For a given stream gauging site, rating tables can be developed that define the relationship between water depth and the stream discharge or flow. The correct application of these rating tables is dependent upon the stream dimensions remaining constant. In reality, river and stream cross sections can change as a result of scouring of the channel bottom, deposition of sediment, growth of vegetation and so forth. To accommodate these changes hydrographers can perform what is termed a Ratings adjustment - a modification of the ratings table that has a period of applicability, and then apply that modified rating table to the water depth to give a more applicable discharge values.

Identify and flag data

1. Select the Level and the period of hydrographic data you wish to review and flag using the Filter Panel or otherwise. Copy your filter conditions for the datasheet.
2. Click Chart in the View group of the Menu ribbon.
3. Drag datetime on to the x axis and your depth variable on to the y axis (See Charts for more details).
4. Paste your filter conditions from the datasheet in the filter panel under the chart.
5. Set Flags which will identify the data that requires a rating shift and which can remain unaltered e.g. Flag = Ratings shift and Flag = Verified data (See Flags for more details).
6. Carry out a Graphical Flagging of the data in your datasheet as desired.
7. Click Save on the Quick Access Toolbar to save your data. Then Close the graphical flagging tool.
8. In the Datasheet tab in the Menu Panel, click Refresh Datasheet. Then in the reloaded datasheet, check that the data flags are showing correctly (cell colours).
9. Go back to your Chart and drag another depth variable onto the y axis.
10. In the Filter Panel for this new depth series, add filter condition Flag Equals Ratings Shift.
11. Right-click on the Chart and select Chart Editor.
12. Go to Series Select your depth variable from the drop down list and adjust the properties as required.
13. When you return to the chart, you should be able to see all the data points to which the ratings shift will be applied.

Develop and apply the modified ratings table

1. Open the Regression
2. Click Conversion Table in the Menu Panel and select the appropriate conversion table (old rating table) to use from that site.
3. Click Gaugings in the Menu Panel. Load all the gaugings or select a gauging by checking the box against its row.
4. Once both the conversion table and gaugings table are selected, the regression chart is drawn and the confidence chart is populated.
5. Identify the outliers in the data set, by hovering the mouse over the plotted confidence point and reading off the gauging information. Use zoom as required.
6. Unselect the box in the gauging list against the gauging identified as an outlier. As a general rule, remove all gaugings that sit outside of the 10% confidence limit.
7. (Optional) You may also exclude data from the original conversion table, depending upon how closely the gaugings follow the original. Unselect the boxes in the conversion table list against the records identified as outliers.
8. (Optional) You can also unselect or select a portion of the Conversion Table. Left click and highlight the rows you wish to remove, then right-click and choose Unselect.
9. Select the Criteria for the modelled regression. You may wish to target only the area of the original conversion table (old rating table) where you have removed points.
10. Select the two depths between which you wish to model and choose the depth increment for the modelled data, for example 0.1m.
11. View the tabulated results in the Analyse Results panel on the right.
12. Right-click on the table to save these results to the Output Table panel.
13. Click Save to save the new rating table with a new name, appropriate x (depth) and y (discharge) labels and a period of applicability which corresponds with the time period of the data that requires the time shift.
14. Choose to assign the new ratings table to the site that the data originated from and then also choose to Apply to Existing Data. In doing so, you bring up another Window called the Data Conversion Window.
15. In the Data Conversion Window, you select the depth variable, the tag you want to use which would be Ratings shift, an output variable which will be Discharge, and select another tag for the discharge data which might be something like Requires final review.
16. Click the Save button and then you are prompted to update the database. Click Yes.
17. Close the Data Conversion Window, and close the Regression Tool.

You can now view the modified discharge data in the datasheet, highlighted as Requires final review.