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This section helps you identify how well the
building controls movement and supports or hinders natural surveillance.
Both movement and surveillance are
key to crime prevention through environmental design principals.
Objectively understanding which areas within the building
are more secluded from view and as a result might be more
vulnerable, can help ensure improvement strategies are targeted
to the locations of
greatest
need.
Surveillance
Why is understanding surveillance important? Surveillance is an important factor when understanding the locations
of crimes and incidents, and identifying risk:
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The risk and fear
of crime can increase as the level of surveillance decreases;
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People are more likely to witness crimes, incidents or disturbances
if the location is well populated or overlooked.
The two main types of surveillance are 'Natural Surveillance' and
'Artificial Surveillance'.
Natural surveillance will be good in an area
if it is well-overlooked by surrounding buildings or staff bases,
or busy (mainly pedestrian) routes.
Artificial surveillance (CCTV)
has to be monitored regularly to be useful for intervening in
crimes - however it can provide evidence after the event has
taken place. This makes it important to find out how CCTV
is being used on site. CCTV that is used solely for evidence
(after the incident has occurred) will far less valuable than
natural surveillance or constantly monitored CCTV.
How will mapping surveillance help me?
Mapping surveillance will help you answer three key
questions concerning your site:
- Which locations have high levels of natural surveillance
by staff?
- Which locations have high levels of natural surveillance
by members of the public?
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Which locations can be overseen using CCTV?
Which locations have neither CCTV nor natural
surveillance?
This will highlight areas which are secluded
from view. It can then be compared to the locations
where crime
and incidents have taken place (identified in
the crime mapping), and the locations of vandalism and
antisocial behaviour (identified in the site
survey).
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Movement and wayfinding
The way that a building or site controls movement is both governed
by the layout of the buildings or site as well as the use of signs
and access control measures.
This section outlines a way in which you can model how well
the design of your site or building supports natural wayfinding
and
where routes are unclear. This is important for crime prevention
as:
It reduces stress to visitors trying to find their way around
This is particularly appropriate to hospitals where
good wayfinding can help reduce the anxiety of patients and their friends or relatives.
It can help control access and movement
For example, a school might wish to locate the main entrance so
the reception can easily be seen upon entry to the site. This would
prevent users from straying into areas without permission but it
would also mean that all visitors can be viewed from the reception
area.
Modelling movement and wayfinding also
helps to identify which routes are likely to be heavily used.
This information can then
be overlaid with the surveillance data to see if there
are key areas which are not under observation. It also helps inform
decisions on signage.
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Creating your surveillance and movement maps
Mapping surveillance and movement by hand is a cheap and quick
method to illustrate the issues outlined above: which areas are
observed and which are secluded; and whether they coincide with
key usage areas.
Download
suggested legends
Left-click on the thumbnails to see an enlargement of
the image, or right-click
and choose 'save target as' to save the picture to your computer.
Mapping
of surveillance and movement by hand
There are six stages to modelling surveillance
and movement. We have used an example from Case Study C, a hospital
A&E department.
Stage 1: Identify surveillance points
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The
first stage is to identify all the locations that provide
cover by natural surveillance, like staff bases, or where
CCTV cameras are present. It is suggested that you use
different colours to distinguish between the locations
of staff providing natural surveillance and CCTV cameras
that provide artificial surveillance. In this example,
a red dot has been placed in the centre of each staff
base in the A&E department. The locations of each
of the CCTV cameras and the direction of view are shown
as green dots (for 360 degree views) or green arrows (for
fixed views).
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Stage 2: Mark view lines
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The
next stage is to take the first point and to draw out
the field of view from it. In the example shown the lines
all face south – this is due to walls on the other
three sides of the room blocking views.
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Stage 3: Shade in the region
surveyed
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The next stage is to colour in the area that
can be overseen from the staff base. It is suggested that
you use a coloured pencil to do this so that the colouring
remains transparent. It is also helpful to use the same
colour as you have chosen to mark the surveillance points – so
in this example the area is shown in pale red.
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Stage 4: Complete regions
surveyed
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Repeat stages 2 and 3 for the other points
for natural surveillance and CCTV coverage so that your
map has a complete picture of all the areas that can be
directly overseen by staff members and / or CCTV cameras.
It is useful to identify through shading or hatching areas
that can be overseen by more than one staff base or camera.
Even though 360 degree CCTV cameras have been shaded as
being able to see in all directions in the example, you
should remember that 360 degree CCTV cameras only point
in one direction at a time.
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Stage 5: Identify movement
and access points
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To help understand how the surveillance impacts
on the use of your building and the vulnerability to crime,
it is useful to mark clearly on the plan both internal and
internal routes. For clarity, the plan on the right has
external entrances marked as filled arrows and internal
entrances marked as hollow arrows, both in black.
