Lines - Parallel Line Features

It is possible to use dimensions that create lines that are parallel to surveyed lines.  For instance, you could survey one face of a wall and create a parallel line that represents the other.  The parallel offset dimension, normally P=, is used to do this and an initial value must be placed on either the first or last point of the feature.  Note that if the attribute check button P,O&W on First Only is ticked, n4ce will not look for the initial offset on the last point of the feature.  The initial parallel dimension will be used until another parallel dimension has been assigned to a point.  Once this happens, this new offset will be used for the next and subsequent line feature segments until the end of the line feature or until another offset is found.

In its simplest form, there will be two parallel lines drawn using the same line style and pen.  The perpendicular distance between them will be the dimension value.  The first of these lines will be drawn on the surveyed line and the other to the left or right.  If the dimension value is negative, the second line will be to the left of the first.

You can further enhance the parallel line by defining the dimension as series of up to three comma separated strings, for example P=offset,prefix,dz.  The first string is the value of the parallel offset.  The second string is the code prefix that is to be used to display the parallel line.  The third string is the height offset from the surveyed feature to the parallel feature.  This can be used to create a combination of channel and kerb lines and you should note that this will only work for features where the height offset is assumed to be constant.

You can only define the parallel line prefix using the initial parallel dimension.  If it is defined in subsequent dimensions, it will be ignored and only the parallel offset used.  However, you can also use a dimension, fixed as FPL=, to define the code prefix that is to be used.  This can be defined as a default code prefix field, so it does not have to be entered when surveying.  This could be used to simulate the survey of one hedge face and using a parallel feature to create the other.

If the attribute check button Close Ends on P&W is ticked, the ends of the two lines will be joined by additional segments to create a closed polygon.  If the parallel line is using a different code prefix to the surveyed line, the closing segments will use the line style and pen of the parallel line.  Note that if you are creating a parallel line to a closed feature, the setting of this check button is ignored.

Parallel	Parallel with Code	Parallel with Code (Closed Ends)

The first of the examples above shows a simple parallel where only an offset value is provided in the parallel dimension.  The second shows a parallel with curved segments where an offset and a code prefix are provided in the parallel dimension.  Using these two examples, the offsets from the first point to its calculated parallel show how the parallel calculations are affected by curve fitting.  The third is the same as the second except the code prefix settings have been modified such that option to close the ends of the surveyed and parallel lines is enabled.

The actual calculations of the parallel line can depend on which comma codes and dimensions have been assigned to a point.  The normal parallel method for straight line segments is to create parallel segments for the two segments before and after each point.  Where these segments would intersect, a point is created.  If there is curve fitting through a point, a tangent to the curve is calculated and a perpendicular offset calculated.  If any point on the base string has a Bearing dimension, BRG, a perpendicular offset is calculated using this dimension.

The examples above show how a parallel line is created where the parallel offset is constant.  However, this parallel offset can be varied along the length of the feature.  How this variable offset is used is defined by the POW Variable check button.  If this check button is not checked, n4ce will use the parallel offsets to ensure that each segment on the parallel feature is mathematically parallel to its parent segment on the surveyed feature.  If this check button is checked, n4ce will vary distance along each parallel segment using the offsets at either end of the feature segment.

Non-Variable Parallel	Variable Parallel	Curve-fitted Variable Parallel

The way in which the parallel offset should be measured is different for each case.  The first example on the left shows a parallel feature where the POW Variable button has not been checked.  The parallel offsets should be measured perpendicular to the line segments and curve-fitting should not be applied to such features as it could produce unpredictable results.  The second example shows a parallel feature where the POW Variable button has been checked.  In this case, the parallel offsets should be measured across the corner rather than perpendicular to the segment direction.

The third example shows how a variable width hedge can be generated using two different line styles together with the FPL= dimension to define a second code prefix.  It also shows that the variable parallel hedges can be curve-fitted to good effect.  If there is a need to display a centre-line between the two parallel lines, you can specify a default field called FCL containing a code prefix to use.  If this is defined, a centre-line will be generated using the line colour and style for that code prefix and is shown in the example above.