INTRODUCTION
The tools available for managing Municipal
information are growing every day, and choosing the right tool for
your municipal operations is a complicated task. One of the emerging
tools are Municipal Geographic Information Systems (GIS). Since
approximately 80 to 95% of Municipal information refers to a physical
object (i.e. roads, sewers, and properties) its easy to imagine why
managing Municipal information based on its geographic location can
be useful.
With a simple click of the mouse on a computer
screen a GIS user can retrieve all the information available on a
Town’s infrastructure, for example, the depth, material and condition
of a section of sewer line. An end to fumbling through old reports
has finally arrived.
A Municipal GIS typically contains
infrastructure and non infrastructure information (mapping and data)
on separate map layers. These layers are turned on and off permitting
users to observe spatial relationships between cultural and non
cultural features. GIS also permits spatial analysis, for example
buffering around a school produces a mailing list of all residents
within this buffer zone; with a click of the mouse this list of
residents could be sent a notice about a impending meeting.
This series of articles describes the issues
involved in designing and implementing a GIS, with an emphasis on
issues important to Municipalities from 1,000 to 100,000 people.
Larger municipalities typically have the internal skills and
resources necessary to evaluate and determine how to develop a GIS
without external assistance.
A GIS can be broken down into three
components, mapping, databases, and software; issues related to each
of these components are discussed in a question and answer approach
using a fictional Clerk Treasurer. Our first topic; “Is Our Mapping
Good Enough”?
Q. Is our mapping good
enough?
Our Town Mapping consists of roads, water,
sanitary sewers with some storm sewers, zoning and assessment maps. I
know the accuracy is not very good as I have measured off them and
sometimes they are off by tens of metres. They also need to be
updated, can I use them as a base for a GIS?
Obviously the best mapping for a GIS is the
most accurate mapping you have available. Unfortunately as you
mentioned, the Municipal mapping hanging on your wall in the Town
office is not very accurate. On the upside, most Towns can obtain
Ontario Basic Mapping (OBM) from the Provincial Government for a
small fee. This mapping is usually accurate to approximately 5 metres
and is an excellent starting base for a GIS. It contains roads,
buildings, major power lines, sometimes contours, and is probably
available for your Town in a digital format that can be brought into
a GIS with little effort. OBM mapping is available in scales of
1:10,000 for most Municipalities, and 1:2,000 for a smaller number of
Municipalities.
Once you have a base for the GIS, either OBM
or Town maps, your infrastructure mapping can be added on top of
it.
Q. Five metre accuracy, what
if I want more accuracy than that?
A very high level of absolute accuracy means
the location of the object in the GIS is very close to its real world
position. An obvious advantage of a high level of absolute accuracy
is the potential to use the GIS base plan for final or preliminary
design purposes, for example, road design. Getting this level of
accuracy into your GIS, in most instances, requires a complete
resurvey of the Town. To maintain your mapping usability it must be
kept at this high level of accuracy through continuous updates
whenever anything changes. The cost of implementation and maintenance
of this type of GIS will prohibit it’s implementation in most
instances, but is an excellent choice if the funds and technical
skills are available in your Municipality to obtain and maintain
it.
Q. What are my other choices
for new base mapping?
Two other common techniques for creation of
this kind of base mapping include using Global Positioning System
(GPS) Technology, and Aerial Photogrammetry. Each provides a moderate
level of accuracy and have advantages and disadvantages, identified
in Table 1.
GPS Derived Mapping
Geographic co-ordinates of your infrastructure
can be obtained relatively quickly using GPS technology. The accuracy
of GPS equipment varies from many metres to sub centimeters,
depending upon the type of equipment and a host of other variables.
The GPS equipment can be installed in vehicles, bikes or carried by
the surveyor around the municipality. The co-ordinates of each
physical object (ie fire hydrants, signs etc.) are recorded at a
quick pace. A big advantage of this technique is it permits the
collection of condition and database information (ie fire hydrant
type and condition etc.) to be obtained at this time using a data
logger.
Once the survey is complete a base plan of the
surveyed items can be produced using this information. GPS field
surveys are relatively quick, but the cost of resurveying an entire
Town in this way may be out of your budget range.
