Introduction to
Geographic Information Systems in Forest Resources |
Exercise: Creating Feature Themes
Objectives:
- Creating new feature themes by query of existing feature themes
- Digitizing a new theme from scratch
- Start ArcView and open an existing project
- Query a theme to select a subset of features
- Convert the selected set of features to
a shapefile
- Create (digitize) a new polygon theme and
modify its legend
- Set the snapping environment
- Add features and attributes to the new theme
- Edit features with Split and Union
- Stop editing and save your edits
- Create a new point theme
- Create a new line theme
- Create a stands polygon theme
- Close the project
Start ArcView and open an existing project
- Retrieve the project create_f_theme.apr.
Save it in your personal directory.
- If ArcView is not running start the ArcView application.
- Use the
Startup Project
to open the project file you just downloaded. When the create_f_theme.apr
project opens, you will see a view containing two themes.
Query a theme to select a subset of features
Forest thinning operations typically occur early in the life cycle of a forest
stand. In this exercise, create a shape file from the stands (ArcInfo
coverage) theme, consisting only of stands between the ages of 15 and 30.
- Click the Query Builder button (
), which
opens the Query Builder dialog.
- In the Query Builder dialog, select Age_2003 from the Fields
list by double-clicking on its name.
- Single-click on the greater-than button (
), and type in the number 15.
- Single-click the and button (
).
- Double-click the Age_2003 field.
- Single-click on the less-than button (
), and enter the number 30.
- Click the New Set button, and dismiss the Query Builder dialog.
- The query should look exactly like this, with parentheses and braces in
the correct positions. If you make a mistake, highlight the query and press
the <DELETE> key, then start over. Make sure you have proper
opening and closing parentheses around all terms, especially if you have manually
edited the expression.
The view will look like this, with stands meeting this age criteria displayed
in the selection color of yellow:
You have just selected a set of features from a polygon theme. You will use
this set of features in the next step to create a new theme composed of only
these features.
Convert the selected set of features to a shapefile
- To convert this selected set into a shapefile, simply select Theme >
Convert to Shapefile from the menu. Name the new shapefile stands1530.
Make sure that the new shapefile is placed in the file system where you want
it to be stored.
- Click Yes when asked to add the shapefile to the view.
- Turn off the original stands theme. Your view should appear like this:
- The new theme is composed of only polygons matching the query criteria.
- Close the Stands view.
You have just taken a set of selected features and created a new shapefile
composed of only those features. Use this technique whenever you want to create
a subset of features from a theme. Sometimes you need a smaller data set for
a specific spatial area, or a set of features that has a specific attribute
value.
Create (digitize) a new theme and modify its legend
Next, create a new theme that will represent four different culvert management
units. Each management unit will be used to schedule routine annual inspection
and maintenance of culverts on the forest.
- Open the Culverts view.
- Create a new theme by selecting View > New Theme from the menu.
- Choose Polygon as the Feature type, and save the new theme
as culv_inv.shp in your personal directory.
The new theme will be added to the view, assigned a random color, and opened
for editing. Also note the dashed line around the checkbox for the theme in
the Table of Contents. The dashed line indicates that the theme is open for
editing.
Before adding any features, change the theme's legend by double-clicking the
theme name in the Table of Contents.
- Click on the Color Palette button
, and with the dropdown
list, make sure Foreground is selected.
- Click the "X" symbol, which will cause the polygons not to be
filled. (If the color does not change, click the white symbol first, then
the "X" symbol).
- Next, select Outline from the dropdown, and select the red symbol.
- Finally, click the Fill Palette button
, and in
the Outline dropdown list, select 2.
Now new polygon features will be displayed with no fill, and a thick red outline.
This way the polygon fill will not obscure the underlying data, but the red
outline will clearly show the position and shape of the polygons.
You have just created a new (empty) polygon shapefile. The legend was also
changed to a transparent symbol so that you can see underlying features.
Set the snapping environment
To assure that new polygon features are added correctly, alter the snapping
environment.
- First, right-click-and-hold the mouse button on the map display of
the view and select Enable General Snapping from the popup menu.
- Click the General Snap tool
.
- Place the cursor on the view, then click and drag a circle that will be
the snapping distance. When new features are added to existing features, and
the new features' arcs terminate within this distance of the existing features,
the new features will snap to the existing features.
Make sure that your snapping tolerances are set whenever
you are digitizing polygons and lines. You can check the snapping tolerances
by looking at Theme > Properties > Editing from the menu.
If you save
a project while a theme or table is open for editing, ArcView will ask you
if you want to save changes. Once the project is saved, any themes or tables
that were open for editing will no longer be in edit mode. In order
to edit the table or theme, you will need to use the Start Editing
menu choice from the Theme or Table menus.
