The coordinates of the fire locations were provided in terms of
latitude and longitude, to the nearest minute of arc. These were converted
to New Brunswick stereographic projection coordinates (Thomson,
Mephan and Steeves, 1977) which was the coordinate system in which
the map of New Brunswick --- which constitutes the observation
window for the pattern --- was obtained. The conversion was done
using a C
program kindly provided by Jonathan Beaudoin of
the Department of Geodesy and Geomatics, University of New Brunswick.Finally the data and window were rescaled since the use of the
New Brunswick stereographic projection coordinate system resulted
in having to deal with coordinates which are expressed as very
large integers with a bewildering number of digits. Amongst other
things, these huge numbers tended to create very untidy axis labels
on graphs. The width of the bounding box of the window was made
equal to 1000 (nameless) units. In addition the lower left hand
corner of this bounding box was shifted to the origin. The height
of the bounding box was changed proportionately, resulting in a
value of approximately 959.
The window for the fire patterns comprises 6 polygonal components,
consisting of mainland New Brunswick and the 5 largest islands.
Some lakes which should form holes in the mainland component are
currently missing; this problem will be remedied in future releases.
The window was formed by ``simplifying'' the map that was originally
obtained. The simplification consisted in reducing (using
an interactive visual technique) the number of polygon edges in
each component. For instance the number of edges in the mainland
component was reduced from over 138,000 to 500.
For some purposes it is probably better to use a discretized
(mask type) window. See ``Examples''.
Because of the coarseness of the coordinates of the original
data (1 minute of longitude is approximately 1 kilometer at the
latitude of New Brunswick), data entry errors, and the simplification
of the observation window, many of the original fire locations
appeared to be outside of the window. This problem was addressed
by shifting the location of the ``outsider'' points slightly,
or deleting them, as seemed appropriate.
The columns of the data frames comprising nbextras
are
[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]
Note that due to data entry errors some of the ``out dates'' and
``out times'' in the original data sets were actually earlier
than the corresponding ``discovery dates'' and ``discover times''.
In such cases all corresponding entries of the extras
data
frame (i.e. dis.date
, dis.time
, dis.julian
,
out.date
, out.time
, and out.julian
) were set
equal to NA
. Also, some of the dates and times were missing
(equal to NA
) in the original data sets.