Here, a qualitative spatial relative direction relation between two o-points is represented by two pieces of information:
- the sector (seen from the first o-point) in which the second o-point lies (this determines the lower part of the relation symbol), and
- the sector (seen from the second o-point) in which the first o-point lies (this determines the upper part of the relation symbol).

Altogether we obtain 8 × 8 base relations for the two points having different positions.

Then the configuration shown in Fig. 1 is expressed by the relation A lf rf B. If both points share the same position, the lower relation symbol part is the word "same" and the upper part denotes the direction of the second o-point with respect to the first one, as shown in Fig. 2.

![0_image_0.png](0_image_0.png)

![0_image_1.png](0_image_1.png)

Figure 1: Qualitative spatial relation between two oriented points at different positions.

The qualitative spatial relation depicted here is A lf rf B.

Altogether we obtain 72 different atomic relations (eight times eight general relations plus eight with the o-points at the same position). These relations are jointly exhaustive and pairwise disjoint (JEPD). The relation **front**
same is the identity relation.

Figure 2: Qualitative spatial relation between two oriented points located at the same position. The qualitative spatial relation depicted here is A rf same B.

## 2.2 Elevated O-Point Relations

An extension of binary direction calculi with a local reference direction can also be motivated with the following conception. In this conception local basic perceptions of cognitive agent are the basis for the agent's ability to distinguish relative distances. In this conception the point which represents the location is "elevated" above the 2D-plane (see Fig 3). From this observer perspective Gibson's insights about natural perspective
[5] motivate the availability of depth clues which let the observer distinguish local distances based on it's height as reference distance. In the case of the **OPRA**mcalculus in which the entities are oriented points called o-points, the elevated entities are called elevated o-points, or eo-points.