Defining a Coverage Grid
Coverage analyses are based on the accessibility of assets (objects that provide coverage) and geographical areas. For analysis purposes, you can further refine the geographical areas of interest using regions and points. Points have specific geographical locations, and STK uses them in the computation of asset availability. Regions are closed boundaries that contain points. STK computes accessibility to a region 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 to select the objects and files that STK will use for defining the coverage area. |
| Custom Regions |
Creates a grid within regions you select. For a description of this option, see the Custom Regions section later in this topic. |
| 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 latitude and longitude point pairs you select. 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 Minimum and Maximum Latitude boundaries you specify.
In scenarios that you created using an 8.0 or earlier version of STK and for which 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 recompute 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 for STK to calculate 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 for STK to calculate 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 you enabled the Auto Recompute option.
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
When Coverage attempts to accommodate your 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 granularity you specified 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 .
To add a file to the Point File List, click 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 also contain altitude values. If a point list file containing altitude is specified, you can override the altitudes of the contained points 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, which is 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, STK discards those points from the current analysis.
Although the specification of exact point locations removes the effects of the point granularity value, STK still uses the granularity 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 to return to the Grid properties page.
- When using custom grid point locations (specified via files), 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.
- If you specify point locations via a point list (.pnt) file, STK uses the optional altitude of the points in the file if and only if you set the point altitude to zero in the Coverage Definition properties. Otherwise, STK uses the point altitude from the Coverage Definition properties. If you specfy point locations via a shapefile (.shp), STK always uses the point altitude from the Coverage Definition properties.
Defining grid constraints
To specify a type of object or a specific object for the points within the grid, click . The Grid Constraint Options dialog box appears.
Custom Regions
If you choose Type as Custom Regions for the Grid Area of Interest for your Coverage Definition, then use the drop-down under Custom Regions to select from the following options:
| Option | Description |
|---|---|
| States/Countries | Select one or more from the list of all states and countries. |
| Area Targets | Select one or more area targets from the list, which the tool takes from all area targets available in the Object Browser. 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. |
| Area of Interest | Choose a starting point object from the provided list, taken from the Object Browser, and specify the Type (Ellipse or Lat/Lon) and the parameters accompanying that type. The provided list includes all targets, places, and facilities. |
| Region File | Click , browse to and open a regions file, and select one or more regions from the file list. Possible file types are region list files (*.rl) and shapefiles (*.shp). This option is not available for Attitude Coverage. |
You can use more than one of these options to add regions. For each individual region you want to include in your Coverage Definition, move (
) it into the Selected Regions list. You can also move (
) items out of the Selected Regions list.
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.
In scenarios that you created using STK 8.0 or earlier in which 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 recompute accesses for this coverage definition.
Using shapefiles and region list files
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.
Select the Check for holes check box to prevent the grid generator from producing grid points within holes in regions defined in region lists or shapefiles.