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


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 

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 

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.

                                        Robert M. Hirsch
                                        Chief Hydrologist

Distribution: A, B, FO, PO, S


* 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