Introduction to
Geographic Information Systems in Forest Resources |
Creating and Modifying Tables
- Clear out your Zip disk
- Adding and Editing Tables
Copy a file to your personal directory
Start ArcView and open an existing project
Add a table to the project
Change the table display
Edit cell values and add a record
Add fields and calculate values
- Selecting and Summarizing Records
Query a table and display the selected set
Get descriptive statistics on the selected set
Modify a selection
Summarize a table
- Joining and Linking Tables
Join tables
Use a joined field for theme display
Link tables
Remove links and joins
- Create a Chart
Create a chart for selected features
Chart multiple fields
Change the chart type
Modify chart elements
Add grid lines
Toggle series and groups
Modify selections
- Export a table
- Close the project
Clear out your Zip disk
- Delete everything from your Zip disk so you will have enough space to work
through this exercise.
- Also take this opportunity to set your working directory to the Zip disk.
Adding and Editing Tables
Copy a file to your personal directory
- Download the file plots.exe and save it on your Zip drive. This is a
self-extracting zip file of a dBase table containing reference trees for the
Pack Forest Continuous Forest Inventory plot centers.
- Double-click on the file name in the Windows Explorer to extract the file.
Extract it to your Zip drive. After you have extracted the file you will see
the plots.exe file and a file called plots.dbf.
- Once it is uncompressed, it will open in ArcView without any translation.
Start ArcView and open an existing project
- Retrieve the project tables.apr and save
it in your personal directory.
- Use the Startup Project to open the project file you
just downloaded.
The project will open with a view document containing the Pack Forest CFI
theme.
Add a table to the project
- Add the table to the project by first making the Project Window active
- Click the Tables icon
, and click
the Add button.
- Navigate to the directory where you saved the table in the first
step.
- Double-click on the file name.
When the table is added, it will open in its own document.
You have just downloaded a table from the web and added it in to an ArcView
project. Most of the tables we have used up to now are feature theme attribute
tables. However, any supported (text, dBase, or INFO) files containing tabular
data can be added in the same manner.
Change the table display
Tables can be altered within ArcView, either by editing, which changes the
source file (on the disk), or by altering the display properties, such as hiding
fields or giving field name aliases (which does not change the source data).
- Change the table's appearance by selecting Table > Properties
from the menu.
- Delete the contents of the Title control, and type in "CFI Plot
Reference Trees."
- Uncheck the fields Section and Plotnum, and type "Plot
ID Number" in the Alias field for the Cfi4_plot field
name.
- Click OK to apply these changes.
- This will rename the table, hide the two unchecked fields,
and alter the displayed name of the Cfi4_plot field.
Note that the field name has changed. You can use this technique to make field
names more understandable. The field name changes only for this table within
this project, and does not alter the data source.
- As you scroll down the table, you will notice that some of the Desc1 field
values are truncated. Move the pointer to the line between the field names
(the pointer will change to a double-headed arrow).
- Click and drag to the right to increase the display width of the field.
You have just altered a table's appearance in ArcView by hiding some fields
and creating field aliases for others. Alter the table appearance when you need
to view fewer fields, or if you want to change the names of fields as they appear
in the table docmument. Remember that changing a table's appearance does not
alter the table's source data.
Edit cell values and add a record
- To edit the values in a table, you must first open the table for editing.
Choose Table > Start Editing from the menu.
Be aware that you will only be able to edit tables to which you
have write-permission. This means that you cannot change tables that are stored
on the CD!
- You will notice that the field names are now no longer italicized. Whenever
a table is open for editing, its field name will be shown in normal font.
- There are a few records in the table with typographical errors. Find the
record where CLUMP" is misspelled "CHUMP."
- Use the Find button
and type in chump.
The record containing "MAPLE CHUMP" will be selected.
- Using the Edit tool
, fix the error.
- Click on the Desc1 value for this record. The value will be highlighted.
- Highlight the "CHUMP" text and then type in CLUMP
and press the <ENTER> key to apply the change.
