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:
The topological overlay process is at recent addition to ArcView 3.x, although it lies at the core of the ArcInfo toolbox. In fact, topological overlay is what ArcInfo and its predecessors, such as Odyssey, were originally designed to do. Topological overlay allows us to ask questions like "Where are locations that are on unstable soils, with a slope in the range of 25-40%, that were harvested within the last 15 years, on low productivity sites, and what is their percentage of area with respect to the entire watershed?"
Topological overlay is a process whereby separate themes sharing the same spatial extent are merged. The landscape architect Ian McHarg developed an analog method, a precursor to the digital implementation within GIS . His approach was to take maps traced on sheets of acetate or mylar, and place one on top of the other, and tape the stack to a window or atop a light table. Areas of overlap will be darker than areas that do not overlap. The GIS performs in a similar manner, except that the input and output is more accurate and precise, and easier to manage.
The reason these operations are known as "topological" overlay is because the overlay process includes the rebuilding of the topological relationships that make themes function. In the GIS, where lines intersect between one theme and another, vertices are created. Where lines or points share the same space as polygons, the lines and points inherit the attributes of the spatially corresponding polygons. New themes are formed which can take on the attributes or coordinate properties of input data sets. Some or all features from the input data sets are passed on to the output. Attribute values from both input data sets are passed on to the output data set.
Topological overlay is different from the Select By Theme operations described in the last lesson. In the Select By Theme operations, ArcView 3.x only looks at the spatial overlap of the features of two themes, and creates a new selected set in one of those themes; no new data sets are produced; no attribute updates are made. In Topological overlay, new themes are created whose geometry and/or attribute structure are altered. Topological overlay allows us to find overlapping features, as well as to quantify the area or length of overlap.
Topological Overlay
Topological Overlay Types
Feature Geometry ValuesBuffering
In ArcView 3.x, topological overlay operations are accessible through the GeoProcessing Extension or other third party solutions, such as Mike DeLaune's Xtools extension. If you use the Xtools extension frequently, you should become familiar with the Xtools users guide.
With the GeoProcessing Extension, each of the different geoprocessing functions is accessed through a GeoProcessing Wizard, similar to other Windows wizards. Here are the dialogs associated with the Dissolve Wizard:
First, the wizard is activated. A series of radio buttons allows the user to specify which operation to perform. Each operation also provides a short description, as well as a graphical image depicting the operation. Help can be accessed for any operation by clicking the More about ... button
Once an operation is chosen, the user selects which themes to act upon, as well as other parameters for the operation. Because each operation is different, the dialog boxes will show different options based on the operation. Also, the dialogs will update to show only valid choices based on the type of operation.
The following cartoon themes will be used to illustrate the functionality of each different topological overlay operation. The sample themes are Ring and Box. Note that the Ring theme has a numeric attribute, while the Box theme has a string attribute. Where the numeric value is undefined, ArcView 3.x will place a 0; where the string value is undefined, there will be no value at all.
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Ring |
Box |
Each of the overlay operations results in new data sets. For output data sets, feature geometry is almost always modified. In cases where geometry is modified, it is possible to update geometry attributes (e.g., area, perimeter, length). the recalculation of geometry, plus the joining of user attributes, is what gives the power to the overlay operations.
Dissolve is the same function as the Spatial Join from Vector Analysis I.
When 2 themes are Unioned, all features from both input themes are combined. All attribute items from the input theme and overlay theme are included in the output. The order of inputs does not matter.
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Note how the null areas from each of the input themes are either null (for a character item) or 0 (for a numeric item). Also note how all input features have been retained, but new polygons have been created at intersections of input polygons. Attributes are merged where there are areas of overlap. Where there is no overlap, only the input attributes are preserved.
Here is an example of a union of some forest stand and soils polygons.
Note how all of the features from both original data sets persist in the output data set. Wherever there are areas of overlap, new polygons are created.
An identification of one of the polygons shows the output data set to contain attributes from both input data sets (stand attributes in red, and soils attributes in green).
The Identity function maintains all features of the input theme, but takes features from the identity theme that overlap with the input theme. The output theme's coordinate properties are dependent on which of the inputs is the identity theme.
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input theme |
identity theme |
output theme |
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input theme |
identity theme |
output theme
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Note how in the identity function, order of precedence is important. In places where there is overlap, all attributes have been joined. Where there is no overlap, only input attributes have been retained.
