What does Quantity by Category mean, and what can you do with this symbology method?
The Quantity by Category layer symbology method lets you do bivariate mapping in ArcMap. Think of it as a way to specify two symbologies for the same layer, each based on a value field. Each symbology defines how to symbolize the variation of the value field. Values in the first field will always be symbolized using unique values. This is the primary symbology "by Category". You have a choice for the second symbology. Clicking the Color Ramp button varies the color (Graduated colors), and the Symbol Size button varies size (Graduated symbols). The secondary symbology is the "Quantity" part of the definition. Combined, they create the "Quantity by Category".
How is the symbol that is used to draw a given feature determined?
The symbol used to draw each feature is built by combining the corresponding symbol in the primary symbology and the corresponding symbol from the secondary symbology. A simple rule to remember is that the symbol from the primary symbology will be used, and either its color or size will be overwritten based on the properties of the secondary symbol. Understand that ANY symbol property that you set on the secondary symbol, other than color or size, will just be ignored. Here are the details on how this works:
· The base symbol always comes from the primary symbology. This symbol's color OR size is modified based on the properties of the secondary symbology.
· If you choose Color Ramp as your secondary symbology, then part of the color definition comes from the secondary symbol. The HSV color model is used (Hue, Saturation, and Value). Saturation and Value come directly from the secondary symbol, and the base symbol (that is, the primary symbol) keeps its Hue value. For example, if the primary symbol is a 12 point trianglular Simple Marker Symbol with H = 200, S = 50 and V = 75, and the secondary symbol is a fill symbol with H = 100, S = 5, and V = 95, then the feature is drawn with a 12 point triangular Simple Marker Symbol with H = 200, S = 5, and V =95.
· If you choose Symbol Size as your secondary symbology, then the only property that comes from the secondary symbology is the symbol size. For example, if the primary symbol is a black triangle of size 12 pts, and the secondary symbol is an orange circle of size 28 pts, the feature is drawng with a black triangle of size 28 pts. If you have a polygon layer, this is a special case, see below.
What do the Background/Rotation/Transparency buttons do on the secondary dialogs?
Clicking the Color Ramp or Symbol Size buttons open some dialogs that should look familiar to you. These are, more or less, the same dialogs that appear if you use the univariate Graduated colors or Graduated symbols layer symbology methods. The Background button appears on the secondary symbology dialog only if you are symbolizing a polygon layer. This is the background fill symbol, but it is NOT used.
When symbolizing using Quantity by Category for a polygon layer, polygons are drawn with the fill symbol from the primary symbology. Graduated symbols are drawn on top of these polygons, and these symbols come directly from the secondary symbology. The background symbol does nothing. The same goes for the Rotation and Transparency buttons that may appear on the secondary dialog. These do nothing. Remember that any symbol property that you set on the secondary dialog, other than color or size, will be ignored.