## 3 Method
### The PRB Model for Field Data Collection  
Due to the complex field environment, DGM has high requirements for the field data collection system in terms of the device, software, battery life, portable display, and interactivity. The existing techniques are not able to satisfy all of the above requirements in a small handheld PDA, so it is necessary to develop a compromise between them. Simplicity and ease of use are important criteria for designing a field data collection system [5]. As polygon objects are difficult to input accurately in the field and further edit indoors, point and polyline objects are preferred as the forms for data entry in the field data collection. To address this issue, the Point-Routing-Boundary (PRB) model was proposed based on the process of geological survey in the field [39].  
In the PRB model (Figure 1), the core \(\mathbf{P}\) points represent the entity nodes to connect geological survey routes and collect information (properties). The \(\mathbf{R}\) lines do not only record the field route track of the geological survey but also represent the segment linking between points and (or) boundaries. Thus, the \(\mathbf{R}\) lines between boundaries usually represent a unique geological unit, which can be used to automatically fill the unit properties of polygonal geological bodies in the stage of map-making. The \(\mathbf{B}\) lines represent the explicit boundary between the geological units and provide implicit topological informa  
Figure 2: The vector data structure in the DGSS.  
tion between geological bodies, such as the properties of the geological unit and contact relationships.  
In the PRB model, the \(\mathbf{B}\) and \(\mathbf{R}\) lines depend on the \(\mathbf{P}\) points. Additionally, the \(\mathbf{B}\) lines depend on the \(\mathbf{R}\) lines. The life cycle of a \(\mathbf{P}\) point starts from \(\mathbf{P}_{\text{n}}\) and ends up at \(\mathbf{P}_{\text{n}+1}\) (Figure 3e). A geological survey route can be regarded as a combination of PRB elements. \(\mathbf{P}\), \(\mathbf{R}\), and \(\mathbf{B}\) elements can be flexibly combined according to the needs for a geological survey. The number of \(\mathbf{R}\) and \(\mathbf{B}\) lines is determined based on the complexity of a geological route. In a field geological survey, the \(\mathbf{P}\) point can also be used alone for field data collection in outcrops (Figure 3a). The two \(\mathbf{P}\) points can be connected by the \(\mathbf{R}\) line to represent two geological survey points and the routing line from \(\mathbf{P}_{\mathbf{1}}\) to \(\mathbf{P}_{\text{2}}\)(Figure 3b). As the pre-designed geological survey route is perpendicular to the strike of the geological framework, \(\mathbf{P}\), \(\mathbf{R}\), and \(\mathbf{B}\) are always combined to represent a geological survey route (Figure 3c), the point combined with a boundary to represent the geological boundary between two geological units, and the boundary on the routing \(\mathbf{R}\) line can be used to represent the boundary within the geological unit. Moreover, the PRB model can represent the intersection of multiple geological units (Figure 3d).  
The PRB model constitutes a data framework for geological route surveys. Other information such as samples, attitudes, fossils, photos, and sketches can be added nearby and attached to the corresponding PRB elements based on the ID number.