Preparing a network#
Preparing a link network to be imported into an AequilibraE project consists of ensuring that all fields necessary for network import exist and are properly filled. These fields are:
link_id
a_node
b_node
direction
distance
modes
link_type
Below we give some directions on how to prepare each field.
Link ID#
The link ID field is necessarily a field with unique values, but it would ideally also be filled with small integers (e.g. a network with 100,000 links does not need to have IDs in the order of 1,000,000,000,000,000), as this save memory and computational time during some of the computations.
To create such field, one can use QGIS’ field calculator as shown below. Please note that the field does NOT need to be named link_id, as you will have the opportunity to indicate your field of choice to contain link ids when it is time to create the project.
Warning
LET’S STRESS THIS !!!!!!
AequilibraE can deal with an arbitrary set of IDs for links and nodes, but we vectorize a lot of operations for faster performance, which means that you will be using a LOT more memory you would if you use large IDs for nodes and links without actually needing it. You may also explode memory or the NumPy Numerical types. A good practice is to keep IDs as low as possible, but in general, if you get too close to 922,337,203,685,4775,807, you will certainly break things, regardless of your system’s capabilities.
Network articulation#
The association of a_nodes and b_nodes to a link layer is what can be named articulation of a network, and is one of the foundational tools in AequilibraE. Please make sure that all your links are LineString geometries, otherwise the articulation is not going to work. If all your links are MultiLineString geometries, you can save your data as a GeoPackage and enforce it to be LineString only.
It is important to note that AequilibraE understands a_node as being the topologically first point of the line, and b_node the last. Topology in GIS involves a LOT of stuff, but you can look at an intro.
Direction#
Links need to have directionality associated to them because links can be bi-directional in AequilibraE, which is not the case for several of the commercial platforms in the market. For this reason, one needs to have a field for link direction with values in the set [-1, 0, 1], where:
If direction = -1, then the link allows BA flow only
If direction = 0, then the link allows flows in both directions
If direction = 1, then the link allows AB flow only
Distance#
The field for distance needs to be numeric, but its value is currently unimportant, as distance values will be overwritten with distance in meters by SpatiaLite. Other units will be possible in the future.
Modes#
In AequilibraE, each mode is represented by a lower-case letter. So imagine we will have several modes in our network, such as cars (c), trucks (t), bicycles (b) and walking (w). In that case, a link that allows all modes will have a modes field equal to ctbw (the order of modes is irrelevant), while a link that allows bikes and pedestrians would have a modes string equal to bw.
The list of modes that will exist in the model, however, comes from the parameter list built into AequilibraE under the section Network –> modes.
To find out how to access the parameters file see documentation on the global parameters file.
Link Type#
Link types can be any string value you would like, but cannot be empty.