- Next, find the record in which the value for Distanc2 is too high.
Make this field active and then sort it, descending. The record whose value
is "1047" should be fixed to read "147".
There is one record which was mistakenly not entered. It should have these
values:
|
Section
|
Plotnum
|
Plot ID Number
|
Desc1
|
Bearing1
|
Distanc1
|
Desc2
|
Bearing2
|
Distanc2
|
|
3
|
828
|
3828
|
DF
|
N34E
|
13.8
|
DF
|
S76E
|
28.8
|
- Because the Section and Plotnum fields were hidden previously,
you will need to unhide them (Table > Properties from the menu,
and check the Visible property for the fields).
- From the Edit menu, choose Add Record.
- Scroll to the bottom of the table. You will see a record whose values are
either 0 or null for the various fields.
- Using the Edit tool, update the values of this record to match the table
above.
You have just made single edits to various values in a table. You may find
editing tables frustrating, because the two main Table GUI tools are the select
tool and the edit tool. If you want to select a record in the table,
you need to use the select tool. However, to make an edit you must use the edit
tool.
Also be aware that when a table is open for editing, you can alter the value
of any cell in the table, whether a record is selected or not.
Add fields and calculate values
The current unit of measure for distances in this table is feet. We need to
add fields for the distance in meters for a group of visiting students from
Switzerland. To do this, first add the new fields.
- If the CFI Plot Reference Trees table is not open for editing, start
editing the table with the menu selection Table > Start Editing.
- Select Edit > Add Field from the menu. When the Field Definition
dialog appears, enter the following:
This field will be numeric, 5 data columns wide, with 2 decimal places. Note
that one of the data columns (in the Width text entry control) will
be taken up by the decimal point. For example, you could store these possible
values in the field:
Before you add fields, you should consider what data will
be stored in the field. This will determine the field definition. In this
case, I have determined that a locator tree will not be more than 99 meters
from the plot center. Adding two decimal places of precision, plus a column
for the decimal place makes a field 5 columns wide.
- Add a field called Meters2 with the same field definitions.
You now have 2 new fields in your table.
Note that when new items are added to the table, values for that field are
null or 0.
- Clear any selected records
so the calculation
we perform will be applied to all records in the table..
- Drag the field Meters1 field title to the left until it is adjacent
to the Distanc1 field. That way you will be able to see the fields
next to each other.
- Make sure that the Meters1 field is the active field (it should look
pushed in).
- Invoke the Field Calculator dialog by clicking the Field Calculator
button
.
- Enter the statement [Distanc1] / 3.2808 as indicated above. You can
do this by double-clicking the Distanc1 field, the / operator,
and then typing in 3.2808 (the conversion constant between feet and meters).
Note you do NOT need to include the Meters1 field in the expression.
By making this field active and using the field calculator, you are telling
ArcView that you want to calculate values for that particular field. As a
reminder, the Field Calculator dialog shows the name of the field that
is being calculated above the expression text entry control.
- OK the change. You will see that the values for Meters1 is
now updated.
- Calculate the Meters2 field in the same manner, but use the Distanc2
field in the Field Calculator expression.
Notice that the Meters2 field becomes updated with the newly calculated
values.
- When you have finished editing the table, select Table > Stop Editing
from the menu.
- When prompted, save the changes.
You have just created new fields in the table and used the field calculator
to populate the new fields with values. The field calculator is used when you
want to add values for based on calculations from other fields, or if you want
to add values for several records at a time.
It is possible to convert values from numeric to string fields and vice versa,
e.g.,
[Distanc1].AsString ++ "feet"
will calculate the active field (assumed to be a string field) to the value
of the Distanc1 field and the string feet, separated by a blank
space.
[Bearing1].Middle(1,2).AsNumber
will calculate the active field (assumed to be numeric) as the middle 2 characters
starting at position 1 of the Bearing1 field.
