Extensions
Extensions provide add-in functions for ArcView. Some extensions are included with the basic ArcView distribution; others are sold separately. This section discusses the simple image viewer extensions, the Geoprocessing extension, and the CAD Reader extension.
To load an Extension, use File--Extensions and check one or more desired Extensions.
Extensions and Project Portability
It's best not to load Extensions that your Project doesn't need. A Project file specifies all the extensions that were loaded at the time it was saved. This can affect the transferability of Projects. If you save a Project with Extensions loaded, and then try to transfer that Project to a second computer with the same data in the same directory structure, ArcView may fail to open the Project correctly if it can't find the specified Extensions. Also, unneeded Extensions just take up system overhead.
Basic Image Extensions
The basic ArcView distribution includes various extensions that read specific image formats. The separate Image Analyst extension handles additional formats used for distributing satellite imagery and airphoto data. Extensions to basic formats include:
Images are useful as backdrops for shapefiles, and are particularly convenient for on-screen digitizing.
Some images store all color information in a single band; others store intensity values for specific colors in multiple bands. ArcView handles either type of image pretty intelligently. When displaying multi-band images in a View, ArcView lets you specify which band should display as red, which as green and which as blue.
Adding an Image to a View
To add an image to a View, first make sure the necessary image Extension
is loaded (with File--Extensions). Then View--Add Theme
or click the Add Theme button 
, choose "Image Data Source" from the Data Source Type pull-down menu,
navigate to the appropriate directory and select one or more images to
load (hold down the Shift key to select multiple images to load at once).
To display shapefiles on top of images, drag the image Themes below the
shapefile Themes in the View legend.
Note that you can add a grid file as a single-band image Theme in your View even if you don't have ArcView's Spatial Analyst Extension loaded. In such cases, you may have to specify an appropriate color table with which to display the Theme. To do so, double click the Theme in the View legend to access the Image Legend Editor. The Image Legend Editor lets you adjust the display of single- or multi-band files in various ways. You can specify which bands in a multi-band file should be displayed as red, green and blue, respectively. The "Linear..." button lets you alter the linear lookup function for each image band by dragging the "handles" for each band's transform line.
In the example above, the Linear Lookup functions for the image covering the left portion of the View have been altered: pixels with zero or low green values have had their green intensity increased, and pixels with low blue values have had their blue intensity reduced or forced to zero. While the image's default color scheme is like that of the image in the right portion of the View, this adjustment gives it a green tinge. You can enhance the contrast of an image by saturating the extreme values: shift the band transforms so that low values are forced to zero and high values are forced to the maximum (255).
Click the "Colormap..." button in the Image Legend Editor to access the Image Colormap Editor screen. The Image Colormap Editor lets you apply a "Ramp," "Random," "Gray" or "Nominal" color table to the image, and lets you "Adjust..." the Red, Green, Blue, Saturation and/or Intensity levels in an image.
Image Registration in ArcView
To be useful in a GIS, images need to be registered to geographic coordinates so that they will line up with other Themes. Although images can be registered to lat-long coordinates, lat-long image pixels are not square or uniformly sized. Most images are registered to a specific projection, and the projection information should always be supplied with the image.
For
example, here is a View of the University of Delaware's campus center in
Newark. Since the campus center spans the Newark East and Newark
West (DE) 7.5-minute quads, the View is a zoomed-in patch of two adjacent
1992 color-IR digital orthophoto images names newarkw_se.jpg and
newarke_sw.jpg.
These are 24-bit color images, so ArcView treats them as 3-band images.
Since the color schemes are pretty consistent, the patch edge where the
west image overlaps the east near the right of the View frame is barely
discernible. A shapefile of land-use/land cover polygons has been
overlaid with yellow outlines and transparent fill.
ArcView aligns an image with other Themes in the same View projection via parameters specified in a text world file that accompanies the image data file. Each image has a companion world file. For example, the world file for newarke_sw.jpg is named newarke_sw.jgw, and is shown below with annotations. It contains just six parameters defining the affine transformation of the raw image to the View coordinates:
1.000 --
pixel x-dimension in map units (meters)
0.000 --
row rotation factor
0.000 --
column rotation factor
-1.000 -- pixel
y-dimension in map units (meters), counting down from the image
origin at the upper left corner.
435600.500 -- x coordinate of center
of upper-left pixel (UTM-18N NAD83 meters)
4393449.500 -- y coordinate of center of
upper-left pixel (UTM-18N NAD83 meters)
You can create or edit a world file for any image with a simple text editor. Check the on-line help for ArcView's world file naming conventions so that ArcView will access the world file automatically when you try to load the image file.
While no projection is specified for the View itself here, all the Themes are in the same UTM-18N NAD83 (meters) projection. Remember that ArcView cannot reconcile inconsistent projections, but it can overlay lat-long shapefiles on projected images if that projection is specified in the View.
