Geography Reference
In-Depth Information
13. Turn on Auto Adjust. Poof. The image disappears. Dismiss the Link table. Select Zoom To
Layer on the TIF file. If you have done everything right, you will see the image, undistorted.
By sliding the cursor around, you can assure yourself that the new coordinates are now being
used. Change the display units to kilometers. Use the Measure tool (also set to kilometers) to
determine the dimensions of the square island in kilometers. _________ by _________. Does
this agree with the UTM coordinates? _________.
Digitizing the Line Boundaries of the Islands
14. View the Link table. Note that the Link Table shows residuals—probably of a few hundred
meters. Like in other transformations this is a measure of how much “less than perfectly
linear” the transformation was. Dismiss the Link Table. Turn off the Georeferencing toolbar.
15. Click Editor. Click Start Editing. Click Create Features. You will create new features with the
Line Construction tool. Bring up the Snapping tool bar. Set Snapping to Snap To Sketch
(on the drop-down menu). Make sure Ends will be snapped to. Under Options, set the snapping
tolerance to 5 pixels.
16. Digitize the square island and the square lake inside it. Open the attribute table of
North_Islands_Lines. The Shape_Length of the outer line should be about 120,000 meters.
What is it? ____________. What is the length of the inner line? ____________. Dismiss the
attribute table.
17. Digitize the other islands. This will be easier and more accurate if you zoom in on each island
to do the digitizing. The island in the southeast has a lake within it, an island in the lake, a
volcano on that island, and a cauldron defined within the volcano. Each time you zoom or pan
you will have to choose the Construction Tool again.
18. Click Save Edits. Make the feature file line symbol a bright color of width 2, zoom to the layer's
extent, and check that everything has been digitized. If not, complete the editing. Click Save
Edits again. Stop editing. Turn off the TIF file; it has served its purpose. Save the map under
Digitize&Transform as North_Islands_map1. Dismiss ArcMap.
Making Polygons of the Digitized Lines
The next task is to make polygons based on the lines you have digitized.
19. In ArcCatalog, be sure you can see the contents of the feature dataset UTM_Zone_2. (This may
require that you highlight the entry and press F5 to Refresh the Catalog Tree.) Open ArcToolbox.
Expand Data Management Tools > Features and then pounce on Feature to Polygon. From the
T/C, drag North_Islands_Lines into the input area. Name your feature class North_Islands_
Polygons and make sure it is being saved to the UTM_Zone_2 Feature dataset within the
Islands geodatabase. Click OK twice and, when the process is complete, examine the results
in the Preview tab. (If not all the polygons show up you may have failed to close one or more
lines in your digitizing. Lines to polygon only works on completely closed figures. You will need
to go back and fix the problem. Zoom up on each island to see if there are any gaps in the lines
surrounding the features. Snapping to the sketch plays an important role in making sure that
lines close completely.)
 
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