An electronic chart topology editing linkage updating method

Abstract

The present application relates to a kind of electronic chart topology editing linkage update method, belong to marine surveying and mapping technical field.By loading electronic chart dataset, topology data structure is constructed;According to topology data structure, topology linkage controller is created;Topology editing function is started, and it is selected to increase, delete or modify a certain topology unit;After completing topology editing, current topology unit is updated, and the topology unit of association is sent linkage update event notice;According to linkage update event notice, topology linkage update is implemented.The present application constructs "topology linkage controller", realizes the topology editing linkage control and update transmission based on event driving, solves the problem that existing basic geographic information platform cannot support electronic chart production, has the advantages such as clear business logic, simple technical method, function robust and reliable.

Description

Technical Field

[0001] This invention belongs to the field of marine surveying and mapping technology, and in particular to a method for linking and updating the topology of electronic nautical charts. Background Technology

[0002] In mathematics, "topology" refers to the characteristics of a graph (or set) that remain unchanged under continuous deformation. "Map topology," a concept in cartography and geographic information systems, refers to the sum of stable spatial relationships (such as coincidence, adjacency, containment, and connection) between spatial geometric instances (such as points, lines, and polygons) after ignoring their specific geometric details (such as length, area, and coordinate position). "Topology editing" refers to various graphic and relational operations applied to spatial geometric instances to construct a map topology that meets requirements. For example, it involves constructing two line features with a coincident relationship into the same set of connected edges and nodes, thereby ensuring the accuracy and consistency of map topological relationships.

[0003] In map topology, spatial geometry is often abstracted into geometric simplexes and geometric complexes. A geometric simplex refers to a single, connected, and homogeneous geometric object in space; a geometric complex is a set of separate geometric simplexes, where the boundary of each simplex can be expressed as a union of other geometric simplexes with lower dimensions within the set. In two-dimensional space, spatial geometry can be divided into three types of geometric simplexes based on their dimension: points, curves, and surfaces. Based on different spatial geometric organization methods and constraints, five topological levels can be defined: 1. Level 1. Unconstrained, consisting of a set of independent points and curves: curves have no reference points (no boundaries), points and curves may repeat; a region is represented by a closed curve. This level can be simply referred to as "topologically unrestricted".

[0004] 2. Level 2a. Consists of a set of point and curve simplexes conforming to the following constraints: Each curve must reference a start point and an end point (which may be the same); curves must not self-intersect; the region is represented by a closed curve, and the start and end points are common points; for regions with holes, all internal boundaries must be completely contained within the external boundaries, and internal boundaries cannot intersect with other internal or external boundaries. Internal boundaries may be tangent to other internal or external boundaries (i.e., at a single point); the outer boundary of the surface must be clockwise (the surface is to the right of the curve), and the direction of the curve must be positive; the inner boundary of the surface must be counterclockwise (the surface is to the right of the curve), and the direction of the curve must be negative. This level can be simply referred to as "point chain topology".

[0005] 3. Level 2b. Consists of a set of point and curve simplexes that meet the following constraints: In addition to Level 2a, the following constraints are added: Each set of simplexes must form a geometric complex; curves cannot self-intersect if no points at their intersections are referenced; multiple coincident geometries are not allowed. This level can be simply referred to as "planar topology".

[0006] 4. Level 3a. Consists of a set of points, curves, and surface simplexes that comply with the following constraints: Level 2a constraints apply.

[0007] 5. Level 3b. Consists of a set of point, curve, and surface simplexes that conform to the following constraints: Building upon Levels 2a and 2b, the following constraints are added: surfaces must be mutually exclusive, and they must provide a complete spatial cover. This level may be simply referred to as "complete topology".

[0008] In the 1980s and 90s, when digital mapping was still in its infancy, many applications adopted data models with topological constraints to reduce storage space. A typical example is the Coverage data model in ESRI's ArcInfo software, which is essentially a "complete topology." Due to the huge success of ArcInfo software in its global rollout, the Coverage data model was widely recognized and used for a long time, and most people equated "topology" with the Coverage data model. However, building a Coverage data model requires maintaining and updating very complex topological relationship tables, such as "face-face," "face-edge," and "edge-edge" relationship tables, making it very difficult to build a robust and practical topology editing software. In the 21st century, ArcInfo has been upgraded to ArcMap, which no longer supports the Coverage data model. It no longer requires explicitly defining and maintaining complex topological relationship tables. Instead, it adopts an object-oriented data model and predefined topology rules to achieve "instant" topology consistency checks at the feature object level, that is, to achieve logical consistency rather than physical spatial geometry consistency, or to achieve topology editing by building temporary "point-edge" topology relationships.

