Shepard (1954) Ternary Classification of Sediments
diagram courtesy Maryland Geological Survey
Sediment Classification Ternary Diagram
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Shepard (1954) Ternary Classification of Sediments diagram courtesy Maryland Geological Survey |
Icon on main toolbar will draw a ternary diagram, but will
currently not plot data, which must be done from a database.
The ternary diagram is really only a two dimensional plot, which is the only reason it will plot on a 2D computer screen or graph. Since the three percentages sum to 100%, only two are independent.
To plot ternary diagram. Example for sediment size classification, but it will work with any three fields in a database.
Open a map. It could be:
New DEM,
such as the bathymetry of a specific area.
New
Vector Map, an outline map of the world.
Predicted
bathymetry, a small scale overview map
showing depth and relief.
Open database from the
map toolbar.
Many of the sediment databases (for instance the USGS East Coast Sediment Data Base, or the Maryland Geological Survey data for the Chesapeake Bay) have a vast number of fields, most of which will not be of interest in an analysis like this. You can hide these with the "Hide" button on the data base table display form. If you have the program option selected to save the database status, these fields will remain hidden on subsequent uses of the data base.
Select "Ternary diagram" option from popup menu that opens with "Stats" button.
Select the three fields. Note on the sample above in which corners they go. If you have the program option selected to save the database status, these fields will remain the defaults on subsequent uses of the data base.
There is an option on the Geology/Geography tab of the Options form about whether the three fields should sum to 100%. This is appropriate if they are in percent. If not, the three will be summed and the percentage calculated. If the points should sum to 100%, they must be in the range of 98 to 102, to account for roundoff errors, or the point will be ignored. Shepard (1954) did not indicate how gravel should be considered in this plot, so it is ignored. If you wanted to include it, you could construct a sand + gravel field in the database and plot that instead of sand. USGS classifies anything with over 10% gravel using a different system (gravel, sand, and fines).
The ternary diagram will be drawn, and it will be color coded to the map, which will also redisplay. The three corners will plot in red, green, and blue, and points between will be scaled accordingly.
| Map showing sediment size distributions in part of the Chesapeake Bay | Ternary diagram of sediment size distributions in the Chesapeake Bay |
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Right click on the ternary diagram,to change the size or whether the grid is shown.
You can ID points on both the map and the ternary diagram by double clicking.
To see gravel.
Pick the "Plot" button menu and then select "Color code by DB field" from the popup window. You will select a numeric field and then plot something like "Gravel_PCT".
If you think the locations with no gravel are hiding points with gravel, you can Filter the database to show only points with a particular gravel concentration.
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| Percentage of gravel on the US East Coast continental shelf, over the predicted bathymetry. | Sediment classification in the Chesapeake Bay near Annapolis, over a shaded relief rendition of a 30 m DEM from the NOAA estuaries program. Same color scheme as the ternary diagram above. |
References:
Last revision 11/3/2014