The Regional Center Point Group
In this article we discuss the various regional centering controls used in the Nearest MountPt Dialog.
The regional center point is used to constrain the location of the Base Stations which will form the NEAR pool to being within a defined region. This allows operating a SNIP node where only a subset of the Base Stations should be considered part of a given NEAR pool.
The primary use of each of these controls is as follows:
Use a Regional Center Point
The Regional Center Point option is used to define a coverage region for the NEAR pool. When checked (enabled), the regional center point values are used to constrain the pool to a geographical region.
When Enabled: All Base Stations within the defined region (which also meet the stream content settings) will be added to the resulting NEAR pool, while those Base Stations outside of the region will not. NTRIP Clients can still connect to any of SNIP‘s Base Stations, as well as the NEAR stream (where the closest connection within the allowed baseline distance will be selected for them).
When Disabled: The NEAR pool will consist of all Base Stations (which also meet the stream content settings) which are present regardless of their location. Any NTRIP Clients using this NEAR stream will be connected to the closest connection within the allowed baseline distance setting.
The lat-lon data shown in the inset image are simply typical. These values represent a large region covering the area in Los Angeles where SCSC development offices are located.
Latitude
This is the Latitude point (in degrees) around which the region’s pool of Base Stations will be selected. The resulting latitude used in the caster table will reflect an average position of the Base Stations actually used.
Longitude
This is the Longitude point (in degrees) around which the region’s pool of Base Stations will be selected. The resulting longitude used in the caster table will reflect an average position of the Base Stations actually used.
Radius Limit
This is the maximum radial distance (in km) from the region’s defined Lat-Lon center point at which a Base Station (not a rover) can be part of the NEAR pool. Those Base Stations within this range (and who have suitable stream content) are used to make up the NEAR pool. The Lat-Lon value in the caster table reflects the average location of all of these stations.
The Regions Center Point vs the Average Point
The Lat-Lon center point which the user enters (and the radius) determines the very precise mathematical circle that is used to determine if a given Base Station is in-or-out for the region’s pool. This is displayed visually with the map feature [the View Map button] and marked “Center” on the map (click image below to enlarge).
Distances to each Base Station are calculated using a Vincenty method (mm accuracy). When parsing is employed, the precise ECEF value (± 0.01 mm) which is sent by the Base Station is used. When the data stream is not parsed, then the coarse position reported in the Caster table is used. (±0.01 deg).
But the Lat-Lon point used in the Caster table entry is an equal weighted average value of all the Base Stations used in the pool. These two values rarely align, and both are shown on the map when the Base Stations are plotted.
This image shows an example of this. The Lat-Lon center was entered to be at 33.8 / -117.5 with a radius of 80km. In this SNIP system these values have resulted in four nearby Base Stations in the pool. The resulting average location is 34.128 / -117.564. This is shown the the above by the NEAR label and is also shown in the Nearest tab in a status line.
One final note: The Lat-Lon value found in the Caster table entry only used as a guide to find the correct data stream. It is not used in the navigation process and will not impact the rover positional accuracy in any way.
The other controls used in the Nearest MountPt Dialog are described here.