The FDZ coverage contains special response requirements for certain
polygons in the attribute table. The special response is primarily
used by the computer automated dispatching system (CAD) to assign
additional equipment or specialized personnel to an emergency.
For example, FDZs that are classified as "Canyon Rims"
require a response at both the rim and base of the canyon while
FDZ polygons with the "Downtown" classification require
special apparatus to deal with high rise structures. SDFD can
produce maps of special response areas using ERMS to generate
these coverages by dissolving adjacent FDZs of the same classification.
The road coverage for San Diego and surrounding areas is the foundation
of the ERMS application. Without an accurate street and freeway
network, reliable routing would not be possible. In the late
1980s, the City of San Diego received funds from a Public Safety
Communications bond to bring the quality of the existing Regional
Urban Information System (RUIS) road coverage up to a level conducive
for routing analysis. The Fire Department staff, in conjunction
with SDDPC, spent over a year editing the connectivity of the
road coverage and verifying the block range addresses. Certain
road segments had to be flagged non-drivable for emergency equipment
based on the street width. The non-drivable classification was
also applied to planned road segments that were in the data base
but did not yet exist. Because speed limits are used for routing,
the assigned speed limit for some arcs were adjusted to account
for equipment limitations based on road curvature or steep slopes.
In addition, nodes in the road coverage were assigned codes
that would allow the later assignment of appropriate turn impedances.
For example, where streets cross over freeways the turn impedance
should prevent the routing algorithm from permitting a turn from
the street onto the freeway and vice versa.
The Fire Department continues to maintain the road coverage by
continually updating this data on the RUIS server. The road coverage
is then copied from the RUIS server to the Fire Department client
on a weekly basis. During this download, impedance values for
each arc are assigned which indicate the time required to traverse
a particular road segment. The impedance time is recorded as
seconds in the road attribute table and is calculated by dividing
the assigned speed limit by the length of the road segment.
ERMS also requires a point coverage of fire station and medical
post locations, otherwise known as quarters. This point coverage
contains 133 quarters (69 fire stations, 47 medical posts, and
17 both fire and medical) which covers all of San Diego and some
of the surrounding area, such as Del Mar, Solana Beach and Poway.
Like the FDZ focal points, quarters points must correspond to
nodes within the road network. This enables ERMS to assign source
nodes during routing calculations from these quarters.
Response districts identify regions in the City of San Diego where
emergency units, trucks or engines, from a particular quarter
will be first to respond to an incident. To create these areas,
the nodes within the road coverage that correspond to quarters
are first selected as the source points (centers) and the FDZ
focal points are selected as the destinations (stops). ERMS then
uses the NODEDISTANCE command to calculate the times taken to
travel from each of the centers to each of the individual stops.
After all the route times have been calculated the boundaries
between adjacent FDZs that have the same "first in"
quarter are dissolved to produce a new polygon coverage. This
coverage, composed of aggregated FDZs, represents the response
districts for San Diego. The Fire Department uses the response
district coverage to identify mutual aid between San Diego and
it's bounding cities. For example, FDZs that are adjacent to
the City of Del Mar can have emergency units from Del Mar as their
first response.
Similar to the creation of response districts, ERMS also utilizes
NODEDISTANCE to calculate the first through fiftieth quarter that
should respond to each FDZ. The FDZs, quarters, and response
times are then downloaded into an ASCII report file. Using custom
C programs, the output of the file is changed into a format readable
by the CAD system.
Allocation analysis can be performed in ERMS either "from"
a selected quarter or "to" a hypothetical incident.
Using the ALLOCATE command (centers out option), routes can
be created from a selected center, usually a quarter, outwards
until the total impedance accrued is equal to a specified time.
The user can either analyze the resulting route or use ERMS to
dissolve all fire demand zones whose focal points intersect with
that route to create a response area coverage. This information
is useful for site location analysis by identifying areas in which
responses can be improved by the addition of a new quarter.
ERMS also applies the ALLOCATE command in a slightly different
way from the procedure described above. Instead of allocating
from a center point outwards, the options of the routing function
are modified to calculate routes into a selected center (centers
in option). With this function the user is able to enter the
number and type of responding units to a node in the road network
that has been chosen as a hypothetical emergency. The result
is a tabular report which identifies the responding quarters and
the times taken to respond.
Routes can be created dynamically within ERMS from any series
of user selected nodes in the road coverage. The route is calculated
from node-to-node in the order the points were specified using
the PATH command. This utility is often used to check the connectivity
of the road network. For example, when a new street is added
to the road coverage, ERMS will be used in this capacity to make
sure that routes can be calculated to each node in the new segment(s).
ERMS allows the user to temporarily make specified road segments
impassable during the calculation of a route. The user can create
a point coverage within ERMS that represent barriers to the routing
process. All the programs within ERMS check for the existence
of this coverage before calculating the route. If a barrier coverage
exists, the node in the road network that corresponds to that
point is selected, and all traversable arcs from that node are
omitted in the route calculation. This feature is useful for
developing disaster scenarios by determining the impact on routing
within the city if one or more freeway overpasses were to collapse
in the event of an earthquake.
Laura Brenner
Public Safety Geofile Coordinator, San Diego Fire Department
3750 Kearny Villa Road
San Diego, CA 92123
Telephone: (619) 573-1325
Fax: (619) 573-1309