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Hot Spot Analysis: Create a custom legend for raster data
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The default legend for the call surface layers.
The default legend for the call surface layers.
A custom legend showing specific significant data values within the progression of colors used to show the call surface layer.
A custom legend showing the mean, specific significant data values, and hot and cold spot regions, within the progression of colors used to symbolize the call surface layer.

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.

Here is an image of the graph we wanted our legend graphic to match up with.
Here is an image of the graph we wanted our legend graphic to match up with.

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:

  1. Create a legend element using only your raster layer; it should look similar to the default legend shown above.
  2. Convert the legend to graphics and ungroup the resulting graphic. Ungroup them once more so you can edit the legend values as well.
  3. Delete the text elements that relate to the title, legend item (layer), and heading.
  4. Edit the legend labels to remove all text except the numerical values. You can also round off the values at this point because the extra digits don't convey any additional sigificant information for the data on this map.
  5. TIP

    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.

  6. If your raster layer was symbolized with a customized color ramp, be sure to save it in a style, as you will need it in the next step.
  7. Delete all the rectangles that contain the legend colors. Then draw a new rectangle graphic element.
  8. Symbolize it by right clicking, selecting Properties, then Change Symbol.
    1. Use a black outline with a line width of 0.4 pts. On this map, this replicates the symbology used for the data frame’s border.
    2. Click Properties again and change the Type to a Gradient Fill Symbol. Use the same color ramp that was used for the raster layer of the hot spot analysis. Set the intervals to 99 and the angle to 270 degrees.
    3. To get an extra wide band of the neutral central color in our color ramp, we added an Algorithmic color ramp in the midst of the ramp we used on the raster layer, giving us five ramps total within our multipart color ramp. Here is an image gradient fill properties we used
      Here is an image gradient fill properties we used.
  9. Add line elements for the minimum, maximum, mean, and thresholds for significance. We set the width of these lines to be 1.0 points.
  10. Since the color ramp outline was less important we set it's width to 0.4 points.
  11. To make sure that the lines are all placed properly, we selected them and aligned them.
  12. TIP

    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.

  13. Last, we added explanatory text to the legend and two text elements that contain a left curly brace "{" and set the size to match the significant areas within the legend.

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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