Defining the Coverage Area | Defining the Grid | Specifying Point Locations Within the Grid | Defining a Custom Region | Defining Point Definition Properties

Defining a Coverage Grid

Coverage analyses are based on the accessibility of assets (objects that provide coverage) and geographical areas. For analyses purposes, the geographical areas of interest are further refined using regions and points. Points have specific geographical locations and are used in the computation of asset availability. Regions are closed boundaries that contain points. Accessibility to a region is computed based on accessibility to the points within that region. The combination of the geographical area, the regions within that area, and the points within each region is called the coverage grid.

Use the Grid properties to define the location of a coverage grid. This includes defining the boundaries of the coverage area and setting the granularity and location of the grid points used to fill the bounded area with grid points. You can also customize the definition of the points within the grid by constraining access to the points.

Defining the Coverage Area

To define a coverage area, choose a grid type under Grid Area of Interest and enter any required values.

Grid Type	Description
Custom Boundary	Creates a grid along a boundary line specified by a combination of area targets, line targets, region files (*.rl), polyline shapefiles (*.shp), and ArcView shapefiles (*.shp). Click Select Boundaries... to select the objects and files to be used for defining the coverage area.
Custom Regions	Creates a grid within regions specified by a combination of user-selected area targets, region list files (*.rl) and/or ArcView shapefiles (*.shp). Not available for Attitude Coverage. Click Select Region... to select the objects and files to be used for defining the coverage area. When using custom regions, it is important to select adequate grid resolution. If a single custom region is smaller than the resolution, STK will generate an alternative grid using a finer resolution so that it ensures at least three samples within the region. In scenarios that were created using an 8.0 or earlier version of STK and you defined a coverage area using a custom region, a small change in distribution of points may occur for near polar regions. In these cases, it is necessary to clear and re-compute accesses for this coverage definition.
Global	Creates a grid that covers the entire globe. Due to the number of points computed when covering the globe, the time needed to calculate data when using this option may be large.
LatLon Region	Creates a grid between the user-specified latitude and longitude point pairs. When specifying point locations in a LatLon region grid, it is usually advantageous to define the grid boundary in a manner that forms a fairly close bound to the distribution of points to avoid artifacts in smooth figure of merit contours caused by interpolation over large geographic extents. For more information, see Specifying Point Locations.
Latitude Bounds	Creates a grid between the user-specified Minimum and Maximum Latitude boundaries. In scenarios that were created using an 8.0 or earlier version of STK and you defined a coverage area using Latitude bounds, a small change in distribution of points may occur for near polar regions. In these cases, it is necessary to clear and re-compute accesses for this coverage definition.
Latitude Line	Creates a set of points along a single latitude line. This option is useful when the coverage is only expected to vary with longitude, as it greatly reduces the number of points to be calculated and, therefore, provides resulting data much more quickly. Enter Start Longitude, Stop Longitude, and Latitude values. The longitude span is the span starting from the Start value and moving eastward until the Stop value (as measured on a circle where -90 and 270 are considered equivalent).
Longitude Line	Creates a set of points along a single meridian. This option is useful when the coverage is only expected to vary with latitude, as it greatly reduces the number of points to be calculated and, therefore, provides resulting data much more quickly. Enter Min. Latitude, Max. Latitude, and Longitude values.

If you define an area based on an existing area target and then modify the boundaries of the area target, STK automatically updates the boundaries of the area if the Auto Recompute option is enabled.

Defining the Grid

The statistical data computed during a coverage analysis is based on a set of locations, or points, which span the specified grid area of interest. The exact location of the grid points are either computed based on a specified granularity or imported from a file.

You can determine the spacing between grid points using the Grid Definition options. These options help you define the fineness or coarseness of the grid if the grid point locations are not specified via a file.

The use of finer grid resolutions typically produce more accurate results but requires additional computational time and resources.

Grid generation near central body poles is handled as a special case for which it is advisable not to use excessively fine resolution.

Specifying Point Granularity

Option	Description
Area	Defines the location of grid coordinates by using the specified area to determine a latitude/longitude spacing scheme at the equator. Not available for Attitude Coverage.
Distance	Defines the location of the grid coordinates by using the specified distance to determine a latitude/longitude spacing scheme at the equator. Not available for Attitude Coverage.
Lat/Lon	Determines the location of grid coordinates by applying the value entered in the text box to the right of the field at the equator. STK stretches grid points in longitude at higher or lower latitudes in an attempt to preserve the area of the grid point.

When Coverage attempts to accommodate a user-defined point granularity value, the granularity actually achieved may vary slightly from the value entered due to the need to provide equal spacing over the specified area.

