USING ARCVIEW

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Step 4: Exploring Themes

Themes in ArcView are displayed as either points (such as showing the location of a city or building, depending on the scale), lines (such as roads or rivers), or polygons (such as gardens, parishes, or countries).

In our current view we have themes that are lines (Parbnd and Jarivers) and polygons (Stcath_s, Ja_coast, bpkprcl.shp, and Bpkcrops.shp) right now, but we will add points later.

As we have seen, each theme in the Table of Contents has a small box to its left. Checking a box draws a theme in the right side of the view, while unchecking the box redraws the map on the right without that theme.

You check or uncheck a box by placing your cursor over it and clicking on it once.

Let us see how this works. Uncheck all the themes so that your view looks like this:

The order in which themes appear in the Table of Contents is important, because themes listed at the top of the Table of Contents are drawn over those listed below them. Thus, a theme that is at the top of the Table of Contents and also displayed as a polygon on the map will cover over any theme listed below it. Consequently, a smaller polygon should be higher in the Table of Contents than a larger polygon if you want to see them both displayed at the same time.

In contrast, a theme that is at the top of the Table of Contents and also displayed as a line (such as a river or road) or a point (such as a city in the map of a country) on the map will only cover over a limited portion of any theme listed below it. Thus, such themes should occur above polygons in the Table of Contents.

In our Jamaica map, click on the boxes for the themes Stcath_s and bpkprcl.shp so that checks appear in the boxes and these themes appear on the map and then zoom into the area covered by Stcath_s using the "zoom to active theme(s)" button. (Be sure to make Stcath_s "active.")

Why can't we see bpkprcl.shp displayed? Go ahead and rearrange the order of the two themes in the Table of Contents so that we can see them both. You can change the order in which themes appear in the Table of Contents by dragging the name of a theme with your cursor to the place in the Table of Contents that you want.

Now click on the boxes for Stcath_s and bpkprcl.shp in the Table of Contents so that they are no longer checked and disappear from the map and then click on the boxes for both Ja_coast and Parbnd so that checks appear in them and they are both displayed in the map (remember that you need to select the correct zoom button to see the entire map of Jamaica).

We can see them both in the map because the line theme (Parbnd) is above the polygon theme (Ja_coast) in the Table of Contents. Now go ahead and drag the theme Parbnd down so that it is below Ja_coast in the Table of Contents. Notice now that we can no longer see the Parish boundaries displayed by the theme Parbnd because they are covered by the polygon theme Ja_coast.

Thus, it is best to have polygon themes at the bottom of the Table of Contents and points and lines above them so that the polygons will not cover the points and lines in the map displayed on the right.

But what happens if you want to display two polygons that cover exactly the same area?

Here is an example. Click on the boxes for the themes Parbnd and Ja_coast in the Table of Contents so that they are unchecked and no longer displayed in the map and then click on the boxes for the themes bpkprcl.shp and Bpkcrops.shp so that they appear on the map. Make one of the two themes active--let us choose bpkprcl.shp--and then zoom into the area covered by the theme as we did above. Your map should look like this:

We can only see the map for bpkprcl.shp because it is listed higher in order in the Table of Contents and thus Bpkcrops.shp is covered over by bpkprcl.shp. To see the map for Bpkcrops.shp, uncheck the box to the left of bpkprcl.shp in the Table of Contents.

To display two polygons at the same time or to change the display of other items on our maps, we need to change the Legends.

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[GO TO STEP 5]