There are infinite ways to build Web GIS sites and OWGIS can not create all of them. The websites that are created by OWGIS follow a set of design principles that we believe can be applied in a large number of circumstances. These design principles were defined by analyzing a few of the currently available Web GIS sites. Sites that display scientific data have, in many cases, a large set of layers that can be overlayed all at the same time. Having many layers available at the same time makes it difficult, for a normal user, to find the specific data to visualize. Moreover, there are several Web GIS sites where the majority of the data are raster (pixel based) layers, which occlude themselves when superimposed with each other, and overlaying raster layers makes a map incomprehensible. With this in mind OWGIS separates layers into three different categories: