Marine Data Literacy 2.0

Providing instruction for managing, converting, analyzing and displaying oceanographic station data, marine meteorological data, GIS-compatible marine and coastal data or model simulations, and mapped remote sensing imagery





Home > 4. Ocean Data View > 4.3 Scatter Plots

4.3 Creating Marine Data Scatter Plots in ODV

1.  Scatter plots are 2- or 3-axis plots of data points, where any parameters may be selected (including depth) for any axis.  The usual X and Y axes are always employed for the physical plotting of data points, but the optional Z axis is represented (in ODV) by the colors of the data points (i.e. a palette has been applied, based on a third variable).  Relationships between parameters can thus be visualized in 2 or 3 dimensions. 
2.  You should see something like this in ODV (after the previous exercise in this group).

NOTE:  There are other scatter layout options which you can explore on your own later.

4.  This figure appears, based on the last graphic in station mode.

Right-click on the graphic and select FULL RANGE.

5.  Here's the complete set of temperature data, versus depth.  Each dot is a single measurement, and right now all the data are shown.

Later on, we'll see how to apply filters to view only desired portions of the data.

You can see there are probably some erroneous values, exemplified by the dots lying far outside the general pattern.

Also, there is a "fork" below 4000 m that may indicate different water masses, or quality control problems.  Only a few points are present, so it is difficult to make any judgement.

6.  Now, let's look at salinity.  Right-click on the graphic and select X-VARIABLE = SALINITY.
7.  This graphic appears.  But a large part of the space is taken up by very low salinity values at the surface (probably coastal measurements).

We need to zoom in to the open ocean values.

8.  Right-click on the graphic and select ZOOM.
9.  Draw the red frame inwards to isolate only the central part of the salinity data.

NOTE:  You can also do this with the SET RANGES command, using appropriate values.  Sometimes it is just easier to do it visually.

When you are ready, double-click inside the red frame.

10.  Now you can see the real variation of salinity with depth.  Notice the well-known salinity minimum near 800 m.

There is also evidence of a "fork" below 4000 m.

There are many stray data points lying far from the general pattern.  This is evidence of some quality control issues.

11.  Now lets see what we can do with some data filters.  Right-click on the small station map and select SAMPLE SELECTION CRITERIA.

NOTE:  There are many functions here you can explore on your own.  ODV is an extremely powerful program that we can only begin to introduce in these short exercises.

12.  The CRITERIA include RANGE filters, where you could set some parameter value ranges from the resource article above.  And QUALITY filters (shown here), where you can specify any combination of the 9 quality flags in the World Ocean Database system of flags used for our collection.


Then click OK.

13.  You can see that this makes a considerable difference, because many of the really bad points (i.e. those lying very far from the general pattern) are not shown now.  Compare this figure with Panel 10.
14.  Now, right-click on the graph and select X-VARIABLE = SALINITY and then Y-VARIABLE = TEMPERATURE.  And also set FULL RANGE again.
15.  This the well-known "T-S Diagram" which always has T(emperature) on the Y axis and S(alinity) on the X axis.
16.  If you click around in the data points, you'll find that most of the salinities below 30 are at the coast, and the very next value below them are above 30.

To see more detail, we have to look to the right of these very low, coastal salinity values.

17.  Use ZOOM (or perhaps SET RANGES) to limit the salinities to a range of 30-34, and reduce the temperatures to 0-30 degrees.
18.  Now you can see the rich detail in the T-S graphic.

NOTE:  You'll need to take a basic hydrography course to learn more about interpreting these figures, and the physical basis behind them.  Various oceanic water masses are represented by the inflection points, and mixing processes are represented by the straight "legs" between them.

19.  Now, let's investigate the relationship between seasons and the shape of the T-S plot, just as an example of the science you can learn here.

Right-click on the small station map, and select STATION SELECTION CRITERIA

20.  In the DATE/TIME tab, select the SEASON from JAN 01 to MAR 31.

NOTE:  You can explore the other station filters on you own later.

Click OK to continue.


21.  Now you can see that a large part of the very warm, very low salinity water is not present in the months Jan/Feb/Mar.

Perhaps these are dry months, regionally, and there isn't any production of low salinity, coastal water.

22.  Use the same method to look at the months of Jul/Aug/Sep and you'll see that the low salinity coastal water is present.

Perhaps these are rainy months.  You can check the climatologies on your own to confirm this.

23.  Now, return to the STATION SELECTION CRITERIA, and on the DATE/TIME tab, click RELAX CRITERIA, and OK to return to the complete set of stations.

The SAMPLE SELECTION CRITERIA, specifying only the good data is not affected by this.

24.  Now we're going to see how ODV handles a third variable in one scatter plot.  Right-click on the graphic, and select Z-VARIABLE = OXYGEN.
25.  Oxygen is represented now by the colors of the dots.  The color scale on the right (which can be controlled by the Z-ZOOM function) refers to oxygen.

Here you can see the well-known oxygen minimum later at temperatures of about 7-17 degrees.

The use of 3 variables gives you a very powerful way to study water masses, and to identify very subtle mixing processes.

26.  Select VIEW > SAVE VIEW AS
27.  Save the view with the name temp_sal_oxy_liberia_wod_tsdiagram (ODV will take care of the extension and the folder location).
28.  This has been an extremely short introduction to one of the most powerful display modes in ODV.  You should explore all the functions you've seen briefly in this quick visit, and you should consider taking a formal course in hydrography to learn more about the water mass identification concepts.