Observation Conventions and Types for Traverse and Network

Determination of Traverse Type

According to the point-numbering convention as established in the table above, traverse types are determined in this way:

Bearing traverse

Two formats are recognised:

1. Strict bearing format
   
   

   Format for every record:

   Instrument Station (At) = Instrument station

   Reference Station (RO) = leave blank or enter 0

   Observed Point (To) = Observed forward point

   HA = azimuth/bearing

   Other column values as required.

2. Angle traverse format but works like bearings

   Format for first record:

   Instrument Station (At) = Instrument station

   Reference Station (RO) = Same as instrument station (At)

   Observed Point (To) = Observed forward point

   HA = Horizontal angle turned from RO - will effectively be a bearing because the reference bearing between the instrument station and the RO (actually the same point) will be zero.

   Other column values as required

   Format for second and following records:

   Current instrument Station (At) = New instrument station = To-point observed in previous record

   Reference Station (RO) = Previous instrument station (At)

   Observed Point (To) = New observed forward point

   HA = Horizontal angle turned from previous station or RO - will effectively be a bearing

   Other column values as required.

Angle traverse

One basic format is recognised:

Format for every record:

Instrument Station (At)

Reference Station (RO)

Observed Point (To)

HA = Horizontal angle turned from RO or previous station

Other column values as required.

However, there are two variations to an angle traverse in which either

1. the reference bearing is calculated from the coordinates of the instrument station and the reference object which both exist as points, or
2. only the point for the instrument station exists and a value is specified for the actual reference bearing, i.e. the point for the RO does not have to exist. This situation may occur when the coordinates of the reference object are unknown and only the azimuth/bearing to it is known.

Angle traverse with starting reference bearing In case 2 for the angle traverse above, it is possible to compute a fictitious location for the RO so that a valid reference bearing can be computed from coordinates. As the first record for the traverse, an observation in the form of the second bearing observation format (see above) may be inserted such as:

Instrument Station (At) = Instrument station, traverse start point

Reference Station (RO) = Same as instrument station (At)

Observed Point (To) = Point number for the RO

HA = Reference azimuth/bearing to the RO

Dist = Enter the distance to the RO if known (a reasonable estimate may be sufficient)

This observation will be treated as a horizontal angle observation as normal, but because the reference bearing between the instrument station and the RO (actually the same point) will be zero, the forward bearing (entered as a horizontal angle) will be the correct reference bearing required and the distance will then allow coordinate values to be computed for the RO - the coordinates will not be correct (unless the distance was correct) but any bearing computed between the first instrument station and the RO along the reference line will be correct. The record following should then be for the first horizontal angle observed as turned between the RO and the first traverse point, but because the RO point number for this observation is now different from the observed To-point in the previous record, a new traverse start will be detected but a correct reference bearing can now be computed for it.

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Azimuth to unfixed RO in least squares adjustment

Commencing a traverse by specifying a reference bearing or by computing false coordinates for the RO as described above, may lead to a difficulty if adjusting the data with least squares. All points to which observations are made in a network adjustment must already exist, either as fixed control points, or as unfixed points which will be subject to adjustment. If an azimuth observation is included (such as for the starting reference bearing for a traverse as described above) to a reference point whose coordinates are unknown but for which fictitious values have been either entered or computed using only an estimated distance, and there is no other observation (eg a distance along the same line or an observation from another point) made to the same point (i.e. it is uncontrolled), then there is no way for Geoida to verify that the point's existing coordinates are within the point tolerance. As a consequence, the least squares adjustment to the network may actually diverge and not reach a solution. This could happen if the original coordinates for the point in question were changed by a swing being applied to the azimuth due to the adjustment of the network, such that the azimuth to the point now computed the point outside of tolerance.

Such a situation could be helped by fixing either the point or the azimuth observation, or by ensuring that there was at least one other observation made to the point, for example, by reversing the direction of any angle observation which had previously included the point as the RO and now making the RO the forward observed point and thus subject to a tolerance test.

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Detecting new traverse starts and side-shots

Bear in mind that Geoida can also handle side-shot observations when included amongst traverse observations.

A side-shot will be determined where:

• the next instrument station is the SAME as the current instrument station
  AND (in the case of an angle traverse)
• the next RO is also the SAME as the current RO (i.e., the instrument setup is unchanged).

A new traverse start is determined by the following circumstances (no adjustment applied):

1. Bearing traverse:
   • the new instrument station is NOT the same as the previously observed foresight point
     AND
   • the new instrument station is NOT the same as the previous instrument station (i.e. the previous observation was not a side-shot from the same traverse)
   
   
2. Angle traverse:
   • The new instrument station is NOT the same as the previously observed foresight point

     and either:
     

     (i)

     • the new instrument station is NOT the same as the previous instrument station
       OR
       
     • the new RO is NOT the same as the previous RO

     (i.e. the previous observation was not a side-shot from the same traverse)

     or
     

     (ii)

     • the new instrument station IS the same as the previously observed foresight point
       BUT
       
     • the new RO is NOT the same as the previous instrument station

     
     

Bowditch adjustment - In addition to the above conditions, when the traverse data is to be adjusted using the Bowditch setting, a traverse section will immediately conclude when an observation is made to a fixed point or to a point already computed on a previous section. See also next heading.

Excessive traverse sections detected - If a set of traverse observation data is broken into what seems to be too many smaller traverse sections, it may be a result of sets of multiple observations (eg directions) from the same setup including side-shot observations to points that are either fixed or already computed on one or more previous traverse sections. This situation would be expected in more complex networks.

To verify if this is the situation, check the number of traverse sections detected when computed in None adjustment mode - if the number of detected traverse sections is now less, Geoida may be wanting to adjust onto points already computed on earlier section/s and also observed in subsequent sections. This is normal and to be expected BUT may be worsened if the observations are not arranged correctly at each setup - the correct arrangement of multiple observations at one setup is un-fixed side-shots first, followed by the foresight to the next traverse station, and fixed points or previously-observed points last. In other words, move any observations to fixed or previously-observed points to the END of sets of angles made from the same stations.

Note that if the observations are directions and Geoida breaks up the data into a greater number of adjustable sections, observations made at the same setups may become separated from the direction observations to their associated reference points. This will not be a problem UNLESS a set's reference observation is NOT exactly zero, in which case the Extract file should be edited and a copy of the reference observation at each setup be inserted prior to any observations following an observation to a fixed point or a point on an earlier traverse section where the traverse section has been closed.

If the survey is a network then it will be far better to adjust by Least Squares anyway - using traverse options will normally be just to check closures etc and derive provisional coords prior to a Least Squares Network adjustment.

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