Refinement

Refinement is a method of cartographic generalization in which the set of features of a particular layer is reduced as the scale of a map is decreased. Maintaining the same level of detail at broader scales would result in an overcrowded and unnecessarily complicated map, so less important features are removed altogether. This technique is especially useful when the topic being portrayed has a clear hierarchy of more and less prominent features, such as roads, rivers, and cities.

Refinement should not be confused with Eliminate or Add operators, which are also useful in GIS. Eliminate operators remove features that are difficult to understand or read or when they become irrelevant to the dataset. Add operators add data to a map as the scale becomes smaller, it is typically reserved for data that is useless at larger scales. Refinement removes or adds features based on a hierarchical ordering and is most useful in GIS when determining symbology.

Refinement is also very similar to another generalization technique, collapsing features, but there is a key difference between the two. Collapsing creates a simple geometric feature from many, such as representing a cluster of trees by a single tree symbol, while refinement just eliminates less important features, such as only showing the largest or most prominent trees.

Selecting what to include/exclude
The scale of the map will determine how much space is available for illustrating and labeling. The hierarchy mentioned above is key in deciding what should be included and what could be excluded. More prominent or popular features should be given priority. For example, a map of the United States should include only its largest cities, not all or even most of them. It wouldn't make sense to illustrate and label every city in the country. If every city were marked, there wouldn't even be enough room to label them with a reasonable text size, let alone room to draw or label any other kind of feature. Refining should follow the hierarchy of most important to least important. It should be noted that what is important for one map may not be as important for another, depending on the intended use.

Implementation in GIS Software
Most GIS software has the ability to filter a layer to select which features therein are displayed, usually according to criteria based on its attributes. For example, in Esri ArcGIS, one can set the definition query for each layer.

There are now many tools available on ArcMap to help refine the symbology on maps. This tool set, called the Cartographic Refinement Toolset, has tools that can create overpasses and underpasses, disperse markers, and align marker to stroke or fill, and many more. . Using tools and refining your maps can create a more unifying look between the generalized features and it can make the map easier for people to understand.