You can read more about text mashing functions in ArcView help for the String
object.
Selecting and Summarizing Records
Query a table and display the selected set
- Open the theme table for the Conifer Volume theme by making the view
active, making the theme active, and clicking the Open Theme Table
button
.
- Scroll through the table to see what items and values exist. The items represent
basal (ba - the cross-sectional area of a tree at 1.2 m) area and Scribner
(board-foot) volume to a 6" top (sv6) for the indicated years.
The items represent either conifer per acre (cpa) or hardwood per acre
(hpa). There are several other items beyond our present concern.
- Select records for plots with high hardwood basal area in 1994 by invoking
the Query Builder
. [Basal area is a
measure of the cross-sectional area of a tree at 1.3 m above the ground.]
- Scroll down to the Ba1994hpa (basal area / acre for hardwoods in
1994) item and double-click it.
- Click the > sign, and type in 100. If you make a mistake, highlight
and delete the query and start over.
- The query should look like the image below (with all parentheses and braces
in the right places. Spaces are optional).
- When the query is complete, click the New Set button, and then dismiss
the Query Builder.
It is important to format your query string in order to avoid syntax errors.
- Only a single record will appear to be selected, although the query actually
selects several records.
- There are several records actually in the selected set. To promote them
to the top of the table, click the Promote button
. Now all selected records will be displayed
at the top of the table.
You will also see that selected polygons are shown in yellow.
- Switch selected sets by clicking the Switch Selection button
.
The table will display the opposite selection.
Also, the opposite set of polygons will be selected.
- Click the Switch Selection button again to return to the originally
selected set.
You have just used a logical statement to select out specific records in a
table. In this case, because the table is a feature theme table, the
corresponding features are selected for the theme in the map display of the
view. You can perform queries on other tables within ArcView, regardless of
the source file type.
Get descriptive statistics on the selected set
Now that we can see which records and plots have high hardwood basal area,
we may be interested in other statistics about the data.
- With the selected set still active, make active the field representing 1994
Scribner volume for conifers (Sv61994cpa).
- From the menu, select Field > Statistics. Descriptive statistics
are listed in the popup. Note that these are descriptive statistics only for
the selected set of records/features.
We can see the mean volume per acre for these records is about 17,000 board-feet
.
- Clear the selected set and generate statistics
again. You will be able to learn much about your data by looking at these
statistics.
The mean volume for all plots is about 10,000 bd-ft.
You have just obtained summary statistics for a numeric field. Understanding
the range and spread of data values for themes is one of the basic things you
should be familiar with. In this, case you calculated summary statistics for
both a selected set of records as well as for all records in the table.
Modify a selection
Sometimes selections are either too broad or too narrow. In the case above,
our selection included plots with no conifer volume at all. We may be interested
in a subset of plots which have high conifer volume and high hardwood
volume.
- First, make a query that selects plots with hardwood basal area (Ba1994hpa)
greater than 80, as shown below.
- Click New Set and then dismiss the Query Builder. This selects all
plots whose hardwood basal area in 1994 was greater than 80 sq in.
- This selects 19 polygons. We will narrow the selection by selecting out
polygons with conifer basal area (Ba1994cpa) greater
than 100, as shown below. Formulate the query in the same manner, but choose
the button instead of
the New Set button. If you make a mistake you can always clear the
selected set and start the query over from scratch ().
- This selects out only 4 polygons (if the records are not promoted, you can
promote them now). What kinds of management activities would you suggest for
these areas?
You have just performed a 2-step query on a table. You will frequently need
to select records that match a large set of criteria from several fields of
data in the table. Use this technique to pare down your selections. If you need
to add records to selected sets, rather than reduce selected sets, use the Add
To Set button.
Summarizing Tables
Summarizing a table gives you the ability to see patterns in your data which
may not otherwise be evident from looking at a raw table. Summaries work by
acting on the unique occurrences of values within a specified field. Summaries
also allow you to generate summary statistics based on numeric fields within
the table that is to be summarized.
The results of a summary are stored in new tables (i.e., new dBase files on
the disk), so you need to specify the output directory and filename for the
new table. These tables always contain the summary field, the count of records
for each unique occurrence of the summary field value, and optional statistics
Summaries also allow you to simplify data in order to create charts, since
ArcView's charts are limited by the number of records placed in a chart.
- Create a new view and add the stands theme from the packgis\forest
directory on the CD.
- Open the stands theme table.
- Make the Species field active, and then select Field > Summarize
from the menu.
- Add the sum of area to the summary table, as well as the average
site index:
- Name the new summary table spec_area.dbf on your zip drive.
- Select Acres in the Field dropdown and Sum
in the Summarize by dropdown, then click the Add button,
and
- Select Site_index in the Field dropdown and Average
in the Summarize by dropdown, then click the Add button.
Make sure to click the Add button whenever you add a summary statistic!
- If your dialog box does not look like this one, you can click the Delete
button to remove mistakes, and then go on to add correct entries
- Specify to save the table in your personal directory as spec_area.dbf.
You can either type in the path to this new table or use the Save As
button to navigate to the directory where you want to save the table,
and enter the file name.
- Click the OK button to create the table.
The summary table is automatically added to the project. It will contain the
fields Species, Count, Sum_Acres, and Ave_Site_index.
The Species field contains the unique values of the species field
in the source table. This field exists in the output table because it was the
active field originally; this field is what we wanted a summary about. Count,
which is automatically created, lists the count of records for each species
in the source table. The other items (Sum_Acres and Ave_Site_index)
represent the sum of acres and average site index per each species type; these
are the individual fields we intended to generate.
Table summaries also work with selected sets; only selected records will be
summarized if there is an active selection. Otherwise all records in the table
will be summarized.
If you create a summary table without adding any summary statistics, the output
table will contain a copy of the summary field with one record for each unique
value in the summary field as well as the Count field, which contains
values of the number of records for each unique value in the source table.
Joining and Linking Tables
Joins and links are touched upon in the relational
data model. In this example, we have the plot location table as well as
the attribute table for the CFI plots ("Conifer Volume"). The common
field between these tables is Cfi4_plot and CFI Plot ID Number,
which is the plot's identification number.
Joining tables
- Make the CFI Plot Reference Trees table active.
- Click on the Plot ID Number field so that it appears pushed in.
- Make the Conifer Volume Attribute table active, and also click on the Cfi4_plot
field so that it appears pushed in.
- With the attributes table still active, click the Join button
.
The CFI Plot Reference Trees table will disappear from the application
window, but the data for each matching record will be appended to existent
records in the CFI theme attribute table.
It is important that the active table be the one you want to join onto. If
you join in the wrong direction, the join will occur, but your data may be
meaningless. If this happened to you, select Table > Remove All Joins
from the menu.
- Scroll to the right and you will see the additional fields. You will also
notice that records in the attribute table which did not have a match with
records from the plots table will have null values for these additional fields.
The tables have not been physically joined (that is, joined
as a single file on the disk), but are only managed this way by ArcView. You
can verify this by adding another copy of the Conifer Volume theme, and open
its table, which will not contain the additional fields created by the join.
You have just joined two tables in ArcView. Use this technique to increase
the informational content of your themes. You will frequently encounter theme
attribute data that are minimal in content, but can be linked to other tables
containing extended attributes. When a project is saved, all of the join information
is also saved, so if you open a project, ArcView will re-establish any joins.
Use a joined field for theme display
For most of ArcView's purposes, the resultant table created from a join functions
just like any other table. If the resultant table happens to be a theme attribute
table, then ArcView gives you the luxury of displaying or analyzing joined data
as though it were an integral part of your theme table.
- Make the Pack Forest CFI view window active, and clear any selected
features .
- Change the legend for the theme so that it is using a Graduated Color display
with the defaults of Natural Breaks and 5 classes, based on the Meters1
field.
