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Maps of the fire weather
conditions are an extremely useful method of displaying this information.
Users who do not have the training in the understanding and interpretation
of the FWI numbers will get an instant appreciation of where the critical
areas are around the country.
The arrangement of each
days maps are a deliberate attempt to guide the user through an overview to
a more detailed breakdown of the specific components driving the fire weather
system. This follows the FWI System structure explained HERE.
-------------> Drill down to more detailed information ------------->
The first set of maps
that this page defaults to are the fire danger overviews. These incidentally
are not part of the FWI system but are components within the Fire Behaviour
Prediction System of the New Zealand Fire Danger Rating System explained HERE.
As the user progresses
from left to right the maps become less an overview and more representative
of specific weather components. The principle behind this is that the user
can get a basic overview from the fire danger maps and if require can "drill
down" to more specific components that show which particular conditions
are driving the fire dangers.
Note that while we
are unable to include data we collect from the Metservice on the public site,
these are included in the creation of the maps.
The Fire Danger Maps
Most people will be familiar
with the FWI system components. There is an additional section containing
different maps providing an overview of the Fire Danger (the Head Fire Intensity,
Fire Danger Class and the Rate of Spread). These are a combination of WEATHER,
FUELS and TOPOGRAPHY - the fire triangle. The weather component comes from
the FWI system, the fuels component is based on the national Land Cover Database
(LCDB) whilst the topographic information is derived from LINZ data.
To represent fuels, the
16 classes of vegetation type in the LCDB were aggregated into the high-level
fuel types in the FBP system currently being developed as part of the NRFA
fire research programme. Fire behaviour equations exist for those fuel type
and have been applied to this application.
The fire danger maps are:
- Rate of Spread (ROS) - The forward progress per unit time of the head fire generally measured
as m/hr.
- Head Fire Intensity
(HFI) - The head output measured in kilowatts per metre of flame length
at the head of a fire. The head of a fire is defined as that portion of
a fire edge showing the greatest rate of spread and fire intensity (i.e.
generally to downwind or upslope).
- Fire Danger Class
(FDC) - derived from the HFI, measures a segment of a fire danger index
scale identified by a descriptive term (e.g. Low, Moderate, High, Very
High, Extreme) and/or a colour code (e.g. green, blue, yellow, orange,
red). For example, an HFI of 4000kw/m or greater means that any direct
attack on the head of the fire will be ineffective.
HOW
THEY ARE CREATED
The maps are merely a
representation of the raw data which you can see on the far right. The raw
data represents an actual value for a specific point on the ground. At the
moment we make our own assumptions as to what area the stations reading apply
to.
Using GIS technology,
the system works like this:
New Zealand is divided
up into a number of square cells to form a grid across the land. In this
case we have chosen a cell size of 2000 metres.
Where a weather station
falls within a cell, the cell is assigned the readings for that station.
Cells around the station have similar, but not exactly the same, values
as the weather station. Cells in-between two weather stations will have
a value somwhere between the two. For example, if two stations have temperature
readings of 10 and 20 degrees respectively then a cell half-way between
them would have a value of 15 degrees (assuming there is flat ground between
them).
When all of the FWI
weather inputs have been "interpolated" in this manner. The
FWI codes and indices can then be "calculated". Mathematical
operations can be performed on overlapping cells to assign a value in
a new cell. For example, to create the Drought Code grid, the entire Temperature,
Rainfall and previous days DC grid can be used as inputs to the calculation
of the DC value using the formula in the FWI
System.
 The system used to create
the maps is calld the Spatial Fire Management System (sFMS) and was developed
in Canada as an application of ESRI's ArcView GIS product. More information
on sFMS can be found HERE.
LIMITATIONS
When basing decisions
on the map information it will help if you are aware of some of the limitations
that should be considered.
SCALE AND RESOLUTION
The maps are only a representation of the raw data. They are intended to provide
a broad-brush overview of the national situation. There is not a high enough
resolution to represent local conditions.
WEATHER STATION PLACEMENT
Traditionally, the placement
of weather stations on our network has been concentrated in areas where there
are significant values, frequent fires and hot, dry climates. This means that
there are areas of the country where there is very little coverage. This makes
the data in that area less represetitive of the actual weather.
INTERPOLATION METHOD
The most appropriate method of interpolating weather data is what is called
a Thin Plate Spline. Imaging a thin sheet of metal placed over the country
which is then distorted up and down depending on values at the weather station
sites. Because there is some "rigidity" in the slpine, some averaging
may occur over sites. This means that the actual value recorded by the weather
station may not be assigned to the weather station's cell but is averaged
over the surrounding weather stations and cells. This will mean that "spikes" or extreme values may not show up as well. Refer to the raw data if this is
suspected.
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