Geography Reference
In-Depth Information
small box around the red square until you see a line of reasonable length. Release the Z key to
return to the sketch tool.
5. Make a few random vertices by clicking. Click the Undo arrow icon in the Standard toolbar.
Click again. What is its effect? ____________________________. Make some more random
vertices, ending with a double-click to finish the sketch. Again click Undo. What is the
effect? ____________________________.
6. Use a different method to again make an east-west line of about 100 meters: this time click (in
about the center of the lower-left quadrant of the window) to make the first vertex, then observe
the Status bar to decide where to place the next vertex. The distance should, of course, be
about 100. The direction may be approximately 0 or approximately 90 (depending on whether
ArcMap is thinking mathematically or geographically). Click.
This is one of those instances in which two standards come into play. Both geographers (e.g.,
cartographers, navigators) and mathematicians commonly use 360 degrees to describe a complete
revolution. There the similarity ends. Northern hemisphere geographers assign zero degrees to north
and the number of degrees increases clockwise; mathematicians (using the polar coordinate systems)
assign zero degrees to the positive Cartesian x-direction (east) and the number of degrees increases
counterclockwise. GIS relies on mathematical depictions of geography. So we have to deal with this
inconsistency. ArcMap allows us to choose which system we want.
7. Choose Editor > Options > Units. Change the Direction Type to South Azimuth. Click Apply, then
OK, and look at the Status bar as you move the cursor. Experiment with the other possibilities
under Units, ending with North Azimuth, Decimal Degrees, and two decimal places.
8. Press F2 to finish the sketch.
9. Open the attribute table of Edit_Play_Lines. Notice that a polyline of about 100 meters is the
only feature referenced by the table. Dock or reduce the size of the table to get it mostly out of
the way.
Next you will make a multipart polyline.
10. Make three more lines of about 100 meters parallel to the first (each 20 or so meters north of
the one before it—look at the y-coordinate on the status bar), but at the end of each, click just
once, then right-click, and choose Finish Part. When all three lines are done, press F2 to finish
the sketch. All three lines will turn cyan, indicating that they are all part of a single selected
feature. Now observe the attribute table. You should see the additional feature as a single
feature of length about 300.
Once you make a multipart feature (as you just did, and as you did earlier by merging features of the
islands), you may disaggregate them if you wish. Here's how.
11. Choose the Edit tool. Click the original horizontal line to highlight it. Then change the selected
features by clicking the three-line feature you just made so it is selected. Chose Customize >
Toolbars > Advanced Editing. On that toolbar, using the Status Bar (in Version 10.0) or ToolTips
(version 10.1), find the tool that “explodes” a multipart feature. Click it and the three lines
become separate features, as you will be able to see from the attribute table.
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