Zero, Zero in a Caster Table

There are several times when the latitude – longitude of a Caster table entry may be the value 0,0 [zero, zero].  This article discuses when this may occur and how the graphical map in SNIP deals with it when there is more than one such entry.

The caster table serves as a guide for help the NTRIP Clients select which data stream they need.  The locations given are somewhat crude (within a few miles), any for many for virtual reference stations, may not be be given at all.   [The actual location given is not used in the navigation process]  This is by far the most common reason to see a map with one or more location displayed at the prime meridian and the equator.

Other common reasons for zero-zero points include:

  • Never having set the actual latitude – longitude in a caster table entry. This may occur when the operator has not fully or correctly provided the caster details.  In SNIP these details are set with the xxx dialog. [SNIP will also detect and automatically correct this for other data streams if the stream is a parsed RTCM 3 type]
  • Relaying a data stream before the caster stream is available. [A transitory event during the first tens of seconds when a data stream is initially starting and before its remote caster data is recovered and used by SNIP]
  • A data stream where presenting a physical location makes no actual sense. [Such as a stream with world-wide orbital corrections]

Because all of these can occur, there can be times when more then one reference station is shown at location zero-zero.  When this occurs, the visual effect is a set of overlapping dots in the Gulf of Guinea south of the Ghana (where the prime median and the equator meet).  This makes it very hard to examine the individual mountPt by further clicking on them.  On most maps found on common NTRIP Client vendors (such as RTKLIB or BNC) and also in SNIP, the display presents some overlapping text.

zerozero

In SNIP some minor additional logic is also implemented when multiple reference stations are found to be at the zero-zero point.   When this occurs, SNIP simply adjusts their displayed position on the map by a few km so that (when zoomed in) they no longer overlap.  The display logic simply adjusted the latitude and longitude values by small amount to offset each point.  By doing this the user can right-click on the mountPt of interest and examine its details further.    Here is an example of this effect with nine data streams that would otherwise overlap.

zerozerocloseup2

See also: this article on the differences between the published caster location and the precise base station location and how these can resulting in different map display details.

See also: this article on the basic map display functions in SNIP.

 

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