The following spatial relationships are used in OpenSIMPPLLE.

Aquatics

The upstream predecessor and the successor aquatic units in the network are identified.

Land

Neighboring land units (similar to vegetation) and the vegetation units that coincide with it are identified.

Vegetation

The set of adjacent neighbors for each vegetation unit are identified. If using irregular polygons it is any unit that shares a boundary. If polygons are uniform, the system can identify a maximum of eight neighbors. Each neighbor is identified as being above, below, or next to by comparing the elevation of the center of the plant community. The elevation has to come from either an elevation field in the cover for the vegetation or a digital elevation model. Neighbors that are downwind are identified by providing a dominant wind direction for the landscape. The distance to the nearest aquatic unit and distance to the nearest road unit are calculated in OpenSIMPPLLE.