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How do I calculate and map a depth-weighted sum in ArcView 3.2?

Once you have your data ready to use in ArcView, you'll need to open the table containing the data you want to map. For our example, we will find the average total water-holding capacity for each mapunit.

To start, open the layer table and we'll create the water-holding capacity for each horizon. With the comp table open, click on Start Editing in the Table menu. Note that all the field headers change from italics to normal type. Now click on Add Field... in the Edit menu. The following dialog box appears,

Field Definition

giving you the chance to give your new field a name, specify the type of data that will be in the field, and control the width of the field. For this case, use the name Wat_Cap and make the field a Number field with 2 Decimal Places. Now highlight the new field and click on the calculator button Calculator Button.

Field Calculator 5

This is a somewhat complex calculation and certainly requires some explanation. The Muid field is a String and the Seqnum field is not so we need to use the .AsString operator to make it into one. Then, once you have two strings, the + operator concatenates them. Now we can summarize. Select the Musym field and click on the Summarize button Summarize Button.

Summary Table Definition 5

In the summarize dialog box we will create two fields, one a summation of the water-holding capacity of each horizon and one holding the Muid field which we will use later to combine the results with mapunit-level tables.

Now we join this table to the comp table. Create a Unique field in the comp table as you did in the layer table. Then highlight the Unique field in both tables. Now, with the comp table active, click on the Join button Join Button.

With this modified comp table open, click on Start Editing in the Table menu. Now click on Add Field... in the Edit menu. The following dialog box appears,

Field Definition 3

giving you the chance to give your new field a name, specify the type of data that will be in the field, and control the width of the field. Now highlight the new field and click on the calculator button Calculator Button.

Field Calculator 7

The calculator allows you to calculate the values for your new field. The formula you see multiplies the Sum_Wat_Cap field (which you calculated earlier and then joined to the comp table) by the percentage of the whole mapunit that the component occupies. Now we need to summarize two fields so that we can do another calculation. A longer explanation of the summarizing procedure is given in this answer. Select the Musym field and click on the Summarize button Summarize Button.

Summary Table Definition 6

In the summarize dialog box we will create two fields, one a summation of the component percentages and one a summation of the calculation field we just made.

In our new table, we will need to perform another calculation so Start Editing it and Add Field... Make sure that the new field is the number type and has a couple of decimal places. Now highlight the new blank field and click the Calculate button. This time you want to divide the sum of the MU_Wat_Cap's by the sum of the component percentages. See if you can get the right formula without looking at the illustration below.

Field Calculator 8

In order to map the newly calculated field, we must join the current table to the table for our view. Choose Stop Editing from the Table menu and then highlight the Musym field. Now open the table for a view Open Table Button. Highlight the Musym field in this table as well and then click the Join Tables button. Now you can create a map based on the calculated values. You can see the results below:

Watcap Map