Geography Reference
In-Depth Information
the name of the stream: Boone (as in Daniel) Creek. The legal description of the line that separates
the two counties is the centerline of that creek. But as you can see, the geographic description of
the dividing line is depicted in two different ways that do not exactly agree. Measure the greatest
distance between the two lines. _______ feet. Dismiss the Identify and Measure windows.
The lack of complete agreement between two geographic datasets can pose annoying problems for those
trying to do spatial analysis. There exist several ways of coping with this problem—either by eliminating
the slivers that are formed between the lines or by using the topological features that come with
geodatabases. For now just be aware that data from different sources, which each attempt to represent
the “real world,” will frequently disagree at some level. This sort of data error something to remember
when you begin working independently with your own data.
You can see why the quality of GIS datasets must be matched to the needs of the user. These county
boundaries might be fine for those delineating watersheds. But surveyors would roll their eyes at
discrepancies of 50 feet. They work with accuracy of inches, and very few of those. Land ownership must
be precisely defined. Local wars have been declared over distances of less than a foot.
45. Change the cursor to Select Elements. Zoom back to the Boat_SP83 layer. Add Mad_Tiger to the
map, in the same fashion as Fay_Tiger, but use a pale blue color of width 3 points. Make a group layer
of the two TIGER files. (How? Check your Fast Facts File.) Click in a blank area of the T/C, and this
time change the New Group Layer name by clicking it and waiting a second or two, and clicking again.
You can then edit the field. Type the new name (call it TIGER_Streams) and press Enter. To get an idea
of the extent of the streams, zoom to the extent of the group layer. Then return to the GPS track.
The Table of Contents: Display vs. Source vs. Selection
So far we have looked at shapefiles, coverages, and vector-based personal and file geodatabases. You
will learn the distinctions between them in detail in Chapter 4. The geodatabase will be the data model
of choice as this century progresses, but all three are important, mostly because, as you use ArcGIS in
your work or other classes, you may find (and be asked to use) extensive datasets in all three forms. The
coverage has been around for decades, the shapefiles for years. The geodatabase, particularly the file
geodatabase, is the database of choice for now and the future. One of the major differences between the
types is the way in which they are stored on the hard disks of computers. If you look at the T/C of ArcMap,
with the List By Drawing Order button active, the different types are pretty much represented in the same
way. If you look at the T/C with the List By Source button active, there are some obvious differences.
46. Toggle back and forth between Source and Drawing Order, using the buttons at the top of the
T/C. Turn off Auto-hiding if it is on and widen the T/C pane horizontally by dragging its right edge
so you can see all the text. See Figure 2-19. When List By Source is pressed, the most obvious
difference is that you not only see each dataset that contains each layer but also the path all
the way to the hard disk. Layers are grouped by their type (shapefile, coverage, or geodatabase)
and by their hard drive location. Note, for instance, that while FORD_VCTR arc and cole_vctr are
both located in the same place on the [___] path and were made into a group layer, they show
up in different places on the List By Source T/C, because one is a shapefile and the other is a
coverage. Neither of the group layer names (TIGER_Streams or GPS_and_Vectors) is anywhere
to be found, since you are now looking at underlying datasets.
The List By Selection button of the T/C lets you determine which layers are “selectable” and which cannot
have features selected from them by clicking on them. The T/C also reports on which layers actually have
selections made from them.
 
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