Setting a classification
Release 9.2 Last modified August 15, 2007	Print all topics in : "Applying symbology"

About setting a classification

When you classify your data, you can either use one of the standard classification schemes ArcMap provides or create custom classes based on class ranges you specify. If you want ArcMap to classify the data, simply choose the classification scheme and set the number of classes.

Defining your own classes

If you want to define your own classes, you can manually add class breaks and set class ranges that are appropriate for your data. Alternatively, you can start with one of the standard classifications and make adjustments as needed.

Why set class ranges manually?

There may already be certain standards or guidelines for mapping your data. For example, temperature maps are often displayed with 10-degree temperature bands, or you might want to emphasize features with particular values, for example, those above or below a threshold value. Whatever your reason, make sure you clearly specify what the classes mean on the map.

How to set a classification

Setting a standard classification method

1. Right-click the layer that shows a quantitative value for which you want to change the classification in the table of contents and click Properties.
   
2. Click the Symbology tab and click Quantities.
   
3. Click the Value drop-down arrow and click the field that contains the quantitative value you want to map.
   
4. Click the Normalization drop-down arrow and click a field to normalize the data.
   
ArcMap divides this field into the Value to create a ratio.

5. Click Classify.
   
6. Click the Method drop-down arrow and click the classification method you want to use.
   
7. Click the Classes drop-down arrow and click the number of classes you want to display.
   
8. Click OK on the Classification dialog box.
   
9. Click OK on the Layer Properties dialog box.
   

Tip

Increase the number of columns shown to see more data values in the histogram.

Editing a class range

1. Right-click the layer for which you want to set class breaks in the table of contents and click Properties.
   
2. Click the Symbology tab.
   
If you have not already set a classification method, follow the steps in "Setting a standard classification method".

3. Click the Range you want to edit.
   
Make sure to click the Range, not the Label.

4. Type a new value.
   
This sets the upper value of the range.

5. Click OK.
   

Tips

You can change a break on the Classification dialog box by clicking a value in the Break Values list and typing a new value. Class ranges will be adjusted. You can click and drag a break in the histogram on the Classification dialog box. You can also type new breaks in the Break Values list on the right side of the Classification dialog box. Any time you insert, delete, or move class breaks, the classification scheme automatically switches to Manual, no matter what scheme you started with. Statistics such as minimum, maximum, sum, and standard deviation appear on the Classification dialog box. If you check Show Mean, the mean is plotted on the histogram; checking Show Std. Dev. superimposes standard deviation lines on the histogram. Checking Snap breaks to data values uses actual data values as class breaks when you insert or move a class break. This option is only available when using a Manual classification method.

Deleting a class break

1. Right-click the layer for which you want to delete a class break in the table of contents and click Properties.
   
2. Click the Symbology tab and click Classify.
   
3. Click the class break you want to delete.
   
The selected break is highlighted.

4. Right-click the histogram and click Delete Break.
   

Tips

Increase the number of columns shown to see more data values in the histogram. To insert a break, right-click the histogram on the Classification dialog box and click Insert break. Statistics, such as minimum, maximum, sum, and standard deviation, appear on the Classification dialog box. If you check Show Mean, the mean is plotted on the histogram; checking Show Std. Dev. superimposes standard deviation lines on the histogram. Any time you insert, delete, or move class breaks, the classification scheme automatically switches to Manual, no matter what scheme you started with.

Excluding features from the classification

1. Right-click the layer for which you want to exclude features from the classification in the table of contents and click Properties.
   
2. Click the Symbology tab and click Classify.
   
3. Click Exclusion.
   
4. Double-click the field you're using to draw the layer on the Query tab.
   
5. Click an operator.
   
6. Click Get Unique Values to see the values for the selected field and double-click the value you want to exclude.
   
7. Click OK to execute the expression and exclude values.
   

Tips

Increase the number of columns shown to see more data values in the histogram. You can click Load to add an existing expression into the Data Exclusion Properties dialog box. You can also check for errors in your expression by clicking the Verify button. If you want to reuse your expression in another query, click Save. To learn how to build a query expression, see Building an SQL expression.

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