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continuous color ramps for raster data in ArcMap

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#1
ehatlebe

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I am having difficulty setting the range of a color ramp for rasters outside the existing data range in ArcMap 10. For example, when working with temperature data with value ranges from 5 to -2 (negative values blue and positive red, in increments of 1 degree), the problem is that zero should be the central value and the blue -2 should be a similar to red 2 instead of red 5. I found the below post a helpful starting point, but trying to go a step further. Any suggestions welcome!

In 10.0 a new option has been added to the raster symbology dialog:
Open the raster layer's Layer Properties sheet and go to the Symbology tab
On the left, under Show, make sure it's set to use the Stretched renderer
On the right, where it has the 3 "classes", with High and Low next to the high and low values click the Labeling button
This will open a dialog titled Advanced Labeling
Set the number of intervals (this is not the number of classes, but the number of breaks between classes; so 2 intervals means you get 3 "classes")
Then click the Generate button
You can edit the color by double-clicking the color patch, or click on the value to change the break point
Once you have it set the way you want OK out of the dialogs to apply the changes
You'll see the new breaks labeled in the TOC and in the legend on the layout


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Dave Barnes
ESRI
Product Engineer
Map Geek

#2
DaveB

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With only 8 values why not use the unique values renderer? You can select a subset of the classes in the layer symbology dialog, right-click and apply a color scheme to just the selected colors. You will want to make sure you have color ramp styles set up for the red and blue colors going in the direction you want them to go. So one would go from light to dark and the other from dark to light.
Dave Barnes
Esri
Product Engineer
Map Geek

#3
frax

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To me it sounds like there are more than 8 values - it sounds to me as if the values are single/double with decimals.
Hugo Ahlenius
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#4
ehatlebe

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Thanks for the feedback Dave. This is good to know, I will keep this information regarding unique renderer in mind for future projects. Unfortunately, there are many values, mostly decimal, and I am still trying to figure out how to keep values, -2 and 2 a similar shade of red and blue when the range of negative and positive values can be very different.

With only 8 values why not use the unique values renderer? You can select a subset of the classes in the layer symbology dialog, right-click and apply a color scheme to just the selected colors. You will want to make sure you have color ramp styles set up for the red and blue colors going in the direction you want them to go. So one would go from light to dark and the other from dark to light.



#5
DaveB

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Sorry, I made the assumption you were talking about integers. D'oh!

The stretched renderer creates equal intervals, and you don't have any control over the ranges, so that's probably not the way to go for this case. I think you can get close to what you need if you use the Classified renderer. It's a bit trickier though. With the Classified renderer you can set the upper limit for each class, the lower limit is defined as anything above the upper limit of the next lower class. So you would need to set the upper limit for each class such that it includes everything above the upper limit of the previous class and nothing above the upper limit for the current class. I don't have data handy to try this out myself and get the correct values, but if you can try experimenting a little I think you can get it. For the zero class, for example, I think you would want the upper limit to be zero and the class below it to have its upper limit set such that it gets everything just short of zero (something like -.0001?).

Once you have the classes set up you can work on the symbols. Set the lightest and darkest colors for both the reds and the bluses and set the color you want to show for zero. Select just those classes using Ctrl + click, then right-click on them and select Ramp Colors. That does what it says, ramps between the selected colors. If you want the colors on the two color ramps to be similar in value or whatever you'll have to sort of "eyeball", adjustingthe outer colors for each ramp proportionally dakerker or brighter or whatever you are using. You can still adjust individual colors afterwards as needed, too.

I hope that helps. After this morning I may not have much access to cartotalk until next week, but I'll be sure to check back here when I can.
Dave Barnes
Esri
Product Engineer
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