Ask A Cartographer

A trick for setting custom class breaks in the Classification dialog.

November 07 2007 | 0 comments
Categories: ArcGIS Methods, Symbology

I am mapping data that is a percentage (% of goal achieved) and my data range from 22% to 91%
(please see attached .pdf file). I am mapping quantities using graduated colors with 10 classes and want my class breaks to be at 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100% so that viewers can see where the data fall on a percentage scale that runs the full distance from 0 to 100%. When I try to set these class breaks (starting with 100% first and working my way down to 10%) the software will not allow class breaks at 20% or 10%, but changes these to 22% or some number close to 22%. It is OK if the colors associated with the 10 and 20% classes never show up on the map other than in the legend. The point is to show the data on an “absolute” scale from 0 to 100%. Any recommendations as to how I can accomplish this?
Thank you.

Attachment: Example of 22-100% range in legend

Mapping Center Answer:

In the Classification dialog, the break values box (in the lower right) click on bottom value in the list, and then type the correct break value (this will set the classification method to “Manual”).

The trick is to start at the bottom and set that break’s value to 90, then the next one up to 80, and so on (the up/down arrow keys are useful to navigate here). The reason to start at the bottom is that the values in the list must be in order and the list won’t accept a number that is out of order. Starting with the highest numbers (at the bottom) and working your way up to the lowest values is the way to go.

Close the Classification dialog. The lowest value in the first range will always be set to the lowest value in your data—“truth in mapping” is why we do that.

Before you fix that you can optionally click on the “Label” heading button in the list of classes and choose “Format Labels” and change the format type to Percentage.

To change the label of the first class, just click on it and start typing to change the range to be 0 to 10.

If you would like to post a comment, please login.

Contact Us | Legal | Privacy |