Setting the snapping tolerances is important to set before you start editing
line or polygon features. You can always reset snapping tolerances at any time,
based on the precision you need.
Add features and attributes to the new theme
- Before adding polygons, place a new attribute field in the theme table.
With the new theme active, open its theme table using the Theme Table
button
.
- From the menu, select Edit > Add Field.
- In the dialog box, add the parameters for the new field as shown:
Accept the "?" dialog generated by ArcView. A new field is created
with a truncated name, but the field alias will be set to display properly.
- Now the theme table has a new attribute, but still no records exist. When
polygons are added to the theme, records will automatically be generated for
each polygon feature. Never add a record to a theme table by
directly editing the table. Records should be added only by adding a feature
in the view.
- Position the view and the table so that they are both visible and not overlapping,
because you will work with both documents switching back and forth. Maximizing
the application window will make things easier. When a polygon is added, you
will subsequently update the record for that polygon.
- Use the Polygon tool
to add the first
polygon. The polygon tool is part of the composite draw tools. If you do not
see this tool, view the draw tools dropdown by click-and-holding the Rectangle
tool 
. You
will not see any polygons in the theme until you add them.
Click at the starting point of the polygon, and then click at each new vertex
in the polygon outline. When the polygon has the shape you want, double-click
to end the polygon. Make sure that the outer edges of the new polygons go
completely outside the Pack Forest boundary; we will clip the management units
later to match
the forest boundary.
After the polygon feature is added, you will see selection handles around
it. If the polygon is incorrect, press the <DELETE> key to remove
it. To select a feature while in edit mode, use the Pointer tool 
and click on the
feature.
- Next, make the table active, and select the Edit tool
.
- Click in the Inspection_Season cell for the selected record and type
Summer.
- Now add the next polygon by using the AutoComplete tool
. Please look carefully at these tool icons,
because some of them look very similar to each other.
- Add a new adjacent polygon by making the first click well within
the existing polygon, then move outside the existing polygon, adding vertices
as needed.
To finish the new adjacent polygon, double-click again within the first polygon.
Make sure to finish the new auto-completed polygon by going back inside the
first polygon to a distance beyond the snap tolerance. (You will see
the circle for the snap tolerance; just make sure the circle is completely
within the existing polygon before double-clicking). Any overshooting lines
will be automatically dropped.
- Each time you add a polygon, it will be selected. Update the records for
each polygon as they are added. You should end up with an application looking
similar to this:
You have just added several polygon features to a new polygon theme. Especially
important is using the Polygon AutoComplete tool, which will allow
you to create new polygons that are adjacent to existing polygons, without needing
to retrace the common boundary.
Edit features with Split and Union
Suppose that now it has been decided that Summer is too busy a season to perform
culvert maintenance. Therefore, the Summer polygon must be split up and
merged with adjacent polygons. Some of the culverts will be placed in the Autumn
polygon, and some in Spring.
- Open the Theme > Properties from the menu, and look at Editing
properties. For the unions and splits we are performing, we will keep the
default rules (Copy). However, be aware that several choices exist
for these rules, and rules can be applied to each field.
- Do not make any changes to the dialog, and then click the Cancel
button.
- First, split the Summer polygon using the Split Polygon tool
. Start the splitting
line outside of the Summer polygon, and end it on the other side of
the polygon, making sure to go farther than the snap tolerance outside the
polygon you want to split. Dangling ends will be deleted. You will notice
that 2 polygons are selected.
- First, make sure there is no active selection (use the Select None button),
and then using the <SHIFT> key and the Select tool
, select the 2 adjacent polygons that are to
be unioned. You will see selection handles around both polygons. You will
also see that two records are selected in the theme table.
- Then select Edit > Union Features from the menu. Do this for both
halves of the Summer polygon. You should end up with an application
window that looks like this:
- If the attribute values are not correct, select the erroneous polygon(s),
and edit records if necessary.
You have just used splitting and unioning to alter the coordinate and tabular
parts of the theme you created in the previous task.
Splitting and unioning polygons can be used when areas or patches change shapes
or size. For example, if a set of contiguous forest stands is harvested, it
may be unioned a single new stand. If a single forest stand develops different
characteristics over time, it may be split into several different polygons.
Stop editing and save your edits
- When you are satisfied with your edits, select Theme > Stop Editing.
Click Yes in the Stop Editing dialog.
- Note that any changes made to the shapefile is saved to the disk. If this
theme is added to another view or project, it will open in the same state
in which it was saved.