[0009] Nautical charts are a type of map that follows the shape of the land. Figure 1While possessing general theoretical frameworks, electronic nautical charts (ENATs) also have specific conventions and exhibit significant international characteristics. Their development is guided and influenced by international hydrographic organizations. For example, current ENAT data exchange formats must conform to the S-57 standard, with a topology level of 2a; while next-generation ENAT exchange formats must conform to the S-101 standard, with a topology level of 3a. The topology level of ENAT data differs from that of Coverage, and to date, topological consistency at the physical spatial geometric level is still required. Therefore, neither ArcInfo nor ArcMap currently supports the standardized production of ENATs. Globally, achieving topology editing and linked updates for ENATs is a challenging task. Currently, only a very few ENAT production software programs possess this capability, such as Caris and dKart. However, these software programs are specifically designed for ENATs, and their complex internal business logic makes them difficult to integrate into basic geographic information platforms, limiting the integration of ENATs into the mainstream geographic information industry.

[0010] The existing technology, "Local Update Technique for Topological Relationships Based on Topological Atomic Events," proposes a technique for local updating of topological relationships. First, it performs intersection chain breaking on all elements within the minimum bounding rectangle of the changing element. Then, nodes added or deleted during element editing are added to the node set generated by the intersection chain breaking. The arcs associated with these nodes and the nodes themselves require topological reconstruction. Other nodes adjacent to these nodes do not require topological reconstruction; only their topological information needs modification. This method simply filters the topological relationships requiring local updates using the minimum bounding rectangle of the changing element, but it ignores the cascading effect of topological relationship updates. Besides elements directly affected by the current changing element, some elements are indirectly affected. Furthermore, chain breaking can change the ID of the topological geometry; for example, moving or merging nodes may cause their IDs to change. Summary of the Invention

[0011] The purpose of this invention is to overcome the shortcomings of the prior art and propose an electronic nautical chart topology editing linkage update method, constructing a "topology linkage controller" to realize event-driven topology editing linkage control and update transmission.

[0012] The technical problem solved by this invention is achieved through the following technical solution: A method for linked updating of electronic nautical chart topology editing includes the following steps: Step 1: Load the electronic nautical chart dataset and construct the corresponding topological data structure according to the different geometric types of the elements, including four types: points, curves, composite curves, and surfaces. Step 2: To receive, process, and transmit editing event information during topology editing, a corresponding topology linkage controller is constructed based on the geometric type of the topology data structure, and topology units are further constructed through topology geometry and the topology linkage controller. Step 3: Activate the topology editing function, select the corresponding operation, and perform topology editing such as adding, deleting or modifying the geometry of a certain element in the electronic nautical chart dataset, and send an editing event notification to the topology unit corresponding to the geometry; Step 4: After topology editing, in order to update the topology units and transmit relevant information of associated topology units, the topology units corresponding to the currently edited geometry are updated according to the editing event notifications received by the topology units, and linkage update event notifications are sent to the associated topology units. Step 5: Based on the linkage update event notification, continue to update the topology unit that has received the notification, and continue to send update events to other topology units associated with the current topology unit, cascading the notification layer by layer, and finally realize the linkage update of the topology.

[0013] Furthermore, the topological data structure in step 1 includes four types of topological geometry: points, curves, composite curves, and surfaces. The relationships between these topological geometries are as follows: curves are composed of control points, excluding endpoints; surfaces are composed of several curves or composite curves; there is a one-to-many relationship between composite curves and curves, and there is also a one-to-many relationship between surfaces and composite curves or curves.

[0014] Furthermore, the relationship between the topology linkage controller and the topology geometry in step 2 is as follows: each topology unit is composed of topology geometry and topology linkage controller. The geometry can be spatial geometry instances such as points, curves, composite curves, or surfaces. The topology linkage controller is further subdivided into four types of linkage controllers: point, curve, composite curve, and surface, depending on the geometry type. Each topology unit will call the corresponding topology linkage controller according to its geometry type.

