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# Dealing with Geometries

Geometry is a key feature when dealing with transportation infrastructure and actual travel. For this reason, all datasets in AequilibraE that correspond to elements with physical GIS representation, links and nodes in particular, are geo-enabled.

This also means that the AequilibraE API needs to provide an interface to manipulate each element’s geometry in a convenient way. This is done using the standard Shapely, and we urge you to study its comprehensive API before attempting to edit a feature’s geometry in memory.

As we mentioned in other sections of the documentation, the user is also welcome to use its powerful tools to manipulate your model’s geometries, although that is not recommended, as the “training wheels are off”.

## Data consistency

Data consistency is not achieved as a monolithic piece, but rather through the treatment of specific changes to each aspect of all the objects being considered (i.e. nodes and links) and the expected consequence to other tables/elements. To this effect, AequilibraE has triggers covering a comprehensive set of possible operations for links and nodes, covering both spatial and tabular aspects of the data.

Although the behaviour of these trigger is expected to be mostly intuitive to anybody used to editing transportation networks within commercial modeling platforms, we have detailed the behaviour for all different network changes.

This implementation choice is not, however, free of caveats. Due to technological limitations of SQLite, some of the desired behaviors identified cannot be implemented, but such caveats do not impact the usefulness of this implementation or its robustness in face of minimally careful use of the tool.

Note

This documentation, as well as the SQL code it referes to, comes from the seminal work done in TranspoNet by Pedro and Andrew.

## Network consistency behaviour

In order for the implementation of this standard to be successful, it is necessary to map all the possible user-driven changes to the underlying data and the behavior the SQLite database needs to demonstrate in order to maintain consistency of the data. The detailed expected behavior is detailed below. As each item in the network is edited, a series of checks and changes to other components are necessary in order to keep the network as a whole consistent. In this section we list all the possible physical (geometrical) changes to each element of the network and what behavior (consequences) we expect from each one of these changes.

Our implementation, in the form of a SQLite database, will be referred to as network from this point on.

Ensuring data consistency as each portion of the data is edited is a two part problem:

1. Knowing what to do when a certain edit is attempted by the user

2. Automatically applying the tests and consistency checks (and changes) required on one

The table below presents all meaningful operations that a user can do to links and nodes, and you can use the table below to navigate between each of the changes to see how they are treated through triggers.

Nodes | Links
--- | ---
Creating a node | Deleting a link
Deleting a node | Moving a link extremity
Moving a node | Re-shaping a link
Adding a data field | Deleting a required field
Deleting a data field |
Modifying a data entry |

### Node layer changes and expected behavior

There are 6 possible changes envisioned for the network nodes layer, being 3 of geographic nature and 3 of data-only nature. The possible variations for each change are also discussed, and all the points where alternative behavior is conceivable are also explored.

#### Creating a node

There are only three situations when a node is to be created:

- Placement of a link extremity (new or moved) at a position where no node already exists

- Splitting a link in the middle

- Creation of a centroid for later connection to the network

In all cases a unique node ID needs to be generated for the new node, and all other node fields should be empty.

An alternative behavior would be to allow the user to create nodes with no attached links. Although this would not result in inconsistent networks for traffic and transit assignments, this behavior would not be considered valid. All other edits that result in the creation of unconnected nodes or that result in such case should result in an error that prevents such operation

Behavior regarding the fields regarding modes and link types is discussed in their respective table descriptions

#### Deleting a node

Deleting a node is only allowed in two situations:

- No link is connected to such node (in this case, the deletion of the node should be handled automatically when no link is left connected to such node)

- When only two links are connected to such node. In this case, those two links will be merged, and a standard operation for computing the value of each field will be applied.

For simplicity, the operations are: Weighted average for all numeric fields, copying the fields from the longest link for all non-numeric fields. Length is to be recomputed in the native distance measure of distance for the projection being used.

A node can only be eliminated as a consequence of all links that terminated/ originated at it being eliminated. If the user tries to delete a node, the network should return an error and not perform such operation.

Behavior regarding the fields regarding modes and link types is discussed in their respective table descriptions

#### Moving a node

There are two possibilities for moving a node: Moving to an empty space, and moving on top of another node.

- If a node is moved to an empty space - all links originated/ending at that node will have its shape altered to conform to that new node position and keep the network connected. The alteration of the link happens only by changing the Latitude and Longitude of the link extremity associated with that node.

- If a node is moved on top of another node - All the links that connected to the node on the bottom have their extremities switched to the node on top. The node on the bottom gets eliminated as a consequence of the behavior listed on Deleting a node

Behavior regarding the fields related to modes and link types is discussed in their respective table descriptions.

See also

- Editing network nodes

    Usage example

#### Adding a data field

No consistency check is needed other than ensuring that no repeated data field names exist.

#### Deleting a data field

If the data field whose attempted deletion is mandatory, the network should return an error and not perform such operation. Otherwise the operation can be performed.

#### Modifying a data entry

If the field being edited is the node_id field, then all the related tables need to be edited as well (e.g. a_b and b_node in the link layer, the node_id tagged to turn restrictions and to transit stops).

### Link layer changes and expected behavior

Network links layer also has some possible changes of geographic and data-only nature.

#### Deleting a link

In case a link is deleted, it is necessary to check for orphan nodes, and deal with them as prescribed in Deleting a node. In case one of the link extremities is a centroid (i.e. field `is_centroid=1`), then the node should not be deleted even if orphaned.

Behavior regarding the fields regarding modes and link types is discussed in their respective table descriptions.

#### Moving a link extremity

This change can happen in two different forms:

- The link extremity is moved to an empty space - In this case, a new node needs to be created, according to the behavior described in Creating a node. The information of node ID (A or B node, depending on the extremity) needs to be updated according to the ID for the new node created.

- The link extremity is moved from one node to another - The information of node ID (A or B node, depending on the extremity) needs to be updated according to the ID for the node the link now terminates in. Behavior regarding the fields regarding modes and link types is discussed in their respective table descriptions.

See also

- Editing network links

    Usage example

#### Re-shaping a link

When reshaping a link, the only thing other than we expect to be updated in the link database is their length (or distance, in AequilibraE’s field structure). As of now, distance in AequilibraE is ALWAYS measured in meters.

See also

- Splitting network links

    Usage example

#### Deleting a required field

Unfortunately, SQLite does not have the resources to prevent a user to remove a data field from the table. For this reason, if the user removes a required field, they will most likely corrupt the project.

### Field-specific data consistency

Some data fields are specially sensitive to user changes.

#### Link distance

Link distance cannot be changed by the user, as it is automatically recalculated using the SpatiaLite function `GeodesicLength`, which always returns distances in meters.

#### Link direction

Triggers enforce link direction to be -1, 0 or 1, and any other value results in an SQL exception.

#### modes field (Links and Nodes layers)

A serious of triggers are associated with the modes field, and they are all described in the Modes table.

#### link_type field (Links layer) & link_types field (Nodes layer)

A serious of triggers are associated with the modes field, and they are all described in the Link types table.

#### a_node and b_node

The user should not change the a_node and b_node fields, as they are controlled by the triggers that govern the consistency between links and nodes. It is not possible to enforce that users do not change these two fields, as it is not possible to choose the trigger application sequence in SQLite