River Sections - Cross Sections

The Rivers Cross option allows you to extract river cross sections from a model.  When selected, a property sheet like that shown below will be displayed.  The normal Models property page is included so that you can superimpose profiles from other DTMs onto the river cross sections.

When surveying river sections, you are expected to use codes in a specific way so that n4ce can understand the data.  The most important thing to note is that all points associated with a section must share the same feature string number.  Four standard items are expected with each surveyed section.

Base Line

The baseline code prefix defines the prefix used for the river cross-section baseline.  This is a two or three-point line feature and the default code prefix is BL.  If a baseline feature with only one point is found, it is ignored.  If you have defined a baseline with three points, the second point is assumed to have a cross-section chainage of zero.  Thus, all points to the left will have negative chainages and those to the right positive.

Left Bank

The left bank code prefix, default LB, defines that used to identify the left bank point on the river cross-section.  A single point is expected but if it does not exist, n4ce will assume that the first baseline point is the left bank.

Right Bank

The right bank code prefix, default RB, defines that used to identify the right bank point on the river cross-section.  A single point is expected but if it does not exist, n4ce will assume that the last baseline point is the right bank.

Water Level

The water level code prefix, default WL, is used to identify the water level of the river.  A single point is expected and if it does not exist, n4ce will assume a no water level for the section.

If there is more than one point for the left bank, the right bank or the water level, the last point that is found will be used.

Strings List

The Strings list allows you to add other features to river cross sections.  Typically, when surveying rivers, you will also be expected to survey hard and soft bed profiles.  You will see that there are two elements to the text strings that are displayed in the list separated by a three character string “ – “.  The first element, HB in the first string, is the code prefix that will be searched for.  The second element, Hard Bed in the first string, is the name that will be given to the profile for this feature in the river cross-section.  You can add, remove or change the order of the strings in the list using the four buttons below the list.  When surveying river cross sections, the hard bed profile is the profile that is normally extended to represent the ground profiles on either side of the river.

Chainage Names

The Chainage Names checkbox allows you to make the naming of the cross sections a little more meaningful.  If this is not checked, the name of each section will be the code of the baseline feature from which it is generated.  If this is checked, n4ce will look for a chainage dimension, CHN=, on the first point of each baseline.  If this is found, the name of the section will use this dimension, for example, ‘Chainage 150’.  Regardless of whether the check box is ticked or not, if the chainage dimension exists, it is stored in the cross-section data.

The example below shows a sequence of river cross sections from BL28 to BL31.  For the section generated from BL28, n4ce will expect to find a left bank point coded LB28, a right bank point coded RB28, a water level point coded WL28 and hard bed and soft bed features coded HB28 and SB28 respectively.  Similar strings should also exist for BL29, BL30 and BL31.

After you have defined your extraction prefixes, they will be checked against the current codes to ensure that each prefix exists in the code table.  All the strings that are to be extracted need to be set up as line features.  The colours and line styles that are assigned to the line features in the code table are the default colours and line styles that are assigned to the profiles in the section view and when plotting the sections.  If one or more have not been defined, a message is displayed and the river section process will be terminated.

If all prefixes have been defined, n4ce will search the current model for section baselines.  A further dialog will display those that have been found and they will appear in the Sections list.  The order of the list is the order in which they were found in the model.  To change this order, you can move individual sections up and down the list using the appropriate buttons.

The Sort button allows you to alphabetically sort the list of baselines.  Care should be taken here as in the example shown, if the baselines are sorted, the order will be changed to BL1, BL10, BL11, etc.  Alphabetically, BL10 would come before BL2.   It is better to code the baselines with the same number of characters in the string number, i.e. use BL01 instead of BL1, BL02 instead of BL2 and so on.  The Delete button allows you to remove unwanted baselines.

Once you have accepted the baselines, the river section profiles will be extracted.  Each point associated with a given baseline will be projected onto that baseline and ordered by distance from the left end.  This means that you can survey a profile starting at both ends.  The sections are then previewed in the normal way.  The water level of the river, if defined, is displayed in the preview using the colour set up in the code defaults.  The example below shows a hard be profile in red, a short soft bed profile in green and a water level in cyan.

Since river sections is a multi-section, there will be a drop list in the toolbar containing the names of all the sections.  Choosing one of these allows you to go straight to a view of that section.

When quitting from the preview window, you will be given the option to save the section in the normal way.