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Stage 6: Add other issues
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It may be of use to mark on the plan the
main locations where crimes occurred – how do these
relate to the natural surveillance levels? As shown on plan
6 there are many areas which have coverage from both from
natural surveillance and by CCTV,
yet there are also key areas, such as the two main entrances
to the left, and the
entrance to the centre right, which are not overseen.
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Computer modelling of movement and surveillance
If you have a very complex plan then it can be difficult
to identify which routes a person will be likely
to take from the main entrance to the building or to clearly
see which areas are more easily observed and which are
secluded. One
way in which this can be tackled is to use a technique
called visibility graph analysis (VGA).
In this context it has
three main uses which are detailed below.
1. Wayfinding
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Identifying the wayfinding from
a given entrance into and through your building. This helps
to highlight the key routes that are likely to be taken
that can be further supported by signage strategies.
The analysis shows the number of changes of visual direction
it will take you to reach a given destination - or in other
words how convoluted the route is to reach your destination.
As shown in the diagram alongside, it takes only 1 change
of direction from the main entrance to access resus, which
is a clinical area that should be difficult to access inadvertently.
However, the main hospital street is over 2 changes of direction
from the main entrance, yet is a route that should be very
easy to find. |
2. Complexity of Routes
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Modelling the overall complexity
of routes within your building or urban area,
to highlight which areas are likely to have high levels
or use, and those that are likely to have less use as
they are in more secluded locations.
The model using visibility graph analysis tells us the
area in square metres that is visible from a pedestrian
standing at any point within the building or street network.
By performing the same calculation for all points in a
grid, measures of the average visibility of an area are
calculated to enable comparisons between alternative designs.
The resulting pattern of visibility can be represented
by colouring each point according to the area of its visual
field. An example of this, again from Case
Study C,
may be viewed by clicking the thumbnail to the left, where
the visibility of each part of public space is represented
in an equal range, spectral colour scale from red (highest
visibility) to blue (lowest visibility). |
3. Natural Surveillance |
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Modelling the level of natural
surveillance from
individual locations, such as staff bases or key areas such
as treatment rooms.
In addition to measuring the total area that can be seen
from each point within the grid, it is also possible to show
the total area that can be overseen from key locations such
as staff bases. This enables you to easily calculate the
level of natural surveillance both on existing schemes and
also when
designing new buildings. It is useful particularly
indoors, as these areas are covered by not only visual but
also aural surveillance.
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This kind of analysis
is very useful as it means that you can test how well your building
or site supports movement
routes and surveillance. This analysis can be applied to existing
buildings and also during the design phase. In the latter possibility,
it can be used to test different design options to show how well
they
support
clear
movement
routes and good natural surveillance. Interpreting the data
Once
you have followed each of the steps to map both natural surveillance
and artificial surveillance, the next stage is to evaluate the
impact upon the control of movement inside your building. Considering
this next to the mapping of crimes and incidents that have occurred
in the past, this will help identify which locations are vulnerable
for crime in
the
future.
Example: Case Study C
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Case Study C with mapping
interpretations (click to enlarge).
In the plan of the hospital, five key
issues have been identified which are as follows: |
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A: Staff members cannot oversee
the paramedic entrance. This means that people can access
sensitive areas such as resus, relatives rooms and major
treatment areas without being directly observed by staff
members.
B: The main entrance cannot be overseen
by staff members and for people entering the department via
this main entrance, wayfinding is unclear as they cannot
immediately see a staff member.
C: The entrance and foyer space to the
back of reception is not observed by staff members, yet people
can access this area unobserved via the paramedic entrance
(there is no access control on the door).
D: The treatment rooms cannot be overseen
from the staff base.
E: Staff cannot survey the corridor that
links the hospital street to the waiting area. Further to
this, people can easily access the treatment rooms in area
D without being seen. |
The key aspects to look for when interpreting your plan are:
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Movement
routes – how well are heavily used routes overseen?
Would you or others be able to distinguish a legitimate user
from an illegitimate user?
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The access routes into your building – are
these controlled through any access control measures? Are
they overseen by staff?
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Where do your crimes occur?
Are there any patterns between crime types and the location?
For example, crimes such as pick pocketing
occur in areas of heavy use, however, bullying, graffiti
and property damage normally occur in areas that are less well overlooked.
top of page Checklist
Have you:
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Created a map that shows natural surveillance
and CCTV coverage of the building interior?
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Created
a map that shows natural surveillance and CCTV coverage
of the building grounds?
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Identified on the
maps locations of access routes and movement routes?
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Compared
the surveillance locations to the locations of
crimes – do
any patterns emerge?
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Download a document
containing all the checklists from this toolkit (PDF 310kB)
Click here for a text-only version of the checklist
Achievements from this step The output of this stage is a map (or series of maps) showing
surveillance, movement and wayfinding on your site. This will help
you to develop a spatial understanding of your site by identifying
secluded areas which might be potentially problematic.
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