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Introducing Geographic Information Systems with ArcGIS: A Workbook Approach to Learning GIS, 3rd Edition by Michael D. Kennedy

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A More Careful Look at Equal Intervals
____ 13. Classify the values again. Choose the Equal Interval method again. Notice that the break lines move back to the positions they had before. These positions are calculated so that each class occupies the same distance along the horizontal axis. What is that distance? ________. The calculation takes the form of “greatest value in the data set minus the least value, divided by the number of classes.” Click OK, Apply, OK. Notice that the resulting map has five categories in the T/C, but only four colors show up in the map, because one class is empty. What is the range of the class that has no values in it? __________ – __________ Notice that the values around 1000 and those around 5000 are lumped together. Look at the Classification window again to notice that one class contains two clusters of values that beg to be separated, while another class is empty. This is a difficulty with equal intervals.

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