When reading data, HEC-MetVue provides an option to limit the spatial extent of data to be read. Providing a spatial extent constraint when reading data provides a means to trim data, thus reducing memory impacts and speeding up data loading/processing. This can be a very powerful tool when analyzing a region that is much smaller than the spatial extents of the available data.
Task 1 will demonstrate how to constrain imported TIN data using a predefined polygon shapefile.
Create a New Map Window.
Right-Click on the "Pearl-Part C" Session and select "Sessions and Maps" → "Add Map Window..."
Name the new Map Window "Extent Constraints" and select Ok.
Open the DSS Data Reader by expanding the "Extent Constraints" Map Window Node in the HEC-MetVue Explorer Window.
Right-Click on the TIN node and click on "Select DSS paths…"
Make the following DSS file selection:
...\HEC-MetVue_DssDataReader\Data\DSS\precip.2021.08.dss
Ensure that the "Temporally" Combine TINs and "Select Specific TIN(s)" options are selected within the DSS Data Reader.
Notice that the "Active Read Constraints/Overrides" are currently set to their default values: Temporal Combine Default (Aggregate) and Spatial Combine Default (Average)
View Full Extents of DSS Data.
Let's first view the full extents of the Lower Mississippi River Forecast Center (LMRFC)precipitation grids.
Filter the B-Part path to "LMRFC" and the F-Part path to "QPE."
Select "Add All" to add all of the QPE grids from 29 to 31 August 2021 to the map window.
Press OK to close the selection editor and view the QPE precipitation data with the full LMRFC extents from 29 to 31 August 2021.
Zoom out if necessary to view the full LMRFC extents for this event.
The full extents of the LMRFC boundaries include about 1.1 million square miles.
Constrain an Imported TIN to a Defined Rectangle.
Next, let's limit the extent of the data within the map window to just the Pearl River watershed in Southern Mississippi and Western Louisiana.
Right-Click on the TIN node and click on "Select DSS paths…"
Expand the "Active Read Constraints/Overrides" menu.
Select "Constrain Imported TIN to Defined Rectangle."
Select "Define Constraint Extents..." to open the "Select Extent Read Constraints" dialog box.
Notice that there are two options for setting constraints:
1. Defined Rectangle: Allows the user to set extents manually by specifying coordinates or copying the extents from the current Map Window extents or the extents of another TIN file that is already loaded within the Map Window.
2. Polygon Mask: References the extents of a specified polygon shapefile when applying read constraints.
For this Task, we will use a polygon shapefile to determine our constraint extents.
Select the "Polygon Mask" option within the "Select Extent Read Constraints" dialog box.
"Browse..." to and Select the following shapefile:
...\HEC-MetVue_DataReaders\Maps\PearlMS.shp
Select OK to close the dialog box.
Press OK to close the selection editor and view the QPE precipitation data using the Pearl River watershed shapefile as a read constraint from 29 to 31 August 2021.
Notice how only the area within the extents of the PearlMS.shp shapefile are displayed. The size of this area is roughly 25,000 square miles–much smaller than the full LMRFC gridded extents.
Also note that the map window processed the data and displayed much more quickly to the smaller, Pearl River extents than it did when viewing the full LMRFC extents.
The data can also be saved back to DSS in its limited extents by selecting the "Save the Current Changes as a Projection on a Grid" icon (
)from the toolbar.