[0015] Furthermore, the topology linkage controller in step 2 includes an event receiving module, an event processing module, and an event transmission module, wherein the event transmission module is connected in sequence. The event receiving module is used to receive messages generated by the topology editing operation; the event processing module is used to perform specific spatial geometric position modification, topology structure update, and data persistence business logic on the nautical chart topology unit according to different event types; the event transmission module is used to transmit the events generated during the editing process in sequence according to the association relationship and order of the nautical chart topology unit, so as to achieve the purpose of cascading update only on local topology units within the association range.

[0016] Furthermore, in step 3, the topology editing function is grouped according to the type of geometry being edited. After selecting the editing type in the menu bar, the selected geometry is selected or edited on the map. Point operations include selection, creation, movement, and deletion; curve operations include selection, creation, editing, deletion, and breakage; composite curve operations include selection, creation, and extension; surface operations include selection, creation, deletion, and trimming; and multi-point operations include addition, deletion, and movement.

[0017] Furthermore, the topology linkage update in step 5 includes upward linkage, parallel linkage, and downward linkage. Upward linkage is used to update and modify topology units that reference the current topology unit and those referenced by higher-level cascaded units; parallel linkage is used to update and modify topology units at the same level that are associated with it; and downward linkage is used to update and modify topology units that are referenced by the current topology unit and those referenced by lower-level cascaded units.

[0018] The advantages and positive effects of this invention are: This invention constructs a "topology linkage controller" that realizes event-driven topology editing linkage control and update transmission, enabling real-time topology editing and dynamic updates. It solves the problem that existing basic geographic information platforms cannot support the production of electronic nautical charts and has the advantages of clear business logic, simple technical methods, and robust and reliable functions. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the electronic nautical chart topology of the present invention; Figure 2 This is a schematic diagram illustrating the relationship between electronic chart elements and topology in this invention; Figure 3 This is a schematic diagram of the nautical chart spatial object model of the present invention; Figure 4 This is a schematic diagram of the chart topology linkage controller of the present invention; Figure 5 This is a schematic diagram of the data editing interface of the S-100 production platform of the present invention; Figure 6 This is a schematic diagram of the data editing interface of the S-100 production platform of the present invention; Figure 7 The invention interrupts the road curve to trigger an upward linkage update of the previous image; Figure 8 The image shows the result of the road curve being interrupted and the upward linkage being updated as described in this invention. Figure 9 This is a record of the point before the road curve is interrupted in this invention; Figure 10 This invention provides a record table of points after the road curve is interrupted. Figure 11 This invention provides a curve record table before interrupting a road curve. Figure 12 This invention provides a curve recording table after interrupting a road curve. Figure 13 This invention provides a record table of combined curves before the road curve is interrupted. Figure 14 This invention provides a record table of combined curves after breaking road curves. Figure 15 This is a diagram showing the process before the parallel linkage update triggered by the merging of submarine cables, as per the present invention. Figure 16 This is the updated diagram after merging the parallel linkage triggered by the submarine cable according to the present invention; Figure 17 This is the endpoint merging pre-point record table of the present invention; Figure 18 This is the endpoint merging point record table of the present invention; Figure 19 This is a curve record table before endpoint merging in this invention; Figure 20 This is the curve recording table after endpoint merging in this invention; Figure 21 The image shown is the previous image before the deletion of the land area triggered by the downward linkage update in this invention; Figure 22 This is the diagram after the deletion of the land area triggers a downward linkage update according to the present invention; Figure 23 The present invention deletes the front and back point record table of the land area; Figure 24 The invention removes the pre-curve record table for land areas; Figure 25 This invention provides a curve record table after deleting land areas; Figure 26 The invention deletes the pre-land area combined curve record table; Figure 27 This invention provides a combined curve record table after deleting land areas. Figure 28 The invention removes the front surface record table for the land area; Figure 29 This invention relates to a surface record table after deleting the land area. Detailed Implementation

[0020] This embodiment further details the embodiments of the present invention in conjunction with the SuperMap basic geographic information platform and accompanying drawings.

[0021] A method for implementing linked updates of electronic nautical chart topology editing includes the following steps: Step 1: Load the electronic nautical chart dataset and construct the topological data structure.