A special grid algorithm, typically employed for a single small custom region, may modify user-selected granularity to ensure sufficient sampling within the region.

Special considerations apply to setting grid point granularity in a custom region.

Specifying Point Altitude

Select one of the following to specify the height of the grid points:

Option	Description
Altitude above MSL	The height is a constant offset from the mean sea level (MSL), which varies as the latitude and longitude of each grid point. This option is available on Earth only.
Altitude above Terrain	The height is a constant offset from the terrain, which varies as the latitude and longitude of each grid point.
Altitude above WGS84	The height is a constant offset from the WGS84 ellipsoid surface measured along the surface normal, where the surface normal varies as the latitude and longitude of each grid point. This option is available on Earth only.
Depth below MSL	The depth is a constant offset from the mean sea level (MSL), which varies as the latitude and longitude of each grid point. A positive value indicates a depth below MSL. This option is available on Earth only.
Altitude above Ellipsoid	The height is a constant offset from the central body's ellipsoid surface measured along the surface normal, where the surface normal varies as the latitude and longitude of each grid point. This option is available on central bodies other than Earth only.
Depth below Ellipsoid	The depth is a constant offset from the central body's ellipsoid surface measured along the surface normal, where the surface normal varies as the latitude and longitude of each grid point. A positive value indicates a depth below the ellipsoid surface. This option is available on central bodies other than Earth only.
Radius	Specifies the location of the grid point by its distance from the center of the central body, a centric rather than a detic measure.

Specifying Point Locations in the Grid

Points within a coverage grid are usually placed by STK based on the point granularity value set. However, you may wish to set each point within a grid for calculation purposes. To identify the location of specific points within the grid, select Custom Locations under Point Locations and click Specify....

To add a file to the Point File List, click Add, and then double-click the new entry in the list. Select the point list (*.pnt) or shapefile (*.shp) that contains the points of interest. Both the point list and shapefile contain latitude and longitude point pairs and the point list file may optionally also contain altitude values. If a point list file containing altitude is specified, the altitudes of the contained points may be overridden by selecting Override as the setting for the Custom Point Altitude Method. This will result in all points being assigned the altitude specified in the CoverageDefinition Point Altitude settings. If the custom point locations do not contain altitude information (always the case for shapefiles and might be the case for point list files), points are assigned the altitude specified in the CoverageDefinition Point Altitude settings.

If you select a file that contains points lying outside of the selected area boundaries, those points are discarded from the current analysis.

Although the specification of exact point locations removes the effects of the point granularity value, the granularity is still used in the computation of the area associated with each grid point.

When the Selected Point Files list contains all of the files of interest, click OK to return to the Grid properties page.

Defining a Custom Region

If you choose to define a coverage area using a file containing point pairs, select Custom Regions as the Grid Area of Interest and click Select Regions... to choose a combination of area targets and/or user-defined boundary points to be used for defining the area.

In the Select Regions window, you can choose to define the area using the boundary points of an existing area target, or to define it using one or more region list files (*.rl) or shapefiles (*.shp), or both. Shapefiles are common in the Geographic Information Systems (GIS) community for sharing geographic data and the format was developed by Esri (the Environmental Systems Research Institute).

Shapefiles and region list files have an advantage over STK area targets because they support defining polygons with holes.  The interior of a polygon is defined to be the area to the right side of the line made by following the points defining the polygon (clockwise).  You can therefore create holes in polygons by creating a polygon that overlaps another and has its points defined in the opposite (counterclockwise) order.

To select an area target to be used as part of the coverage area, highlight it in the Area Targets list and use the right arrow or double-click the name to add it to the Selected Regions list. More than one area target can comprise a single coverage area.

To use an area target when defining the coverage area, the area target must already be a part of the current scenario.

Area targets used to define custom regions should have a longitude span no greater than 180 degrees. This limitation includes area targets encompassing a polar region.

To select one or several region lists (*.rl) or shapefiles (*.shp) as part of the coverage area, click Load Region File.... When the Select File window appears, select the file(s) of interest and click Open. Coverage adds the file(s) chosen to the Selected Regions list.

Select Check for holes to prevent the grid generator from producing grid points within holes in regions defined in region lists or shapefiles.

When using custom regions, it is important to select adequate grid granularity. If a single custom region is smaller than the resolution, STK will generate an alternative grid using a finer granularity to ensure at least three samples within the region.

Once you have selected all of the area targets, region lists and/or shapefiles that comprise the coverage area, click OK to return to the Grid properties page.

Defining Grid Constraints

To specify a type of object or a specific object for the points within the grid, click Grid Constraint Options.... The Grid Constraint Options display.

E-Mail AGI Technical Support

   Visit AGI.com