Geoscience Reference
In-Depth Information
One of the possible explanations of the differences described above is that they
are the consequence of different methods of calculation of self-potential. To test this
hypothesis, a comparison of self-potential values calculated within particular
models was prepared (Fig. 3 ). Even a
first glance at the maps enables one to
disprove the hypothesis. The differences between the values obtained from the
models being compared are totally the inverse of that expected with this explana-
tion. In general the self-potential values obtained within the municipal model (M1)
are higher than in other models while the potential accessibility values are lower
(cf. Figs. 2 and 3 ). In the case of Warsaw and most other big cities (so-called
'
) the self-potential values obtained from the municipal model
are much higher than those in both grid-based models (and especially those in
model M2). In consequence, the method of calculation of travel time between
municipalities or data aggregation method should be treated as the main source of
the differences of A i values, rather than the method of calculation of self-potential.
subregional centres
'
5 Conclusions
The potential accessibility analysis for the Mazovia region shows both signi
cant
differences in the accessibility values obtained from the three models tested and a
relatively stable spatial pattern when the results are standardised according to the
population-weighted average. The latter suggests that the potential accessibility
indicator is, to some extent, independent of the aggregation mechanism applied for
the investigation. This applies in the case of comparison of differences of acces-
sibility values over space. Nevertheless, the differences are clearly visible when
investigating the overall level of potential accessibility. The municipal model (M1)
provides comparatively low values of potential accessibility indicator for all spatial
units, while the application of the population-weighted average travel time model
(M3) and particularly the grid model (M2) result in signi
cantly higher values.
The results are in line with the assumptions made on the basis of the concept of a
smoothing process. The application of larger units (municipalities in the model
M1), provokes more smoothing than other models, thus the values should be lower.
The application of the M3 model causes signi
cantly less smoothing in comparison
to the M1 model, but more in comparison to the M2 model, thus the results are in-
between the others, closer to the latter than to the former one. Nevertheless, the
most important seems to be the fact that the higher number of relatively short-
distance trips provides the higher A i results, even though the mass ascribed to
destination nodes is substantially lower. This explains the difference between
models M1 and M2. The difference between models M1 and M3 is the consequence
of the different method of calculation of travel time between each pair of units. The
results show that the (population-weighted) average travel time (M3) is signi
cantly
lower than the travel time derived directly from the O-D matrix between nodes that
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