[0022] like Figure 1As shown, the topological data structure includes four types of topological geometry: points, curves, composite curves, and surfaces. The relationships between topological geometry are as follows: curves are composed of control points, excluding endpoints; surfaces are composed of several curves or composite curves; there is a one-to-many relationship between composite curves and curves, and there is also a one-to-many relationship between surfaces and composite curves or curves; endpoints participate in the formation of curves as topological units, while ordinary control points only have geometric meaning and do not participate in topological construction.

[0023] like Figure 2 As shown, each point record in the point topology table contains the ID and geometry of the (end) point.

[0024] Each curve record in the curve topology table contains the curve's ID, the curve's geometry, the start endpoint ID of the curve reference, and the end endpoint ID of the curve reference.

[0025] Each composite curve in the composite curve topology table consists of one or more records. Each record contains the ID of the composite curve, a list of curve IDs referenced by the composite curve, the ID of each referenced curve, and the direction of the referenced curve. The complete geometry of the composite curve can be logically assembled through the geometry and direction of the curves it references.

[0026] Each surface in the surface topology table consists of one or more records. Each record contains the surface ID, the ID of the curve or composite curve referenced by the surface, the reference type, the curve direction, and inner / outer loop indication information. Reference types include curve type and composite curve type; curve direction includes positive and negative types; inner / outer loop indication includes inner loop and outer loop types. The complete geometry of a surface can be logically constructed using the geometry and direction information of the curves it references.

[0027] Step 2: Create a topology linkage controller.

[0028] like Figure 3 As shown, the topology linkage controller and spatial geometry have the following relationship: each topology unit is composed of topology geometry and topology linkage controller. The geometry can be spatial geometry instances such as points, curves, composite curves or surfaces. The topology linkage controller is further subdivided into four types of linkage controllers: point, curve, composite curve and surface, depending on the geometry type. Each topology unit will call the corresponding topology linkage controller according to its geometry type.

[0029] like Figure 4 As shown, the topology linkage controller consists of the following modules: (1) Event receiving module, used to receive messages generated by topology editing operations, such as endpoint coordinate modification, curve geometry modification, and other geometry modification messages that reference the curve; (2) Event processing module, which is used to perform specific business logic such as spatial geometric position modification, topology structure update, and data persistence on nautical chart topology units according to different event types; (3) Event transmission module, which is used to transmit events generated during the editing process in sequence according to the association relationship and order of the nautical chart topology units, so as to achieve the purpose of cascading update only for local topology units within the association range.

[0030] Step 3: Activate the topology editing function and select to add, delete, or modify a topology unit.

[0031] like Figure 5 and Figure 6 As shown, the topology editing function is grouped according to the type of geometry being edited. After selecting the editing type in the menu bar, you can perform corresponding selection or editing operations on the selected geometry on the map. Point operations include selection, new creation, move, and delete; curve operations include selection, new creation, edit, delete, and break; composite curve operations include selection, new creation, and extend; surface operations include selection, new creation, delete, and trim; and multi-point operations include add, delete, and move.

[0032] Step 4: After completing the topology editing, update the current topology unit and send a linkage update event notification to the associated topology units.

[0033] Step 5: Implement topology linkage update.

[0034] Topology linkage updates include the following three types of operations: upward linkage, used to update and modify topology units that reference this topology unit and those referenced at higher levels; parallel linkage, used to update and modify topology units at the same level that are associated with it; and downward linkage, used to update and modify topology units that are referenced by this topology unit and those referenced at lower levels.

[0035] 1. Upward linkage Case scenario: The road is composed of a single composite curve, which is composed of two curves. One of the curves is broken into two, and then the break point is moved.

[0036] When a curve is broken, the event receiving module of the original curve topology linkage controller receives a curve break message. The curve event receiving module then sends the event to the event processing module. The curve event processing module creates a new endpoint at the break point, modifies the endpoint of the original curve to this new endpoint, and creates two new curves with this endpoint and the start and end points of the original curve as their respective endpoints. At the same time, the original curve is deleted. The curve event propagation module continues to propagate the event upwards to the event receiving modules of the composite curves that reference the original curve. The composite curve event receiving module sends the event to the event processing module. The composite curve will add references to two new curves and delete the reference to the original curve. The composite curve will no longer be referenced by other composite curves or surfaces. At this point, the event processing of all associated geometric topology linkage controllers ends.

