Quality Assurance Measures for Serving Real-time Water Data on the World Wide Web
In Reply Refer To:
Office of Surface Water
Mail Stop 415
Sunset Date: February 28, 2005
February 28, 2000
WATER RESOURCES DIVISION POLICY MEMORANDUM NO. 99.34
SUBJECT: Quality Assurance Measures for Serving Real-time Water Data on
the World Wide Web
This memorandum establishes a policy for the implementation of
quality-assurance measures for real-time water data being served on the
World Wide Web (WWW). It builds on the information provided in Water
Resources Division (WRD) Memorandum No. 95.19 and No. 97.17. Memorandum
95.19 established a policy that the nature of provisional data available
on the WWW would be clearly identified and Memorandum 97.17 provided
guidance for use of the rt_www software for serving real-time streamflow
data.
Including the WWW in our real-time data delivery system has significantly
increased the visibility of these data. Although WRD has been providing
real-time streamflow data to selected cooperators for over 25 years, our
feedback shows that the public is now using our data in ways we never
anticipated. Unfortunately, our ability to quality assure these data has
not kept pace with our ability to provide it. Erroneous and spurious data
(such as spikes due to transmission errors) that appear on our web pages
are problematic for two reasons. First, these data can lead users to make
bad decisions involving lives or property. The second is that bad data,
even if not used for a decision, undermine the credibility of the vast
majority of good data we are serving. When the bad data persists on our
web pages for several days, it suggests a lack of attention to quality
assurance. Although provisional data statements clearly warn users about
potential errors, it must be our goal to avoid serving data that we know
to be seriously in error.
Ensuring that the data provided to the public are the best we can offer
will require a concerted effort to review data before and after it is
available on the web and make appropriate corrections to the displayed
data. Our approach to ensuring the quality of our real-time data must be
made up of two parts. First, we need to use the software tools available
to automatically prevent bad data from being served in the first place.
This memorandum describes some of the tools that are currently available
for this automatic checking. These tools can be very effective if used
correctly and even better tools are being developed to enhance this
process. However, even the best automatic process will not eliminate all
of the spurious data. Thus, we need to supplement the automatic processes
with frequent visual inspection of hydrographs followed-up with rapid
deletion of the bad data.
WRD Policy Memorandum No. 99.34
Frequent inspection should be taken to mean at least once per work day
although many of our offices already have procedures for checking even on
weekends and holidays. However, at times when users most rely on our data
(floods and droughts) even more frequent checking may be appropriate. It
is far better for us to indicate that the data are missing, and that we
are aware of the problem and will work to resolve it, than it is to
continue to serve the bad data.
Quality Assurance Measures for Serving NWIS Data on the WWW
The NWISWeb software, once implemented, will be used to serve our historic
and real-time surface water, ground water and water quality data on the
WWW. Information on NWISWeb development, requirements documents, and
quality-assurance measures associated with the current release of NWISWeb
software will always be available from:
The QA measures detailed below apply to the surface water WWW data
currently being served using the rt_www software. The NWIS Web software
will continue to use these measures.
The rt_www rev 2.0 software will automatically mask from web display any
unit-value data that is flagged by ADAPS as exceeding the thresholds for
Very-high-value, Very-low-value, Very-rapid-increase, and
Very-rapid-decrease. When any data for a day is masked in this way, a
small black triangle will appear at the top axis of the graph indicating
the condition.
Also included in rt_www 2.0 is a web-based "quick-view" utility which
provides data reviewers an easy way to sequentially view stage (and
optionally discharge) graphs for monitoring data quality. This utility is
especially useful for reviewing graphs from the field using a
cellular-capable modem. When viewed on the district LAN, graphs can be
sequentially drawn to the screen once every 3 seconds allowing all
real-time stations to be reviewed as often as necessary. The visual
review of the real-time data using the quick-view utility is useful for
catching subtle errors that may pass through the threshold filters.
Minimum Quality Assurance Procedures
Memorandum 97.17 established policy for the daily review of data and for
making corrections when necessary. The “quick-view” utility can be used
to conduct a review of the data once a day under normal conditions.
During flood or drought, keep in mind that there is higher visibility of
data and errors are potentially compounded, therefore, it is important
that data being served be checked and corrected, as necessary, more
frequently. A completely spurious data spike during a
flood should not be displayed on the WWW for more than 24 hours. During
more normal
conditions, such a spike should not be displayed for more than a few hours
into the next work day.
WRD Policy Memorandum No. 99.34
At a minimum, the ADAPS Very-high-value and Very-low-value must be set for
every station with data displayed on the WWW in real time. The values for
each of the threshold filters must be defined by individuals that are
knowledgeable about the hydrologic and historical flow characteristics of
each site.
The Very-high-value should be set to eliminate values that are greater
than the highest possible value; therefore, a value that is 0.10 units
higher than the highest possible value that can be measured by the sensing
and recording equipment at the site would be appropriate. For a
streamgaging station, this might be 0.01 feet higher than the bottom of
the instrument shelf.
The Very-low-value should be set to eliminate values that are less than
the lowest possible value; therefore, a value that is 0.10 units lower
than the lowest possible value that can be measured by the sensing and
recording equipment at the site would be appropriate. For a streamgaging
station, this might be 0.10 feet below the point-of-zero flow of a stable
control.
The VERY RAPID INCREASE and VERY RAPID DECREASE threshold can be used by
the rt_www software to provide an alert about a possible erroneous jump
between successive unit values. Currently, because there is no standard
method to determine valid rapid increases and decreases these thresholds
are not required, although a knowledgeable streamgager can determine these
values for each site by examining the historical unit-values record and
then calculating values for both of these thresholds.
The requirement to review data served on the WWW at least once a business
day under normal conditions and the establishment of Very-high-value and
Very-low-value thresholds in ADAPS are policy of WRD. Other guidelines
for quality assuring real-time data were distributed with OSW Technical
Memorandum No. 99.07 as part of the addendum to Open-File Report 94.382,
"A Workbook for Preparing Surface Water Quality-Assurance Plans for
Districts of the U.S. Geological Survey, Water Resources Division."
Using the two-part approach: automatic screening to prevent bad data from
appearing, coupled with visual screening and editing at least once per
work day, we can significantly lower the amount of erroneous data we
serve. This will be of benefit to the public we serve and maintain the
high level of credibility that USGS streamflow data rightfully enjoy.
/signed/
Robert M. Hirsch
Chief Hydrologist
Distribution: A, B, FO, PO, S
USGS:WRD:tyorke:jr:2/28/00:5303
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* Robert M. Hirsch
* Associate Director for Water
* 409 National Center rhirsch@usgs.gov
* U.S. Geological Survey 703-648-5215
* Reston, VA 20192 fax 703-648-7031
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