- Select the Color Ramp to Orange monochromatic. Note that some
of the polygons do not appear. These are polygons for which the classification
field is null (there was no match on the join).
- Add the \packgis\forest\boundary data source as a polygon theme with
a contrasting color, and make sure it draws before the CFI plots.
- Open the Theme Properties for the theme, and alter the Theme Name
control from the current value to Plot - Tree Distance. This will change
the theme name in the legend.
If there is a positive correlation between the distance of plot trees from
the plot center and difficulty in finding the plot center, the darker plots
will be more difficult to find. You might alter your field work schedule accordingly.
Note that some of the features disappear; these are the features that did
not have matching values between the two tables that were joined. If you view
the table, you will see null values for these records for the Meters1
field.
You have just used a joined field for altering a theme's display. If you have
tables of extended attributes that can be joined to your theme tables, you can
use those extended attributes for display of themes.
Linking tables
Tabular links are similar to joins, but with links, the tables remain separate.
However, selected records in one table also select related tables in associated
tables. Also, in contrast to joins, links are often made between tables which
have a one-to-many relationship. In this section, we will link a species table
to the stands attribute table. Links are performed when you have a one-to-many
relationship between your destination and source tables.
- Make the stands attribute table active, and then make the Species
field active.
- Make the spec_area table active, and then also make the Species
field active.
- From the Table menu, select Link.
- Make sure you are using the Select tool, and make a selection from
the table.
You will notice that the tables do not change in any way. However, when you
select a record from the spec_area table, you will notice that corresponding
records in the stands table also become selected. Be sure to look at
the view document as you make these selections.
Part of the value of linking tables is that it allows you to make very rapid
selections on data. You can easily select, in order, DOUGLAS-FIR stands,
then MIXED-REDCEDAR, then RED ALDER. Imagine how much longer
this would have taken if you had used the Query Builder to make a selection
based on species.
You have just linked tables together. Always use linking when you have a one-to-many
relationship between the destination and source tables.
You can also link tables together both ways so a selection on either table
will select related records in the other table. Also, you can string links together
across multiple tables so a record selected on one table will select records
in 2 or more related tables.
Remove links and joins
Do not remove any joins or links yet, but read this ...
If any joins or links are active, you can easily remove them by making the
tables active that contain links or joins and selecting Remove All Links
or Remove All Joins from the Table menu. If these menu choices
are greyed out, this means you have no joins or links on the active table.
You will need to do this when your links or joins have gone in the wrong
direction.
Create a Chart
Create a chart for selected features
- With the REDCEDAR and MIXED-REDCEDAR stands still selected,
and with the spec_area table active, create a chart by clicking the
Create Chart button
. (If you need to,
select all of these stands using the technique shown above.)
- Change the Name of the chart to Area & Site Index.
- Select Sum_Acres, and click the Add button to place it in
the Groups list. The groups are fields containing numeric values we
wisht to display on the chart.
- Make sure that the Label series using: control is set to Species.
- Click the OK button.
- The chart will open, displaying the number of acres in each species type.
As you can see, only records that are selected are shown in the chart. In
this way, the charts are dynamically linked to the tables.
- Clear the selection on the spec_area table, and look how the chart
changes. You will need to increase the size of the window containing the chart
document in order to see all the species types in the chart legend:
You have just created a chart from a table. Charts in ArcView are always derived
from tables.
Chart multiple fields
To chart more than one field at a time, alter the Chart Properties.
- Click the Chart Properties button
.
- Select Ave_Site_index and then click the Add button.
- Now the chart will display both groups.
You have just altered your chart so that it displays more than one field at
a time.
Change the chart type
To change chart types, select different buttons on the Chart Button Gallery
. You have the choice
of Area, Bar, Column, Line, Pie, and XY Scatter charts. Within each type, there
are several choices.
- Select the Area button, and choose the first large button in the Area
Chart Gallery.
