River Long Section Menu
The River Long option allows you to extract a long section from a series of river cross sections. You will be asked to select the river cross sections from which the long section is to be extracted. The dialog is the normal section selection dialog with only the river cross sections listed.
After you have selected the cross sections, you will be asked to define the profiles which are to be generated in the long section using the dialog shown below.
The Available list shows those which are in the river cross sections and have not been selected. The Selected list shows those which you have already selected. Use the arrow buttons to move the profiles from left to right and vice versa. The single arrows allow you to move the selected profile and the double arrows allow you to move all of the profiles.
The Update Chainages checkbox allows you to store the chainage that is calculated along the long section for each cross section. This is of use if you did not specify the chainage of each cross section on its baseline.
The Centre Line drop list allows you to specify how the centre line of the river is to be extracted from the cross sections. This is also used to calculate the distance along the river.
Mid LB & RB - The centre point will be calculated as the midpoint of the left and right bank points. If the section baseline for a section has three points, the centre point will be taken.
Lowest Point - The centre point is calculated by finding the lowest point on the profile. This will normally be found in the hard bed.
CHN= - When the river cross sections were generated, it was possible to extract the chainage dimension from the baseline points. This option allows you to use this value as the chainage along the long section.
Once the long section parameters have been defined, it will be extracted from the cross sections. It will then be previewed in the normal preview window allowing you to save it or not.
Increasing the Frequency of River Long Section Bank Points
Sometimes, there is a requirement that the frequency of left and right banks points on a river long section is greater than the frequency of the required cross sections. If this is the case, a simple linear interpolation between the section points will not be good enough. If this is a requirement of the river long section, then we must use some of the other tools available in n4ce to create the desired result and some work will need to be carried out in the office before going out on site.
The first step is to acquire the centre line of the required section of the river. This can be digitised from the OS plans or could have come from a previous survey. Ordinary cross sections along the centre line of the river can then be taken at the appropriate spacing. The co-ordinates of the cut lines of these cross sections should be saved into a co-ordinate block using the option to save cut line co-ordinates described later. The codes for each of these cut lines will use BL01, BL02, etc.
The next step is to determine which of the cross sections are to be surveyed in the field. For instance, the first section may be on the co-ordinates for BL01 from above whilst the second may be at BL07. The required survey of the selected sections should then be carried out ignoring left and right bank points. At the same time, a linear survey along the left and right banks should be carried out at about the required frequency of points. The code assigned should be the normal code prefixes used for left and right bank points.
The next step is to create a model of the surveyed sections, all the baseline points including those that have not been surveyed and the left and right bank strings. There should only be one left and one right bank string with no string numbers.
Finally, the river cross sections should be extracted from the model, ignoring the fact that some of the baselines will not have cross section profiles. You should then select the option River Banks to create the left and right bank points needed for the long section. You will be asked to select the river cross section file and then define a co-ordinate block to store a copy of the new left and right bank strings. The left and right bank points are automatically inserted into the river cross sections so they can be extracted for the river long section.