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ANNEXURE 1: QUALITY CONTROL METHODS

E POSITIONAL ACCURACY

You need to check how well spatial features are located geographically. By overlaying the study
area and various secondary layers of information (e.g. administrative boundaries, roads and
streets, rivers) on the target population, their positional accuracy can be determined. A random
sample of about 10% of:
 Spatial features from the target population dataset can be extracted and overlaid on a base
map or ortho-image to further check the positional accuracy. The positional accuracy of
government service points and road segments can also be checked in this way.

 Service points and road segments will determine the positional accuracy of spatial features
in these datasets. Ideally, positional inaccuracies of government service points and road
segments should be systematically corrected. These corrections can all be done using GIS
software.

F OTHER QUALITY CONTROL CHECKS

Other quality control measures that you should consider include:

❏ It is important to ensure that all spatial datasets used in the accessibility studies are provided
in a Cartesian coordinate system in metres because their geographic coordinates are
provided in angular units (degree, minutes, and seconds) and, therefore, they are easier to
work with when doing accessibility modelling.

❏ The datasets should be provided with metadata, which enables users to get a more
thorough understanding of the datasets.

❏ The datasets should also be provided in a suitable GIS format (e.g. Geography Markup
Language or Shape file format that can be easily incorporated into the accessibility study.

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