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Interactive Double Mass Analysis
User's Guide

Version 1.2
March 15, 1999

1. Overview

Double mass analysis is a technique commonly employed to determine corrections to hydrometeorological data to account for changes in data collection procedures or other local conditions. The changes may result from a variety of things including changes in instrumentation, changes in observation procedures, or changes in gage location or surrounding conditions. The purpose of this Interactive Double Mass Analysis (IDMA) tool is to provide an effective and efficient way for users to perform consistency checking using double mass analysis and to produce the correction factors that are used in subsequent processing by NWSRFS preprocessor tools to account for any discovered inconsistencies.

Computations of mean areal time series for use in calibration and ESP have traditionally been made using the appropriate preprocessing software (MAP, PXPP, MAT etc.). The preprocessors have traditionally taken point time series as input and have provided facilities for estimation of missing data, and for consistency checking and correction in addition to the mean areal computations. The IDMA tool has been designed to run in conjunction with the preprocessors in an iterative fashion. It performs the consistency checking function, leaving the estimation, correction and mean areal computations to the preprocessors.

Note: Throughout the IDMA tool, the correction factor is the actual value that is applied to the raw data by the preprocessor. This slightly departs from the current manual approach used within the preprocessors where the user may have to manually compute a correction to a correction factor and then multiply the two to obtain a new value.

Figure 1.1 Data Flow Diagram Showing Interaction Between IDMA and Preprocessor
As shown in Figure 1.1 the preprocessor software generates a data file that contains accumulated data values for each station and the associated group base, as well as flags indicating whether the values were estimated or not. This data file along with the input card file used by the preprocessor are used by the IDMA tool. The IDMA tool then allows the user to perform consistency checks and to calculate correction factors. The correction factors are then returned so that the preprocessors can perform the corrections, re-estimate missing data and perform the mean areal computations. Note that there is an interdependence between the estimation of missing values in the preprocessors and the correction factors computed in the IDMA and so there needs to be iteration between the two processes until the user is satisfied that an appropriate balance has been reached.

The estimated data flag in IDMA is displayed if any value in the month has been estimated. The flag does not mean that the whole month is estimated, just tha there is at least on value in the month that is estimated.

The steps that the user needs to go through to use IDMA in conjunction with a particular preprocessor are:

  1. Run the appropriate preprocessor software to create the input file for IDMA
  2. Run the IDMA (which can make suggestions for break points based on a statistical analysis of the data)
  3. Determine appropriate break points in the double mass curves
  4. Determine appropriate correction factors for each selected period
  5. Save all correction factors to the input card file
  6. Rerun the preprocessor software to generate corrected data and re-estimate missing values
  7. Rerun IDMA to check corrections
  8. Repeat steps 4 through 7 as necessary
  9. Rerun preprocessor software to produce mean areal time series
The double mass curve is a plot of the deviation of a station's accumulated values versus the average accumulation of the base group. In this context, deviation is computed as the accumulation for a station minus the average accumulation of the base group. Any break point in the curve suggests a possible change at the station in relation to the base group. The IDMA tool displays double mass curves in sets. Each set may contain up to 5 stations. Each station has its own color and the user has the ability to view or hide the station on the plot. Estimated and observed data for each station are distinguished by different colors. The observed data is displayed in the same color as the station, while the estimated data is displayed in white. Station moves are indicated on the curve by downward arrows. If a seasonal analysis is being performed, and a station move occurred outside the season being displayed, then a V will be positioned on the first month of the year that the move occurred. To find out when the move occurred, the user can view the station information display and get the dates of the station moves.

To calculate correction factors, the user must first choose the period of record that will serve as the base. In the other words, this is the period to which other periods will be corrected. Often, this base period is the most recent period. The user may enter a correction factor, have the IDMA tool to compute the correction factor or change the computed correction factor. After all break points and correction factors are determined, the user needs to save all breaks points and correction factors to the input card file for the preprocessor. The IDMA tool also provides an option for automated detection of potential break points. The user may use this option as an initial consideration or reference.

The user may view the station history information and point information during consistency checking. The station history information shows the station moves, dates for the data values, the accumulated data values and the accumulated base values. This information is shown for each station currently being displayed on the double mass plot. The point information shows the station moves, the date of the particular point, the particular accumulated data value for the point, the accumulated base value for that point and whether the station data value is estimated or not. In addition, the user may draw a reference line along the mass curve to identify if any corrections are necessary and assess if the corrected data are acceptable after corrections.

In summary, the GUI-based IDMA tool provides the user the following capabilities and features:


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National Oceanic and Atmospheric Administration
National Weather Service
Office of Hydrologic Development
1325 East West Highway
Silver Spring, MD 20910

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Page last modified: August 24, 2006
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