Line Separation (Batch QC Tab Parameters)

Use the Line Separation tab (QCPATH GX) to perform quality control on flight line separations.

Line Separation tab options

Lines	Select the flight lines as, Survey lines or Tie lines. Script Parameter: QCPATH.LINESEL
Nominal line separation	Specify the nominal line separation. This is a required field. Script Parameter: QCPATH.NOMSEP
Maximum separation	Specify the maximum line separation. This is a required field. Script Parameter: QCPATH.MAXSEP
Allowed separation	Specify the allowed separation between lines (for a given distance – see below). This is a required field. Script Parameter: QCPATH.ALLSEP
Over a distance of	Specify the over a distance of parameter such that, if the Allowed separation value (see above) is exceeded for this specified distance then the line will be selected as out of survey specification. This is a required field. Script Parameter: QCPATH.DIST
Recalculate distance	Select to recalculate distances. Script Parameter: QCPATH.RECALC

Application Notes

Current Database and Selection of Lines

This GX assumes that the current database is the one containing the actual survey data. The database is called the survey database. If you wish to process a different database, you may browse to add it to the project or select one from the workspace list.

It is advised that you carry out quality control on the survey lines (first letter of the survey line begins with "L") and tie lines (first letter of the line begins with "T") separately. By default, survey lines are selected (checked). Select the Tie lines option in the Lines parameter if you want to check the separation between tie lines.

The Separation Calculation

Before the quality control test for compliance of line separation can be run, it is necessary to determine the spacing between lines for each point on the lines. This dialog performs this function, and saves the results to two channels: Closest_Left and Closest_Right. If the channels do not exist, the calculation is automatic, independent of the value entered for "Recalculate Distances".

A nominal or average line spacing is required, as a grid is created internally to store flight path information. First, the lines are scanned to determine an average direction for the survey This calculation takes into account the fact that lines may be flown in opposite directions). Separations are calculated perpendicular to this direction. The lines are scanned to determine their relative positions and paths through the survey area. Then, for each position, the perpendicular distance to the closest adjacent line is determined, for both sides of the line. (Because the lines are not necessarily parallel, these distances are not necessarily the shortest distance to the other line). Two channels are created, Closest_Left and Closest_Right, which give, respectively, the distance to the adjacent lines lying to the left and right of the current line, as seen proceeding in the average line direction. For instance, if the average line direction is 0° azimuth then "Right" is at 90° and "Left" is at -90°. Where no line is found in these directions, the distance is left as a dummy value. This usually occurs near the ends of lines, and on the "outside" of the lines at the edges of the survey. Once the initialization is completed, and the distance channels have been created, the compliance stage may be run any number of times, with different values, without re-calculating the distance channels.

The line path grid created internally to facilitate the distance calculations uses a cell size of one-half of the Nominal Line Separation, and can store information for up to two different lines at a time. This is adequate to handle crossing lines, and surveys with a reasonable amount of deviation in the line paths. If there are sections of the survey where more than two lines cross a cell of this size, the algorithm may break down, and miss the third (or other) line when locating adjacent lines. To remedy this, decrease the Nominal Line Separation to ensure that no more than two lines will cross any square measuring half this distance on an edge.

Quality Control Specifications and Indicators

This dialog enables two separate specifications to be applied to check whether the separations between actual flight lines are acceptable or not.

1)	At each readings location, the distance to the nearest position on the nearest line should be less than the Maximum separation.
2)	The distance of a flight line to the nearest line may be greater than the specified separation for a distance less than the specified distance.

You can apply one or both of these specification tests. To disable a test leave the entry blank.

The Compliance Test

Once the distance channels have been created, they may be used to test line separation compliance. The dialog creates two flag channels, called Flag_SepL and Flag_SepR, which correspond to the two distance channels. These channels contain numbers from 0 to 3 inclusive to indicate the result of the quality control test on flight line separation. Note that a line may be in compliance with respect to a line on one side, but not in respect the nearest line on the other side.

 Indicators and Meaning

Calculation of Distances

When the specification of Allowed Separation over a specified distance is input, the cumulative distance along the actual survey locations is used instead of the distance along the ideal line path.