, and areas that are not visible are shown in red.
The Spatial analyst also enables another sample extension, the Visibility
Tools. This adds a few new GUI controls, another Line of Sight tool
and the Visibility
tool
. This Line of Sight tool
acts like the standard Line of Sight tool, except that it also generates a chart
showing the surface profile along the chosen line. Here is a line selected (starting
at the black dot), along with the surface profile along the line.
It is also possible to create surface profiles from existing features or graphics.
Graphics or line features are selected in a view, then the use of the Profile
Graph tool 
(in the Layout GUI)
creates a profile chart. (This surface profile was not generated from the line
above.)
The 3D Analyst includes powerful tools for visualizing landscapes. One of the most powerful is color hillshading. This displays elevation in user-defined colors, such as green > yellow > red color ramps, but also applies analytical hillshading to the view. This creates a visual model of the landscape in which features appear to have volume. Compare the two 3D scenes, one with analytical hillshading and one without:
The option for shading is specified by the check box in the Theme > 3D Theme Properties dialog.
The sun angle for analytical hillshading is set in the 3D Scene > Properties dialog.
The watershed delineation tools used in the section on Hydrologic Modeling allows the creation of
catchment areas. For any point within a watershed, it is known where the ultimate
outlet point is. But where does the actual water flow across the surface? With
a TIN surface model loaded and the only active theme in a view window,
the Steepest Path tool 
becomes available.
When a point is clicked on the view, a graphical line is added to the view,
following the path of steepest descent (the black dot at the starting point
is shown for emphasis only).
This graphical object can be copied and pasted in a new shapefile and added to a 3D scene.
Many physical processes and management objectives are related to slope and aspect. Slope is defined as the (change in elevation / change in planimetric distance). Sometimes slope is defined in degree measure, and sometimes in percent measure. Aspect is the compass direction of the slope, facing downhill.
For TIN surface models, slope and aspect are automatically calculated as attributes of each triangle. However, for elevation grids, slope and aspect are not native attributes. It is easy to calculate slope and aspect for elevation grids, using the Surface > Derive Aspect and Surface > Derive Slope menu choices.
ArcView calculates slope in degrees, which can be converted to percent slope by multiplying the tangent of the slope (measured in radians) by 100%.
Here is a view containing a slope grid. Notice that the steepest slopes are located in the valleys and canyons of the Mashel and Nisqually rivers.
Aspect classes are shown here. Note how aspect changes abruptly at stream channels and ridgelines.
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