CoastWatch Data Analysis Tool Help: Geographic and mask overlays

In this section:

What are overlays?

CDAT uses data overlays to help give context to the data view. An overlay can show where latitude and longitude lines fall, where physical boundaries such as coastline, political borders, and bathymetric contours lie, and can also mask off certain parts of the view so that only the relevant data is visible.

Adding an overlay to the view

The following figure shows the current overlays and the saved overlay groups:

You can add one of several types of overlays to the data view using the CDAT toolbar buttons as follows:

Coastlines
Land/water boundaries derived from Global Self-consistent Hierarchical High-resolution Shorelines (GSHHS) data: http://www.ngdc.noaa.gov/mgg/shorelines/gshhs.html.
Latitude / longitude grid
Latitude and longitude grid lines with labels. By default, the grid spacing adapts to the view, but can be set manually.
Row / column grid
Data row and column grid lines with labels.
Country and state borders
Lines are derived from CIA WDB-II data. By default, only international borders are shown — state borders can be added manually by editing the overlay.
Topographic and bathymetric contours
Contour data is computed from ETOPO5 elevation data: https://www.ngdc.noaa.gov/mgg/global/etopo5.HTML. By default, only the 200 m and 2000 m bathymetric contours are shown, but the overlay can be modified to include any set of topographic or bathymetric contours.
Shape files
Line and polygon data stored in ESRI shapefile format or simple text files with lists of lat/lon values. Currently, shapefile support is limited: point data cannot be displayed, shape overlays cannot be saved to an overlay group, and shapefile rendering may be slow for large shapefiles.
Single layer bitmask
A single-color mask that uses a data variable and integer mask value to mask certain data pixels that contain the same bits as the mask value. For example, a bitmask can be used to mask clouds as gray.
Multilayer bitmask
A multiple-color mask that uses a data variable and color for each bit to mask certain data pixels that contain the bit values with the corresponding color. For example, a multilayer mask can be used to analyze the detailed output of a cloud masking algorithm.
Expression Mask
A single-color mask that uses a mathematical expression and color for each bit to mask certain data pixels for which the result of the expression is true. An expression mask is slower to compute than a bitmask or multilayer, but is much more flexible. See the User's Guide cwmath command for the syntax of the expressions. Only variables imported when the data was opened can be used in the expression.

Overlay list

Once added to the list, overlays can be edited, renamed, set visible or invisible, moved up or down in the list, and removed. To edit an overlay's properties, double-click the overlay in the list or select the overlay and click the Edit button. You can change simple line properties such as color and thickness directly from the list. To rename an overlay, edit the overlay's name text field. Overlay visibility can be changed by checking or unchecking the visibility box on the left. The overlay list represents a vertical stack and overlays are rendered in order from bottom to top. You can use the Move Up and Move Down buttons to rearrange the overlay stacking order. Finally, you can remove overlays by selecting a set of overlays from the list using Ctrl-click (⌘-click on the Mac) or Shift-click, and clicking Remove.

Overlay groups

Overlay groups are a convenient way of saving a set of overlays to be used later. Create a new overlay group using the Create button, selecting the overlays to save to the group, and then entering a group name. Click the Load button to load an overlay group or just double-click on the group name. The overlays are restored with the same properties they had when saved, and stacked on top of any other overlays already in the list. Delete an overlay group by clicking Delete.

CDAT comes with a default set of overlay groups that you may find useful; see the preferences and resources section for more on where overlay groups are stored and other things you can do with them. If you make a mistake and delete or modify these default groups, you can click Restore to recover them.

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