Image Catalogs
If you have many images that tile together to cover an area, you may want to experiment with an image catalog, which is just an ArcView Table that specifies the name and extent of each image. Create a new Table with five fields:
The GeoProcessing Wizard
ArcView's geoprocessing tools perform sophisticated spatial logic operations on shapefiles--operations that can only be done in a GIS. These tools treat maps like elements in Venn diagrams, and extract features based on any logical relationship: AND, OR, NOT, XOR.... As with Venn diagrams, you can often achieve the same result by various sequences of different operations.
Load the Geoprocessing Extension, then access the Wizard via View--GeoProcessing
Wizard. Each of the listed tools creates a new Theme from
one or more input Themes. From the main Wizard window, select the
operation you want to perform; the Wizard explains and guides you through
the operation. (You will see later on that most of these standard
spatial logic operations that are more easily performed with grids using
Spatial Analyst.)
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Dissolve polygon or line features based on an attribute. This is the same operation as a Summarize including a "Shape" field Merge: all features sharing the same value of the specified attribute are combined into a single feature. Specify the shapefile to be dissolved, the Attribute field on which to do the dissolve, and the path and name of the new shapefile that will store the combined features. Specify other attribute fields to be merged, and a merge rule (sum, average, etc.) for each field. Note that Dissolve will combine all features with the same value in the specified field even if they aren't contiguous. |
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Merge features from two or more shapefiles (of the same type) together into a new shapefile. You can structure the output Theme's attribute table based on one of the input Themes: then if other input shapefiles have the same attribute fields, the utility copies the field values for their features to the equivalent features in the output shapefile. Non-matching attribute fields are omitted from the merge. This utility is mostly meant for patching together Themes covering adjoining areas. Specify the shapefiles to be merged, the input shapefile on which to model the output shapefile's attribute table, and the path and name of the new shapefile that will store the merged data. |
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Clip selected or all features in an input line or polygon shapefile with selected or all features in a polygon overlay shapefile. This utility uses the overlay shapefile as a "cookie cutter," and creates an output shapefile that contains the portions of the input shapefile's features that fall within the selected (or all) polygons of the overlay shapefile. Specify the input and overlay shapefiles, and the path and name of the new shapefile that will store the clipped versions of the input shapefile's features. |
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Intersect selected or all features in an input line or polygon shapefile with selected or all features in an overlay polygon shapefile. This utility splits the features in an input shapefile with the features in the overlay shapefile; only the split features fall within the spatial extent of both parent shapefiles are retained. Each new split feature derives attributes from both its parent features. Specify the input and overlay shapefiles, and the path and name of the new shapefile that will store the intersection features. |
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Union selected or all features in an input polygon shapefile with selected or all featurees in an overlay polygon shapefile. This utility splits the input features by the overlay features and retains all the split features with two parent features, as well as features with only one parent feature. The output shapefile has attribute fields from both input shapefiles. Specify the input and overlay shapefiles, and the path and name of the new shapefile that will store the union features. |
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Assign data by location (same as spatial join). This utility assigns attribute values from features in an input (source) shapefile to the point, line or polygon features in an overlay (destination) shapefile that are nearest to or inside the input features. The spatial join rules for "nearest" and "inside" apply. Specify the shapefile to assign data from, the shapefile to assign the data to. The Wizard will report the join rule to be used, and add the new data to the destination shapefile attribute table. |
The CAD Reader Extension
As noted in the beginning of this course, vector GIS evolved from CAD technologies, and there are many applications where GIS and CAD overlap. ArcView supports the import and processing of some standard CAD formats, including native AutoCAD drawing files (.DWG), and standard CAD interchange files (.DXF).
To import a CAD file to a View, load the Cad Reader extension via File--Extensions.
Then View--Add Theme or click the Add Theme button
and select the appropriate .DWG or .DXF directory. Note that while
an ArcView shapefile can have only one type of feature (polygon, line or
point), a CAD file may contain multiple feature types in different layers.
ArcView treats a multi-layer .DWG or .DXF file as a directory with separate
point, line, polygon and annotation files. Add all the feature types;
you will see the CAD file added as a single Theme to your View.
Now use the "Drawing" window within Theme--Properties to list all the layers in the CAD Theme and select the particular layer you want. If you want more than one layer from a CAD Theme, use Edit--Copy Themes to copy the CAD Theme and select another layer from it. Use Theme--Properties to rename these Themes so you don't get them confused. Once you get the layers from a CAD Theme added to your View, you can treat them like conventional shapefiles. To convert a drawing layer to a shapefile, use Theme--Copy to Shapefile.
If your CAD drawing doesn't line up with other Themes in the View, you can create a (text) world file matching X-Y points in the drawing to X-Y points in the View. Then specify the world file in the "Drawing" window under Theme--Properties. If you specify a single point in the world file, ArcView simply shifts the drawing without rescaling or rotation. If you specify two points, ArcView displays an affine transformation of the drawing.