Save your changes frequently, even if you do not stop editing. It takes a lot
less time to stop and save your changes than it does to recreate a data set
in case ArcView crashes during your editing session.
Create a new point theme
Creating point themes is similar to (but less complicated than) creating polygon
themes, except that point and line graphics tools are used, rather than polygon
editing tools. Attribute tables are altered in exactly the same manner.
- Create a new view. Rename the view from View1 to Crossings.
- Add the roads and streams themes from the \packgis\forest
directory from the CD.
- Create a new point theme that represents stream and road crossings within
the forest. From the menu, select View > New Theme. The feature
type should be Point. Save the new theme on your Zip disk as crossings.shp.
The theme will be open for editing, but will lack any features.
- Before adding any points, alter the structure for the theme table, so that
it will be able to hold the proper attributes. Open the theme table (), and move it so
that you can see both the view and the table. You will see that the table
only contains the Shape and ID fields, and no records.
- Add two fields, one called Stream, and one called Road. Bake
both of these fields Type: String , with Width: 25. For each
point, we will encode the stream name and road name for the intersection.
You will need to resize the table in order to see all the fields.
- Zoom into a location near a crossing that will allow you to place a point
precisely. You will need to zoom in several times to achieve this level of
precision. Using the Draw Point tool (
), add a point at the crossing. You
will notice that as soon as you add the new point, a record will be added
to the table.
Note that the point is selected (see the graphic selection handles) and the
corresponding record is selected (shown in yellow).
- Now, make the other themes active, and click on the new point with the Identify
Features tool (
). You
will see the records for the streams at this location. Make a mental note
of the names of the stream and road.
Dismiss the Identify Results dialog.
- Make the table active, and using the Edit tool (), alter the values for the new point, entering
the values you saw in the last step.
For the point I chose, there was no name for the stream, so I left the value
blank.
- Zoom back out, and repeat the process for a few more points. You get the
idea....
Create a new line theme
- Create a new view called NewRoads.
- Add the \packgis\forest\ortho_96.bil image theme from the CD. Make
sure to specify Image as the Data Source Type.
- Create a new line theme (called new_roads.shp, on your Zip disk)
- Set the Snapping tolerances, as you did before.
- Zoom in and start digitizing some roads based on what you see in the image.
You will want to maximize the view and the application in order to achieve
better precision and accuracy.
- Continue adding a few other road segments
You will also need to zoom in and out as you add features.
Create a stands polygon theme
- Create a new view called Stand Digitizing and add the same orthophoto
image theme you added in the last step.
- Add the boundary theme and alter its legend to make it unfilled as you did
before with the culvert inventory polygon theme
- Create a new polygon theme on your Zip disk called stands.shp.
- Zoom into the portion of the forest shown in the white outline below:
- Now you should see some clearly visible stands and recent cut units.
- Enable general snapping and set the snap tolerance as you did before.
- Open the theme table for the new Stands.shp theme in preparation
for adding a few fields.
- Add 2 fields (Origin_yr and Species) with the definitions
shown below:
- You should now see that the structure of the table is set up, but there
are no records yet (because no features have been added)
- Switch back to the view, and use the Polygon draw tool
and trace the first stand.
- Update the record for this polygon (origin year = 1995, species = Douglas-fir
[PSME = Pseudotsuga menziesii])
- Use the Auto-complete polygon tool
to create an additional polygon
- Update the values for this polygon's record as well (Origin = 1930, Species
= Western hemlock [Tsuga heterophylla])
- Add a third polygon
- Alter the record
- You have just found that one of the stands needs to be split, and part of
that stand merged with an adjacent stand. Select the northwestern stand of
the group of 3 and split it into two pieces using the Polygon split
tool .
Note that the record has also been split:
- Use the pointer or select tool now to select the northernmost and easternmost
2 polygons (use the <SHIFT> key for making multiple selections).
- From the Edit menu, select Union Features. You will see that
your polygons have been unioned, and the records have also been merged.
- Continue to add more polygons and records if you have more time left to
get more practice.
- Stop editing (Theme > Stop Editing) and save your edits.
You have just used a digital orthophoto image as a backdrop for creating two
new vector data sets (roads and streams). There are a large number of digital
orthophotos for Washington state available from the Map Collections in Suzzallo
library. Additionally, the WA Department of Natural Resources maintains a large
collection of digital orthophotos that can be purchased directly from the DNR.
Close the project
Make the Project window active and select File > Close Project. If
you wish to save the changes to this project, choose Yes, and place the
project file on your Zip disk. If not, click No.
Because you have several themes that are open for editing, ArcView will ask
if you want to save changes to the themes. Choose Yes.
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