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GENERIC STEP-BY-STEP APPROACH STEP THREE

C Assess the quality of the spatial information

The preparation of the data is one of the key activities of undertaking accessibility modelling. Once the
different datasets have been sourced they will need to be cleaned and validated. The premise being
that the outcomes of the accessibility modelling would be significantly influenced if the information is
not complete or incorrect. The challenge is that departments often underestimate the amount of effort
and time it takes to clean, validate and improve the quality of the data.

It is important that all existing service points are included and that their locations are captured correctly.
Quality control assessments should include standardising the datasets; checking for duplicate
geographic co-ordinates; ensuring that spatial features have a unique identifier; ensuring that the
datasets are in a local geographic coordinate system, and checking to see that the datasets are as
complete as possible. Details about quality control measures are provided in Step 2 of the Guideline.

D model
Incorporate the spatial information into the accessibility

A key activity in the data preparation phase is to import the spatial datasets from your database into the
accessibility modelling software that will be used in the study.
Several commercial GIS software packages have spatial modelling capabilities. It is important that you
should select a software package or combination of packages or tools that consider(s) the combined
effects of:

 The access distance standards of your department.

 Population parameters (capacity) of services points.

 Actual distances travelled along a movement network.
 The extent of the target population.

 Currently available service point infrastructure.

E Improve movement network connectivity by using Feedlinks

and Delaunay networks

Road network datasets are often incomplete. Conduct a connectivity analysis in the GIS software to
see if any road segments are not connected properly to the road network. If such segments exist, you
need to connect them to the road network.

Furthermore, smaller roads and paths used by communities to access other communities, service points
or major roads are sometimes not included in the network dataset. It is therefore necessary to create links
between these communities, service points and the main road network.

These linkages are established by creating Feedlinks and Delaunay networks. Feedlinks are straight
lines generated by the GIS software that are used to connect all service points and communities to the
main road network using the shortest distance (see Figure 19).
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