Changing symbology

Concepts:
   raster colormap
   unique values classification
   stretched symbology

When you use the landcover data, you may find that the symbology used is not what you would like to see.  In the case of landcover data, a raster colormap is used, which assigns a color to each pixel value automatically.  A colormap is fixed, the colors cannot be changed (at least not easily, one has to remove the colormap and load a new one which is beyond the scope of this class).

However, the layer can have a different classification applied, where an entirely different system of assigning colors to pixel values is used.

One such approach, using categorical data such as landcover data (where pixel values represent discrete categories), is the unique values classification in ArcMap.  This is similar to a colormap, except that it is defined in the layer in ArcMap and can be easily changed by the user.  This is the method used in both the sample layer file and MXD distributed with the data (and the layer file used by DEP users).  The user defines what color is assigned to each pixel value, and which values are shown and which ones are not.  It also can be used to simplify a classification without changing the data (i.e. 'clumping' classes).

To create a unique values classification, right-click the layer in ArcMap and bring up the layer properties.  Click on the 'Symbology' tab, and select "Unique Values" from the list.  The original colormap will be automatically translated, so the colors stay the same.



For data with continuous values, such as NLCD imperviousness and canopy data (more on these data layers later), using the stretched symbology is a better option.  With those data, you typically want to indicate that higher percentages have more of a certain color.  For a range of values from 0 to 100, for example, you may want to indicate the highest (i.e. 100% canopy closure) with one color, and the lowest (i.e. 0% canopy closure) with another, with all other values indicated by a ramp between the two extremes.

To create a stretched symbology, just choose "Stretched" on the "Symbology" tab, and choose the color scheme.  Be sure to set the stretch type to "Minimum-Maximum".




30-meter NLCD 2001 canopy data shown using a min-max stretch.