Method used in determining distances
Distance calculations for the Deemed Distance Calculator are made using
Geographic Information System (GIS) technology, a technology for
managing and analysing computer-based mapping information.
Source Data
Source Data for the Deemed Distance Calculator is primarily based on
the Australian Surveying & Land Information Group's TOPO-250K digital
product, with updates and corrections from various other sources.
The TOPO-250K product is primarily sourced from the 1:250 000 scale
National Topographic Map Series (NTMS) and the Royal Australian Survey
Corps' Joint Operation Graphics (JOG) map reproduction material
(repromat). In the worst case, a well defined point is out of position
by 300 metres. As the TOPO-250K data were digitised from existing map
production material, some features may be subject to cartographic
displacement.
Corrections to errors and revision of road position and classification
have been applied to reflect changes since the data were first
published.
Road Classification weighting
In order to determine the shortest practicable route between two locations,
a weighting system is applied to road data based on the roads' classification.
The classifications used in the calculation and their relative weighting
are:
| Freeway |
|
1.0 |
| Highway |
|
0.7 |
| Secondary Highway |
|
0.5 |
| Major Road |
|
0.4 |
The implications of this relative weighting is that where alternate
routes exists between two locations, a Highway, for example, must be
less than 0.7 times (70% of) the length of an alternate Freeway to be
used in the calculation in preference to the Freeway. The other classifications
are used similarly according to their relative weighting.
As an example, a country town on a major highway is bypassed by a freeway.
The bypass length is 17 kilometres and the alternate, older, route through
the town along the highway is 13 kilometres. Even though the freeway
bypass is 4 kilometres longer, it is used by the calculator in preference
to the highway because of its higher relative weighting value.
Map Projection
A map projection is a systematic representation of part of the surface
of a round body, especially the earth, on a plane. Any attempt to represent
a surface curving in all directions on a flat surface leads to distortion
of one form or another, including distortion of distance. No map projection
can maintain linear scale throughout the projection and as such distort
distance to some extent.
The projection chosen for the Deemed Distance Calculator in order to
minimise distance distortion as much as possible is the Equidistant
Conic projection.
Acknowledgements
GIS consultancy by LANDINFO, the Spatial Division of Sinclair Knight
Merz, Canberra.
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