Classifying numeric data that includes a "No Data" code:

If your data layer's legitimate values range from (for example) 0 to 100, but you have a set of "no data" records coded (for example) -99, ArcMap will report the minimum data value as -99, and will include those -99s in any calculations of means, quantiles, etc. If you need to know what the true min/max/mean values are, or you want to set symbol classes based on true quantiles or some other statistical method, you must tell ArcMap that a particular set of values in your data layer should be excluded from the symbol classification:

  1. Under the "Symbology" tab in the layer properties, choose "Classify..."
  2. Click the "Exclusion..." button.
  3. Under the "Query" tab, find the field you are using for your map and double-click it to add it to the "SELECT FROM" box.
  4. Click the "=" sign to add it to the "SELECT FROM" box.
  5. Now type -99 after the equals sign in the "SELECT FROM" box. Your expression should look something like:
  6. Now click on the "Legend" tab.
  7. Check the checkbox that says "Show symbol for excluded data"
  8. Click on the colored box after the "Symbol" text to set how you want your "no data" areas to display.
  9. Type "No Data" (or similar) in the "Label" box.
  10. Click OK to close out the "Exclusion" window.
  11. Click OK again to close out the "Classification" window.
  12. Click one last OK to close out the "Symbology" window. You must close out all of these windows for the "no data" exclusion to get absorbed into ArcMap's engines.
  13. Now re-open the layer's symbology window ("Properties" -> "Symbology" tab). Go back into the "Classify" window and note the new (correct) range of values that excludes your "no data" value. Also note the new "No Data" class in your layer's legend. Symbolize your data as usual.

More info : Classifying numerical fields for graduated symbology

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