You can use representation to control the appearance of map layers. It is used most commonly with vector data, but can be also be used for raster (raw or thematic) data. A default representation is applied to any vector layer that is not linked to a representation style table (RST). An RST is a table that contains a key and the style associated with that key. The key or the style is also known as a REPCode. The key links the style in the RST to the record in the layer. Each layer can link only to one RST, but you can link an RST to many layers. You can continue using the default, create a new RST, or link to an existing RST.
In vector or thematic raster representation, keys can be added as attributes, such as in the REPCode field, or you can use an existing attribute as the key. When an RST is linked to a layer, the attributes used as keys in the layer are identified and the RST is searched for the corresponding keys. When a match is found, the associated style is applied to the shape, for vector representation, or area, for thematic raster representation, in the layer.
For example, you can associate an RST to the attributes in a field called LandCover in a polygon vector layer or in a thematic raster layer. In the LandCover field, you have a variety of land covering types, such as forest, water, and urban. In the RST, the key is the LandCover type with which you have styles associated. When a layer is linked to the RST, the shapes or areas are assigned the style according to their key.
In raw raster representation, styles are linked to pixel values. For example, you might have a DEM where the pixel values represent elevations. You could specify a unique color (or shade of gray) for each unique pixel value. However, you would more likely specify several elevation ranges—for example, less than zero, between 0 and 1000 meters, between 1000 and 2000 meters, and so on. Each representation entry can be assigned a graduated color, so that as the elevation values increase, the color of the style becomes darker.
A style is composed of one or more parts. Each part has a primitive and a priority. The primitive is the building block of the style. A primitive is based on a point, line, or polygon pattern that is designed according to a number of parameters, such as color. The priority determines which part is placed in front of the other parts in the style. You can control the order by assigning a priority number to each part. A part with a higher priority number displays in front of a part with a lower priority number.
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