[0037] In this case, the road element is composed of composite curve No. 369, which is composed of curves No. 1074 and No. 1075. Curve No. 1074 references endpoints No. 1569 and No. 1570, and curve No. 1075 references endpoints No. 1570 and No. 1571. That is, they all reference endpoint No. 1570.

[0038] like Figures 7 to 14 As shown, curve 1075 is now broken. The event receiving module of the topology linkage controller for this curve will receive the broken curve message. This module then sends the event to the event processing module. The curve event processing module will create endpoint 1572 at the breakpoint, create curve 1076 composed of endpoints 1572 and 1571, and create curve 1077 composed of endpoints 1572 and 1570. Curve 1075 will be deleted. The curve event transmission module then sends the event to the event receiving module of the topology linkage controller for the combined curve 369, which references this curve, and further to the event processing module of the combined curve. The combined curve will then add references to curves 1076 and 1077, while deleting the reference to curve 1075. After moving endpoint 1572, the linkage update of curves 1076 and 1077 will be triggered sequentially upwards, further triggering the linkage update of combined curve 369. Since there are no other upward references to this combined curve element, the upward linkage update ends.

[0039] 2. Parallel linkage Case scenario: There are two unconnected submarine cables. Moving one end of one cable to the end of the other will merge the two ends, and the two curves will be connected together and have a common point in the topology.

[0040] When an endpoint of one curve is moved to the endpoint of another curve, a threshold is used for judgment. When the co-point condition is met, a delete endpoint event is sent to the point event receiving module of the redundant point's topology linkage controller. The point event receiving module then sends the event to the event processing module, which deletes and persists the redundant point information. The point event transmission module then sends the event to the event receiving module of the curve topology linkage controller that references that endpoint, and so on, until all associated geometric topology linkage controllers have finished processing their events. Parallel linkage is essentially an upward linkage based on the linkage update of associated geometric topology units of the same type.

[0041] In this case, there are two submarine cable elements, No. 2005 and No. 2006. Submarine cable No. 2005 references curve No. 1069, which in turn references two endpoints, No. 1561 and No. 1562. Submarine cable No. 2006 references curve No. 1070, which in turn references two endpoints, No. 1563 and No. 1564.

[0042] like Figures 15 to 20 As shown, at this point, the two submarine cables need to be connected together. Drag endpoint 1562 to endpoint 1563. The interaction layer automatically captures the endpoints after determining that their coordinates are within the threshold range and coincident. It then sends an event to the event receiving module of the topology linkage controller for endpoint 1562. The point event processing module then removes endpoint 1562. The point event transmission module then sends the events to the event receiving modules of curves 1069 and 1070 respectively. The event processing module for curve 1069 changes the ending endpoint 1562 to 1563 and updates the geometry. The event processing module for curve 1070 changes the starting endpoint 1562 to 1563 and updates the geometry. There are no further references to either curve upwards, thus concluding the parallel linkage update.

[0043] Curve record table after endpoint merging (ENDNODEID of endpoint with RCID 1069 adjusted and updated)

[0044] Geometric coordinates of curve No. 1069 before endpoint merging

[0045] Geometric coordinates of curve No. 1069 after endpoint merging (changes in endpoint coordinates)

[0046] Geometric coordinates of curve No. 1070 before endpoint merging

[0047] After endpoint merging, the geometric coordinates of curve 1070 remain unchanged. 3. Downward linkage When a surface is deleted, the event receiving module of the surface's topology linkage controller receives the surface deletion event. The surface event receiving module sends the event to the event processing module. The surface event processing module deletes and persists the surface information. The surface event transmission module then sends the event to the event receiving module of the curve or composite curve topology linkage controller that constitutes the surface, and so on, until all associated geometric topology linkage controllers finish processing the event.

[0048] In this case, feature #331 in the land area references surface #40, which in turn references composite curve #60, which in turn references curves #63 and #65. Feature #330 in the depth area references surface #39, which in turn references composite curve #59, which in turn references curves #63 and #64. In other words, both the land area and the depth area reference curve #63.

