What: The default legend that ArcGIS creates for a raster layer is somewhat more limited than those for vector layers. Therefore you may want to customize it to suit the needs of your map. For this map, a custom legend was created because we wanted to give the map reader more information and more specific information about the data mapped. This legend represents the results of a hot spot analysis, which identifies clusters of points with values higher in magnitude than you might expect to find by random chance. The values are z scores which indicate the statistical significance of clustering for a specified distance. To determine if the z score is statistically significant, you compare it to the range of values for a particular confidence level, for this analysis, +1.96 or -1.96. Significant clustering occurs when the colors are bright blue or red. Learn more about hot spot analysis in the ArcGIS online help. This legend is visually connected to a graph that shows the data distribution, allowing the statistically significant breaks in the distribution to be seen on the graph.
Why: A customized legend gives you more power over its graphic appearance; however, it may require that you "disconnect" the legend from the data. The disadvantage of doing this is outweighed, in this cases, by the importance of presenting needed information and doing so in a graphically pleasing fashion.
How: To create a customized raster legend, you will pretty much have to make one from scratch. But you do need the data values from an ArcGIS legend, so a good approach is to create a raster legend, and then modify it. Here are the steps for creating a custom legend:
You can either edit the text in the Raster Layer Properties Dialog on the Symbology tab by changing the High and Low values, which will help by showing more consumable text in the table of contents and the text in the default legend will also use these values. However, for this map, since we needed to convert the legend to graphics anyway, and create custom values, we just edited (rounded) the text in the graphic elements of the custom legend.
To more efficently align graphic elements use the Graphics toolbar, which contains all the tools you will need, and they will be just one click away, rather several clicks deep in the Draw toolbar's Draw menu.
You should now have a customized legend that more fully describes the data and links visually to the graph. Of course, you can use the same set of steps to make customized legends for other raster based maps.
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