Example 1.3: Displaying Classes in a Layer
By using non-spatial attribute information in the data, we can create a map like this:
This is what the mapfile looks like (Example1-3.map):
The mapfile structure, by objects, looks like this:
MAP (states_poly) LAYER----------|---------LAYER (states_line) | | (land) CLASS---|---CLASS (water) |-CLASS | | | STYLE-| |-STYLE |-STYLE
Our mapfile still only has two layers but the polygon layer has been broken down into two classes. Let's have a look at the additional parameters:
How do we know which attribute to use? It is recommended to use ogrinfo to display basic attribute info in your vector data-look back at example 1.1 (the last few lines after "Layer SRS WKT:" show the attribute names and types). In this case, since database records in shapefiles are stored in DBF files, we can also open it in a spreadsheet program such as LibreOffice, or in desktop software such as QGIS. If your data came with a metadata (and it should!), you can skim through through that metadata file for attribute information..