When creating a long or cross section there are a variety of options available to determine how the section is created. Sections can either be created from a digital terrain model, or from objects in the database that have elevations.
When creating a section from a model it is possible to use any model in the current database, or alternatively, another database can be opened and a model selected from within this database. Sections created from a model will, by default, attempt to define a continuous surface along the nominated region of the control line. Holes in the surface of the model will result in holes in the surface of the section. An elevation will be computed at every intersection of the control line against each triangle side. This will ensure that the shape of the surface faithfully reflects the shape defined by the model. At the same time, it is possible to nominate objects within the database that you require the elevation to be computed at. These could be any objects that cross the control line. For example an underground pipe may cross the control line and the invert elevations of this pipe may also be known. If you were to request that elevations were to be computed at crossing lines, then the elevation at the point of intersection with the pipe line and the control line would be computed on the surface of the model. This intersection point will be flagged in the section as being a crossing line. Note that the elevations of the pipe itself are not used, as this would alter the shape of the section and cause it to dip down onto the pipe. What is recorded is the elevation directly above the pipe.
Sometimes a digital terrain model may not be available, or it is not convenient to create one from the information available. In this case it is still possible to create sections if there are sufficient objects in the database with elevations. The most typical example of this would be to use the key points in a control line that has elevations. Another typical case would be where points have been picked up close to a control line. In this case it is possible to nominate a distance to search either side of the control line, or an offset to the control line, for points with elevations that can be used to form a section. This technique can also be used with objects that do not have elevations, provided that sufficient objects are found that do have elevations. In this case the surface will be formed from the objects that have elevations and then the locations of the remaining objects will be determined and an elevation computed by grading between known elevations in the surface. These locations will not change the shape of the surface, but an elevation will be able to be reported for these points.
When forming profiles from objects with elevations, it is possible to nominate that the points are not to be joined together to form a surface. This would be the case for crossing pipes or overhead cabling. As these objects would be independent of each other they would be treated as point objects in the section.
In the creation of cross sections, it is also possible to generate a closed section, when creating the section using objects with elevations. This is typically used to generate tunnel profiles where string lines have been generated along the length of the tunnel, or wherever sufficient points have been picked up around the tunnel to be able to determine a profile.
Cross sections are always generated at floating section markers, perpendicular to the primary alignment, and for fixed section markers, along the bisector at the key point. Note that the orientation of the section marker symbol is for display purposes only and has no influence on the direction of the cross section generated.