[0049] like Figures 21 to 29 As shown, when land feature #331 is selected and deleted, the land feature's topology linkage controller event receiving module receives the event and sends it to the surface event processing module. The surface event processing module processes surface #40, which is directly removed because it is not referenced by other objects. Subsequently, the surface event propagation module sends the event to the composite curve linkage controller #60, which is referenced by this surface. Since this composite curve is also not referenced by other objects, its event processing module directly removes it. The composite curve event propagation module then sends the event to the curve linkage controllers #63 and #65, which are referenced by this composite curve. Because curve #63 is still referenced by depth region #330, it is not processed. Curve #65 is directly deleted because it is not referenced by other geometry or features. Similarly, the curve linkage controller event propagation module continues to propagate the event to the endpoint linkage controllers #628 and #629, which are referenced by it. Since these two endpoints are still referenced by other geometry and features, they are not processed. At this point, the downward linkage update is complete.

[0050] It should be emphasized that the embodiments described in this invention are illustrative rather than limiting. Therefore, this invention includes, but is not limited to, the embodiments described in the specific implementation. Any other implementations derived by those skilled in the art based on the technical solutions of this invention are also within the scope of protection of this invention.

Claims

1. A method for linked updating of electronic nautical chart topology editing, characterized in that: Includes the following steps: Step 1: Load the electronic nautical chart dataset and construct the corresponding topological data structure according to the different geometric types of the elements, including four types: points, curves, composite curves, and surfaces. Step 2: To receive, process, and transmit editing event information during topology editing, a corresponding topology linkage controller is constructed based on the geometric type of the topology data structure, and topology units are further constructed through topology geometry and the topology linkage controller. Step 3: Activate the topology editing function, select the corresponding operation, and perform topology editing such as adding, deleting or modifying the geometry of a certain element in the electronic nautical chart dataset, and send an editing event notification to the topology unit corresponding to the geometry; Step 4: After topology editing, in order to update the topology units and transmit relevant information of associated topology units, the topology units corresponding to the currently edited geometry are updated according to the editing event notifications received by the topology units, and linkage update event notifications are sent to the associated topology units. Step 5: Based on the linkage update event notification, continue to update the topology unit that has received the notification, and continue to send update events to other topology units associated with the current topology unit, cascading the notification layer by layer, and finally realize the linkage update of the topology.

2. The method for linked updating of electronic nautical chart topology editing according to claim 1, characterized in that: The topological data structure in step 1 includes four types of topological geometry: points, curves, composite curves, and surfaces. The relationships between the topological geometries are as follows: curves are composed of control points, excluding endpoints; surfaces are composed of several curves or composite curves; there is a one-to-many relationship between composite curves and curves, and there is also a one-to-many relationship between surfaces and composite curves or curves.

3. The method for linked updating of electronic nautical chart topology editing according to claim 1, characterized in that: In step 2, the relationship between the topology linkage controller and the geometry is as follows: each topology unit is composed of topology geometry and topology linkage controller. The geometry can be spatial geometry instances such as points, curves, composite curves, or surfaces. The topology linkage controller is further subdivided into four types of linkage controllers based on the geometry type: point, curve, composite curve, and surface. Each topology unit will call the corresponding topology linkage controller according to its geometry type.

4. The method for linked updating of electronic nautical chart topology editing according to claim 1, characterized in that: In step 2, the topology linkage controller includes an event receiving module, an event processing module, and an event transmission module, wherein the event transmission module is connected in sequence, and the event receiving module is used to receive messages generated by the topology editing operation. The event handling module is used to perform specific spatial geometric position modification, topology structure update and data persistence business logic on the nautical chart topology unit according to different event types; the event transmission module is used to transmit the events generated during the editing process in sequence according to the association relationship and order of the nautical chart topology unit, so as to achieve the purpose of cascading update only on local topology units within the association range.

5. The method for linked updating of electronic nautical chart topology editing according to claim 1, characterized in that: In step 3, the topology editing function is grouped according to the type of geometry being edited. After selecting the editing type in the menu bar, the selected geometry is selected or edited on the map. Point operations include selection, creation, movement, and deletion; curve operations include selection, creation, editing, deletion, and breakage; composite curve operations include selection, creation, and extension; surface operations include selection, creation, deletion, and trimming; and multi-point operations include addition, deletion, and movement.

6. The method for linked updating of electronic nautical chart topology editing according to claim 1, characterized in that: The topology linkage update in step 5 includes upward linkage, parallel linkage, and downward linkage. Upward linkage is used to update and modify topology units that reference the current topology unit and those referenced by higher-level cascaded units. Parallel linkage is used to update and modify topology units at the same level that are associated with it. Downward linkage is used to update and modify topology units that are referenced by the current topology unit and those referenced by lower-level cascaded units.

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