Tuesday, September 23, 2014
In 2007, Halifax Regional Municipality (HRM) transferred wastewater and stormwater assets to Halifax
Water, establishing the first regulated and integrated water, wastewater and stormwater utility in
Canada. Even prior to the transfer, HRM realized that the methodology for stormwater revenues had to
be reviewed. Halifax Water embarked on a formal cost of service study to change the way stormwater
charges were historically collected in an effort to make it more equitable to their customers, and more
cost-effective to administer. The first thing this study concluded was that a new rate structure needed
to be established to separate costs for wastewater and stormwater services.
This was different from the
current billing structure that
allocated revenue to stormwater
as a proportion of wastewater
revenue that was billed based on
volumetric water consumption.
An added benefit of separate
stormwater charges was to
hopefully influence customers
to reduce the amount of
stormwater that was getting into
the wastewater system. This
would be advantageous because
inflow and infiltration accounted
for a significant amount of the
wastewater treatment capacity.
With the understanding that a larger amount of impervious surface on a property increases the amount
of stormwater runoff, Halifax Water decided to use a pilot program to determine the feasibility of using
remote sensing technology to map impervious surfaces. The goal was to demonstrate a cost-effective
approach to assess stormwater usage without relying on costly and labor-intensive manual surveying.
The use of remote sensing not only offered the potential of lower cost, but also improved accuracy, and
provided regular monitoring on a regional scale. Basing the billing structure on impervious surface also
offered a higher correlation to cost causation than a structure based on water consumption or a per-lot
charge. Today Halifax Water has 98,000 customers which include 15,000 stormwater only customers.
PILOT PROJECT FOR IMPERVIOUS SURFACE MAPPING
The pilot project was launched with three different software providers to not only determine the
feasibility of the project, but to evaluate the strengths and weaknesses of different software to
determine which would be best equipped to complete the larger scope of work. Exelis VIS completed
the pilot study for Halifax Water using pansharpened 8-band multispectral Worldview-2 imagery.
Exelis VIS developed semi-automated methods to map impervious surfaces of the three small study
areas identified by Halifax Water using its commercial ENVI and IDL software.
The pilot study not only demonstrated the effectiveness of the approach, but the products Exelis VIS
created for Halifax water were 96% accurate. Using ENVI and IDL, Exelis VIS demonstrated that the
project could be completed in a cost-effective manner without compromising accuracy or detail, and
was therefore awarded the contract to produce GIS-ready impervious surface mapping products for the
entire stormwater boundary established by the HRM.
GETTING DOWN TO BUSINESS
The mapping approach that Exelis VIS employed for the project was based on the methods developed
during the pilot study, with a few exceptions. During the pilot study it became apparent that that the
map precision of the Worldview-2 imagery was not sufficient when compared to base imagery already
contained in Halifax Water’s GIS. Even after Digital Globe provided samples of their highest precision
orthorectified map product that didn’t require customer-provided ground control, Halifax Water
accuracy requirements still were not met. Exelis VIS then ordered imagery in a format that was suitable
for further georectification: Level 2A “ortho-ready” format. Because the ultimate use of the data was
for spectral analysis, Exelis VIS also requested the imagery be produced using only nearest neighbor
resampling -- a further effort to retain as much radiometric precision as possible.
With the Worldview-2 data calibrated, orthorectified, and pansharpened, the enhanced resolution
multispectral data were then used to identify impervious surfaces through a combination of multiple
processes. A vegetation index was used to take advantage of the strong, and fairly unique signature of
vegetation in the 8-band multispectral data. Because it’s rare to see a strong vegetation signature on
man-made impervious surfaces, this index image is used as an initial, rough estimate of the pervious
surfaces. Other processes including vegetation masking, soil mapping, unsupervised (auto-clustering),
and supervised classification were also employed. N-Dimensional spectral angle metrics were used for
measures of spectral similarity, and an anomaly detection algorithm was used to reduce false positives
caused by spectral artifacts.
TRACKING THE SOURCE OF ERRORS
Visual interpretations of the mapping results
(without ground truth) showed that the largest
errors in classification occurred for soils that
occasionally get categorized as an impervious
surface. In some cases these soils have unusually
bright signatures, a feature that is characteristic
of urban materials, and erroneously get filtered
out in post-classification processing. This may be
more common for altered soils, such as those mixed
with sand, or those compacted due to construction
activity. To help minimize this source of error,
Exelis VIS made adjustments to the post-processing
to ensure that flagged spectra were secondarily
assessed for the likelihood of being a soil ground
In the final analysis, selected parcels were ground-truthed with an actual survey of impervious area, the variance between the survey results and satellite imagery was 0.16%, indicating 99.84% accuracy for the project.
Based on the impervious surface mapping results, Halifax Water determined there should be two
classes of rates: one for residential customers; one for Commercial, Industrial, and Institutional
customers (ICI Customers). This decision was based on analysis that concluded it was possible to
develop a residential average. Residential lot sizes differ but the ratio of impervious area to pervious
area does not change much. ICI customers, on the other hand, have much more variances in lot size,
and nature of the property use. As it was not possible to develop an average impervious area, it was
decided to charge based on the impervious area of the actual lot.
Using impervious area as a billing determinant has provided a transparent and objective system
because it is measurable and does not change a lot over time. There are some on-going challenges with
respect to what gets identified as impervious area, however Exelis VIS and Halifax Water are continuing
to work together to improve the system.
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