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Hot Spot Analysis: Color match symbols in vector and raster layers
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The left map shows colored circles and the right maps shows a colored raster surface that echoes the colors of the circles.
This data frame shows graduated circles colored to show the data values
A custom legend showing specific significant data values within the progression of colors used to show the call surface layer.
This data frame shows a raster surface using the same colors as the circles..

What: On this poster, we used the same colors to symbolize the graduated symbols and the raster surface. The graduated symbols use bivariate point symbols in which color relates to the results of the hot spot analysis and size relates to the number of 911 made from each point location. The raster surface shows an interpolation of those points. Since the data frame with the raster surface is directly related to the data frame showing the graduated point symbols it was symbolized using a color ramp that matches the colors for the points.

Why: When you want your readers to compare two related maps, you can help them with visual cues. A really good one to use is color. Map readers will have an easier time comparing the two distributions since the colors are the same.

How: To create these maps with matching colors, you will need to set the point symbology then edit a color ramp for the raster layer so it uses the same colors in the same locations. Here are the steps you can follow:

  1. First, set the symbology for the points. We set the symbology to ten classes using the colors of the rainbow. This color scheme is called a "spectral color progression". Save each of the colors to a style so you can use them to specify the properties for the color ramp that symbolizes the raster layer.
  2. Right click the raster layer, select Properties and click on the Symbology tab.
  3. Set the symbology Type to "Stretched".
  4. Since we decided to use a spectral color progression for the point symbols, we needed a color ramp to match. In ArcGIS, this is created as a multi-part algorithmic color ramp. To create this so it matched point symbols colors, we edited the color ramp through the Layer Properties dialogs instead of the Style Manager. This allowed us to see the changes interactively on the map so it was easier to fine tune than repeatedly opening and closing the Style Manager.
  5. Scroll down to find any diverging color ramp to edit. A diverging color ramp is one that has brighter colors at the two ends and a faily subdued color, like white or yellow, in the middle. These are used to place emphasis on the extemes at both ends of the data range, as well as critical values in the middle. This is an appropriate type of color ramp to use with the data that was used to make our map.
  6. Verify that you selected a diverging color ramp by right clicking the color ramp. This displays a menu that can be used to toggle the Graphic View on and off. When off, it will instead display the name of the ramp, which can sometimes be a very useful way to search for the ramp you want. If you selected a color ramp from the you will be able to use this method to check that the color ramp you selected is indeed diverging.
  7. When you right-click on the color ramp in the symbology tab you get a menu.
    By right-clicking directly on the color ramp in the Symbology tab of the Layer Properties dialog box you can access the proprities, save, or toggle between graphic view and showing the color ramps' names.
  8. Right click the color ramp you chose and select Properties.
  9. Double click the first color ramp. Now you can edit the colors in each part of the multi-part algorithmic color ramp. For our map, we changed Color 1 to the blue that was saved in Step 1, and then we changed Color 2 to the light yellow that was saved in Step 1.
  10. Click OK and repeat the above step for each part of the color ramp. Note that you can also add a new part to the ramp and you can move it up or down in the display order using the arrows at the right.
  11. Move the Layer Properties window out of the way, but DO NOT close it. Click Apply rather than OK to see how this color ramp looks on your map. For our map, the first thing we noticed was that the colors in the ramp for the raster data didn’t quite match the colors of the points in the middle portion of the color ramp. The orange was more vivid, and the cyans and greens were missing.
  12. Here's how we edited the colors:
    1. "To edit the oranges, we displayed the color ramp properties again and change the Algorithm to "Lab LCh". Click OK.
    2. To edit the cyans and greens, we changed the Algorithm of the second color ramp to HSV.
    3. TIP

      There is a Knowledge Base article that describes the different color ramp algorithms.

  13. To save this new color ramp in your personal style, right-click the color ramp (as shown above) and select Save to Style. Give your color ramp an appropriate name and click OK. If you don’t do save the color ramp this way, you will lose all the edits you just made and have to start over again.
  14. Then apply the new symbology to the raster layer.
  15. Visually compare the two maps. Where red points exist, red pixels should correspond. We noticed that they didn’t quite match on our maps; some of the red points were located in orange areas on the raster map, and some of the blue points were in green areas. To fix this, we edited the the Histogram of the raster layer. Here is how we edited the histogram:
  16. The default histogram for the raster.
    The default histogram for the raster.
    1. In the Symbology tab of the Layer Properties click the Histograms button.
    2. To edit the red colors, we added a point by clicking on the line in the histogram tool. To help understand where to click on the line, imagine your color ramp is displayed horizontally across the bottom of the histogram. Click the spot where the orange is that you want to become red. That will add a vertex; now select and drag that vertex vertically to the top. The customized histogram displays in magenta over the gray original histogram showing you how the distribution of cells using your color ramp has changed.
    3. To edit the blues, click on the line at a location that corresponds to where blue would be and drag the vertex straight down to the bottom of the histogram. The edits we made are displayed on the histogram below.
    4. The custom histogram showing the changes for red which are related to the vertex at the top of the data space, and blue which are related to the vertex at the bottom.
      The custom histogram showing the changes for red which are related to the vertex at the top of the data space, and blue which are related to the vertex at the bottom.

      To remove a vertex from the histogram, click on it and drag it towards either the upper right or lower left of the histogram. That will cause the vertex to disappear by the time you reach either corner as you drag the vertex.

  17. Until your map looks right, you can go back and repeat the necessary steps to further edit the colors and their assignments to the data.

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