Photogrammetry Derived
Mapping
Another option is to produce mapping through
aerial photography. Normally it is not possible to scale off of
aerial photographs due to the variance in scale throughout the
photograph. However the science of photogrammetry permits moderately
accurate measurements to be made off photographs. A moderately
accurate base plan (0.1 to 1m+ accuracy) is created by drawing off of
the photography. Therefore anything that can be seen in the
photographs can be mapped.
Its not quite as easy as this, as special
control must be introduced into the photographs depending upon the
technique. The level of detail obtained and absolute accuracy is a
function of camera platform height (50m to 3,000 m from blimp,
ultralight, or plane), the type of camera system (including film size
and lenses), and the software/hardware capabilities of the vendors
technique. Therefore some infrastructure components may not be
mappable, particularly with high altitude photography. In all cases
underground components can not be mapped in this way.
One big bonus of this technique is that the
aerial photography can be brought into the GIS as a picture (raster)
underlay to add more capabilities to the system. For many people a
photograph really adds to the usability of the system. Photogrammetry
based surveys are usually a moderate to high cost and therefore may
also be outside of your budget range.
Table 1 - Pros and
Cons of Mapping Techniques
|
Item
|
GPS
|
TSS
|
Photogrammetry
|
OBM
or Town Maps
|
|
Cost
|
M-H
|
H
|
M
|
L
|
|
Completeness
|
H
|
H
|
H
|
M-H
|
|
Accuracy
|
M-H
|
H
|
M-H
|
L-M
|
|
Level of Detail
|
V
|
V
|
H
*1
|
L-M
|
|
Addition of Attribute
Information
|
H
|
H
|
L
|
L
|
|
Implementation
Period
|
M
|
H
|
L-M
|
L
|
|
Engineering
Capability
|
L-M
|
H
|
L-H
|
L
|
|
Maintenance/Update
Cost
|
M
|
H
|
H
|
L
|
|
GPS: Global Positioning
Systems
|
L = LOW
|
|
TSS: Total Station
Surveys
|
M = Moderate
|
|
OBM: Ontario Basic
Mapping
|
H = High
|
|
*1 Surface features only
|
V = Variable
|
Q. What if I can’t afford to
resurvey?
Another concept in GIS mapping accuracy, is
relative accuracy. A high level of relative accuracy means the
location of the objects in the GIS are accurate in their location
relative to one other, but all the while they may be up to 10’s of
metres away from their actual real world location. For example, the
sewers can be illustrated on the appropriate side of the road, the
telephone poles shown offset from the road, and the property lines
matched to the road allowance. The location of each element appears
to be accurate, but in reality the components may be up to many
metres away from their correct position. What you can’t do with this
kind of system is solve a property dispute, or create engineering
drawings for final design purposes. Apart from these limitations,
this type of GIS has much of the functionality of systems based on
more accurate mapping.
Q. How does this work?
Each piece of mapping (sewers, zoning and
property etc.) are reviewed and rated according to their accuracy
from high to low. Each map is re-created in digital form (through
scanning or digitizing tablet) into a CAD environment. A process
called drawing
conversion. Starting with the most
accurate map, each map is placed on top of one another. The scales
are synchronized and common features between the maps are used to
match the mapping sets so features on the less accurate maps can be
transferred to the more accurate maps. This process continues for all
of the available mapping until a GIS containing all the mapping is
completed. Often OBM maps are the most accurate maps that are
available, and therefore all other maps are matched to it.
Q. Will it be good
enough?
That’s a subjective question, and the answer
will depend on your Towns needs. Since this process of map
standardization is only improving your existing mapping; if your
mapping is currently meeting your needs, it stands to reason that it
will meet your needs in a GIS.
Another of the benefits of this type of GIS is
the ability to get up and going to quickly, giving time for you to
become acquainted with the technology. The accuracy of the base
mapping can be improved in the future on an incremental or Town wide
basis with one of the three techniques discussed earlier. Unless your
mapping is complete, you can also expect to do some work in adding
new subdivisions and correcting errors if you go with this
option.
Once your mapping is complete, its time to add
the data to the GIS.
Check out our Municipal map
update service if your not ready yet
for a GIS!