GDAL
GNM Architecture

This document is intended to describe the purpose and the structure of Geographic Network Model classes. GNM is the part of GDAL and provides the methods of creating, managing and analysing geographical networks.

The key purpose of GNM classes:

General concept

Any real-world network can be represented as a set of vector data, which can be itself represented in GDAL as a GDALDataset. In GNM this data consists of two parts. Network's topology (graph), network's metadata (name/description), set of special feature identifiers, etc. belong to the "network part", while the common for GDAL layers, features, geometries belong to the "spatial/attribute part". In order to work with the datasets of different formats the following classes were designed in GNM:

Network: GNMNetwork represents an abstract network. The network data and spatial/attribute data in a dataset of some format in fact can be not separable (just additional layers/fields/tags), while the concrete implementation of GNMNetwork "knows" which data from the whole dataset refers to "network part" and is able to operate it. GNMNetwork allows user the following:

Format: GNMNetwork inherits GDALDataset and looks like OGRDatasource with additional functionality. There are a set of GDAL drivers for networks. The generic network implementation in GDAL provides additional functionality like rules, virtual edges and vertices. Also, while editing the feature the network control the network rules and other specific, and can deny saving edits. The other network drivers (pgRouting, OSRM, GraphHopper, etc.) should provide the basic functionality via the GNMNetwork class.

Network formats

To add a native support of the existed network format (like PostGIS pgRouting, Oracle Spatial Networks, topology in GML, etc.) to GNM the developer should implement the corresponding GNMDriver-GNMNetwork interface. But there is also a capability to use the generic network format, which is already implemented in GNM as a special class. It can be extremely useful when there is a need to create and use a network in the format that initially does not have its "network part" (like ESRI Shapefile) directly.

GNMGenericNetwork: GNMGenericNetwork is a concrete implementation of the GNMNetwork. GNMGenericNetwork intends to support the most GDALDataset drivers (depends on the corresponding driver capabilities). Technically the network format abstraction is achieved with the help of GDAL abstraction: datasets and layers approach. GNMGdalNetwork aggregates a GDALDataset instance where the "network part" is represented as a set of "system layers" (wkbNone geometry, specific attribute fields) and the spatial/attribute data is regarded as the set of "class layers" or "classes" (layers with geometries and attributes, as usual). The "network part" is created and maintained by GNMGenericNetwork automatically and provides methods to work with it.

The way of describing real-world networks by GNMGenericNetwork intends to be a generic, because:

See the GNMGenericNetwork class documentation for more details.

The network of common format has also the following important features:

Network analysis

The network analysis in GNM is implemented in GNMNetwork object.

GNMGenericNetwork holds the graph in memory in STL containers and provides basic algorithms which return the results in the array-form (e.g. std::vector full of path's edges and vertexes GFIDs). But the caller get a result as OGRlayer there features get from layers consist the network. Also some additional fields created (VERTEX/EDGE indicator field, GFID, layer name, etc.). The caller have to free the result OGRLayer via ReleaseResultSet


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