- The chart will be modified to look like this:
- This chart is not appropriate for this type of data. Although there is a
lot of flexibility in choosing different chart types, it is up to the user
to decide which chart is appropriate for the type of data and the intended
audience.
- Switch back to the column chart type. Feel free to experiment with different
chart types.
Modify chart elements
The chart as it is shown now needs some alteration to make it more informative.
- Use the Chart Element Properties tool
.
- Click on the Title text to invoke the Chart Title Properties
dialog.
- Alter the title to read Area & Site Index, and OK the
change.
- Because of the scale of the data, we need to show a smaller range of data
in the Y-dimension. Click on the Y-axis to invoke the Chart Axis Properties
dialog.
- Alter the Scale max to read 250.
Now the chart is scaled so that the data values for site index are easier
to read. Beware that the reader knows that the data have been cut off at 250
for Sum_acres or it may appear that the actual values for 3 records
are 250, not the true values. In this case, you may want to include a chart
with the default Y-axis scale along with this chart.
If you ever set the Scale max for a chart, you will need to make sure
that you include a description! The chart now looks like the maximum Y value
is 250, even though the actual values are much higher. If you do not include
a description, the chart will be misleading.
- Next, click on the legend to open Chart Legend properties.
- Click at the empty spot left of the cartoon of the chart to alter the legend's
position and OK the change.
You have just customized the look of the chart to make it more informative.
Add grid lines
To make charts easier to read, it is often customary to add grid lines.
- Click on the Y-axis with the Chart Element Properties tool.
- Click on the check boxes for Major grid and Minor grid.
- Alter Major unit and Minor unit to read 100 and 20,
respectively.
- OK the change.
Now the chart has gridlines and tic labels on the Y-axis.
These changes to the chart make it easier for the reader to compare values.
Toggle series and groups
The chart as is shown displays acres and site index in different groups. This
makes it easy to compare different species to each other within each group.
However, sometimes it is useful to compare two attributes to each other for
the same record. This is done by toggling the fields and records in the chart
(
).
When this button is pushed, you will see the change reflected in the chart.
Toggling series and groups lets you modify your comparisons. If you are interested
in comparing within groups or between groups, you will alter the group/series
position.
Modify selections and alter legend
Because charts are dynamic representations of tabular data, any selections
made on the table will be reflected in the chart. We will alter the selection
on the source table so that we see only the major species groups.
- Switch the active document to the spec_area.dbf table, make the Sum_acres
field active, sort ascending, then select the 4 largest-area records. (To
do this, press the <SHIFT> key, then click and drag the first
4 records using the Select Records tool
). The selected records will be shown in yellow.
- Make the chart active again and you will notice that only 4 records are
displayed.
- Using the Chart Element Properties tool, click the legend in the
chart document, and then click the central position in the Chart Element
Properties dialog. Also update the Series Labels as shown:
- Finally, click on the X-axis to open the Chart Axis Properties dialog
for the X-axis. Alter the Group labels, check boxes, and Axis label
as shown below:
When the chart is updated, you will be able to drag the legend to any position
on the chart document.
You have just altered the legend and series labels for a chart. Because field
names are frequently cryptic, you should label groups and series so that the
chart is understandable to the reader.
Export a table
- Select all records from the stands attribute table where the stand
age in 2001 is greater than 100.
- Alter the table properties so that several fields are hidden, like so:
- From the File menu, select Export.
- Specify Delimited Text as the Export Format.
- Save the file to your personal directory, as stands_gt_100.txt.
- Open the file from the Windows Explorer. You should see the file open with
Notepad or Wordpad. Note that the file only contains the selected records,
and only the visible fields.
- Import the text file into Excel. You may need to configure Excel so that
the records become parsed out into individual fields using the Excel import
wizard.
You have just exported a table from ArcView into a generic format. You can
use this technique to save tables for import into other applications, such as
statistical analysis software.
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
save the project on your Zip disk. If not, click No.
Syllabus