Here is an example of an identity performed on stands as the input theme with soils as the identity theme. The output theme has the same spatial extent as the original stands theme, but if you look carefully, you will see there are additional polygon boundaries, formed by the overlap of soils polygons on stand polygons.
The attributes are also joined in an identity, as shown in the results of this identify:
The Intersect operation is similar to the other overlay commands, but only areas which are common between the two inputs are included in the output. The order of input theme and intersect theme does not matter.
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The only areas that exist in the output are areas that are present in both inputs. For these areas, all attributes from both inputs are preserved.
Here are the same input data sets as before, with an intersect performed on the stands and soils themes. The output data set (in blue) is spatially limited to the area in common between the input themes.
The intersected data also have attributes from both input data sets.
Update replaces overlapping parts of the input theme with features from the update theme.
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As with Identity, order of precedence is important with Update. Only the attributes in the input theme exist in the output theme. Where there are no values for particular fields, the values will be null (blank for string fields, 0 for numeric fields).
Here, the soils data have been updated by the stands data. The new theme has new polygons from stands that have been added and overwritten any existing soils polygons (note the difference between the southern part of the original soils polygons and the newly updated theme).
Here is an identification of one of the new polygons in the southern part of the new data set. Polygons, or sections of those polygons, that existed in the original soils data retain their original attribute values, but those that have been updated lose their soils attribute values.
Clip clips out parts of the input theme with the outer shape of the clip theme. Only attributes from the input theme are retained.
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Clip's order of precedence also matters.
Here are the roads for the Pack Forest area, clipped by the administrative boundary. Only the original road attributes remain.
An intersection of roads and the boundary would have created a data set with the same spatial properties as this output, but boundary attributes would also have been placed in the output attribute table.
Erase removes parts of the input theme theme based on the spatial properties of the erase theme. Attributes from the input theme are passed to the output theme, and none of the erase theme's attributes are transferred to the output. As with some of the other topological overlay operations, the order of input and erase theme matters.
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Here is the stands theme erased, using the soils as the eraser. Any areas in the stands theme that overlap with soils polygons are erased. The output theme table has the same structure as the input (stands) theme table.
One of the main reasons for performing any type of topological overlay is to obtain quantitative measurements of the overlap between themes. Length of line features and area and perimeter of polygon features is an inherent property of the features. ArcView 3.x 's GeoProcessing extension does not automatically update length, perimeter, or area values within
There are several methods of updating feature geometry values within theme attribute tables. The most convenient methods use scripts embedded within extensions, such as the XTools extension.
It is also possible to calculate/update feature geometry values manually with the Field Calculator. The [Shape].ReturnArea calculation will get the area value of each record's shape. The [Shape].ReturnLength will get the length of line shapes or the perimeter of polygon shapes.
Buffering
Buffering creates polygons which are buffered a specified distance from input points, lines, or polygons. Buffers are used to locate and quantify the areas within a specified distance of existing features, such as the area of a riparian management zone, the no-touch zone around a spotted owl nest, or the possible area serviced by a water or sewer network. There are many options for using buffers, including variable-width buffers based on attribute items. For a full treatment, please see the on-line documentation.
Like the GeoProcessing Wizard, buffering also has its own wizard. The Buffer Wizard allows you to specify a number of parameters, e.g., the theme to buffer, the buffer distance or field, and whether to create a shapefile or simple graphical elements.
Here are a few examples of some buffers generated in ArcView 3.x
Concentric buffers around points, with creation of a new shapefile:
| section corners |  |
| buffered in 3 concentric 1000-ft rings, added to the view as a new shapefile |  |
Streams buffered by distance related with DNR Water Code:
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streams |
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buffer lookup table |
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variable-width buffer based on WA-DNR stream type, added to the view as a graphical element |
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boundary polygon |
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buffered both inside and out to 1000 ft; buffer added as a polygon graphical primitive (not a theme, but only simple graphics) |
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buffered outside to 500 ft; buffer added as polygon graphical primitive |
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The University of Washington Spatial Technology, GIS, and Remote Sensing Page is provided by the College of Forest Resources and the College of Ocean and Fisheries Sciences through Unit-Specific UIF. Site administrator: Phil Hurvitz. |
Copyright © Phil Hurvitz, 1998-2003 |
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