Maps can become cluttered with excessive amount of map data. HEC-MetVue allows the visibility of map attributes to be controlled based on the geographic extents of the data being displayed in the map window. Visibility thresholds are best set from the context menu of the map window although they can also be set using the attribute schema dialog. Visibility thresholds are always defined by attribute type.
For example, suppose that the maps being viewed contain map data for Interstates, U.S. Highways, State Highways, Primary roads, and Secondary roads. At a very wide view perhaps only interstates are visible. As the view is zoomed in, perhaps U.S. highways become visible (high view threshold) and symbols show where major cities are located. Zoom in even more and other roads become visible, the symbol showing where the city is located is removed (low view threshold) and replaced with an outline of the incorporated limits. Zoom in further and streets in subdivisions and flood zones become visible.
Since the zoom thresholds are associated in the attributes of a particular schema, once defined, they are defined for all map windows. (Remember that attribute schemas, palettes, and scales are global objects.)
Define upper visibility thresholds for an attribute as illustrated below:
- Zoom on the map to the maximum extent where attribute should begin to display.
- Right-click on the map window to bring up the map window context menu.
- Select 'Map Element Visibility Thresholds'.
- Select the desired attribute to set the upper threshold for and select 'Set High View Threshold'.
To define the lower visibility threshold for an attribute, do the same as above for the upper threshold except select 'Set Low View Threshold' instead for the final step.
If neither of the thresholds have been defined then the 'Remove View Thresholds' menu item will be grayed out.
