Exercise 4: Display Images in a View
In this exercise, you will learn how images can be added to a view, how to use them as a backdrop, and how to modify their display.
Image data can come from photographs, remotely sensed data, scanned data, satellite data, and graphics. All images are raster-formatted data stored as a grid of columns and rows. Each row-column intersection is called a cell (or pixel). Associated with each pixel is a value. Pixel values represent reflectance values, such as the amount of energy being reflected by the earth in a satellite image or the brightness values of a scanned document. However, pixel values can be interpreted as colors or grayscales for display purposes in ArcView® GIS. Images can be used as source data for themes in ArcView. They have no associated features or attribute (theme) table.
Note: This exercise requires that you have access to a digital orthophoto of Winnebago County in MrSID format. If you are a DNR employee, you may have this on your local server. It has also been copied to the DNR Water Management Specialist GIS Training compact disc.
Step 1 – Start ArcView and open a project
If necessary, start ArcView. From the File menu, choose Open Project. Move to the c:\esritrn\arcview\wiscexer directory and open wiscexer4.apr.
When the project opens, you see a view called Winnebago County. This view already contains the Floodplain.shp theme.
Step 2 – Add image data sources (Digital Orthophoto)
First, you will add a digital orthophoto of Winnebago County using the MrSid Image Support extension. Orthophotos are generally compressed due to file size. This extension enables ArcView to view compressed files with the *.sid file extension.
From the File menu, choose Extensions… to display the Extensions dialog box. Scroll down until you see the MrSid Image Support extension. Move your cursor over the box on the left and ‘check’ it on. Click OK.
Click the Add Theme button to display the Add Theme file browser.
In the Data Source Types dropdown list, select Image Data Source. Then move to the r:\dnrview\wtm91_dops\winneb directory. Select Winneb.sid and click OK. (Note: If you are following this exercise from another location other than the UW-Madison Land Information and Computer Graphics Facility, you may find the digital orthophoto for Winnebago County in a different location. It may be on your local server or on the DNR Water Management Specialist GIS Training compact disc.
Image themes can be used as a backdrop for feature based themes. The order of display of image themes is important. Because these themes are represented by solid color cells, any data under the cell cannot be seen (unless the cell is transparent).
Turn on the Winneb.sid theme. Drag this theme to the bottom of the table of contents so that the Floodplain.shp theme displays on top of the orthophoto.
Next, change the fill of the Floodplain.shp polygons so that they the underlying features show through.
Double click on the Floodplain.shp theme to bring up the Legend Editor. Double click on the symbol 100 year fp to display the Fill Palette.
Scroll down and choose a diagonal hatch fill. Now click on the paintbrush at the top-right of the fill palette to invoke the color palette. Choose Background from the pulldown menu and then select the transparent symbol (the box in the upper-left corner marked with an X). Repeat these steps for 500 year fp and floodway. When finished, click Apply and close the legend editor and the color palette.
Step 3 – Modify the image display
Winneb.sid is a single band image. Other types of satellite imagery may contain multiple bands, where each band represents a particular wavelength range of energy reflectance from the earth’s surface. First, you will adjust the contrast of the image, and then change the color.
Turn off the Floodplain.shp theme. Double click on the Winneb.sid theme to display the Image Legend Editor.
In the Legend Editor, click the Linear… button to display the Linear Lookup dialog box. Try changing the contrast of the image by slightly adjusting the slope of the line or moving it slightly to the left or right. (A steeper slope increases the contrast; a flatter slope softens it. Moving the middle handle to the left increases the brightness; moving it to the right darkens the image.) Click Apply after each change to see the results. Make as many changes as you like.
When you are done, close the Linear Lookup dialog box and click Default in the Image Legend Editor. This will return the contrast to the default settings.
Step 4 -- Add image data source (DRG)
Next, you will add a 7.5 minute USGS quad map to the Winnebago County view. These are referred to as DRG’s (Digital Raster Graphic) and are *.tif files.
Click the Add Theme button to display the Add Theme file browser.
In the Data Source Types dropdown list, select Image Data Source. Then move to the c:\esritrn\arcview\wiscdata\ directory. Select o44088a7.tif and click OK.
Turn on the o44088a7.tif theme. Drag the Floodplain.shp theme to the top of the table of contents and turn it on so that it displays on top of the DRG.
Step 5 – Modify the image display
Now you will make the white and green pixels of the quad map transparent so that the underlying orthophoto is revealed.
Turn off the Floodplain.shp theme.
Double click on the o44088a7.tif theme to display the Image Legend Editor.
In the Image Legend Editor, click the Colormap button.
In the Colormap dialog box, double-click on the second (white) symbol to open the Color Palette. From the Color Palette, select the transparent symbol (the box in the upper-left corner marked with an X). Close the Color Palette window.
Double click on the sixth (green) symbol and make these pixels transparent also. Close the Color Palette window and click Apply in the Colormap window. Close the Colormap and Legend Editor windows.
Now you should see the orthophoto in the background with the contours, roads, etc. from the DRG displayed on top. If you have extra time, zoom in on the hill on the left. Try to identify the different color groupings that make up the contours and change them all to a uniform brown.
Step 6 – Close the project
Click on the Project window to make it active (or choose wiscexer4.apr from the Window menu). From the File menu, choose Close Project.
Click No when ArcView asks if you want to save your project.
END
4
Wisconsin ArcView Exercise #4 - 03/16/00