Datums in DTM

Standard Datum Volumes

The Datums Standard option allows you to calculate volumes over a series of datum values using the current DTM.  When selected, a dialog box like that shown below will be displayed.

The Range group define the range over which the datum volumes are to be calculated.  The Interval field in the Datums group defines the interval used to calculate volumes.  Using this value, the maximum and minimum datums within the DTM triangles can be calculated.  There four range options are.

All - Volumes will be calculated across the full height range of the DTM triangles using the maximum and minimum heights of the DTM.

Between - Volumes will be calculated between a maximum and a minimum datum.  These are defined in the Maximum and Minimum fields of the Datums group.

Above - Volumes will be calculated from the maximum height of the DTM down to a datum value specified in the Minimum field.  An example of this would be the calculation of slices through a spoil heap.

Below - Volumes will be calculated from the minimum height of the DTM up to a datum value specified in the Maximum field.  An example of this would be the calculation of void slices in an excavation.

The table below shows a graphic representation of each of the four datum range options.

All	Between	Above Minimum	Below Maximum

When entering the maximum and minimum values into the appropriate fields in the dialog, it is up to you to ensure that they are a multiple of the datum interval.

The datum volumes are then calculated and displayed in a report dialog.  This displays each datum together with the prism volume results of the DTM when compared to each datum.  It also gives the change in volume between each of the calculated values.

The areas shown in the results dialog are slope areas.  Therefore, if you were calculating the datum volumes for a pond, the values in the Area Below column would each represent a wet area whilst the values in the Area Above column would each represent a dry area.

The Report button allows you to generate a text report window which can be printed, saved or copied to another application.  The report window contains the same information as the Datum Volume Results dialog.

The Plot Table check button allows you to plot a table of the results in the dedicated CAD backcloth.  If you plot a table, the datum volumes table dialog will be displayed.  There are only two check buttons in this dialog and they control the display of the volume deltas and the areas.  An example is shown below.

Flood Datum Volumes

There is a check button in the Datum Volumes dialog called Flood Volumes which allows you to specify that the results will be presented in a slightly different way.  As the name suggests, this display is more relevant to calculations for flooding volumes but the calculations that are carried out are the same.  When the results have been calculated, the dialog shown below is used to display the results.

The Flood Vol column displays the volume of water when the model is flooded up to the appropriate datum.  The Flood Area columns display the surface are of the water at that datum.  The Air Vol column shows the volume of air from the datum up to the maximum datum specified for the calculations.

You should note that the option to swap cut and fill results is irrelevant for flood volumes as only the volume to fill the model to a given datum is considered.

Projection Datum Volumes

The Datums Projection option allows you to calculate the volumes of cut and fill between two surfaces and then to segment those volumes into datum slices.  You will be asked to select the DTM to which the current volume is to be compared using the normal DTM selection dialog box.  You will then be asked to constrain the calculations using the same dialog as that for standard datum volumes.  The only difference is that there is not a check button allowing you to plot a table.

The table below gives a graphical representation of each of the datum range options.

All	Between	Above Minimum	Below Maximum

Using the examples in the table above, if we assume that the dome model is the current model and the dish model is the projected surface, the areas of cut are hatched from top left to bottom right.  The areas of fill are hatched from bottom left to top right.  Once again, when entering the maximum and minimum values into the appropriate fields in the dialog, it is up to you to ensure that they are a multiple of the datum interval.

After you have defined your datum range, a projection between the models will be carried out and the analysis will take place to produce the required results.  They will then be presented in a dialog like that shown below.

The Cut Above column shows the volume of a cut above the datum value in the Datum column, whilst the Cut Below column shows the volume of cut below that datum.  The Fill Above and Fill Below columns show the same information for fill volumes.  For any given datum, the above and below values will always add up to the same total for either cut or fill.

The dCut column shows the change in the cut above and cut below between two consecutive datum values.  The dFill column shows the same information for fill.  These values are the volumes of cut and fill in a horizontal slice through the two DTMs as shown below.  Again, if the dome surface is the current DTM and the dish surface is the projected DTM, the change in cut volume is represented by the hatch from top left to bottom right in the centre.  The change in fill is represented by adding both the areas of hatch from bottom left to top right at either end.

At the top of the results, dialog is the Group drop list.  When dropped down, this contains a list of all the DTM groups in the current model. 

Selecting one of the items in the list will change the display of the results so that only the cut and fill volumes for the selected group are displayed.  As well as the names of these group in the current DTM, there are two additional items.

All Triangles - this item changes the results display to show the results for all the triangles in the DTM, whether they have been assigned to a group or not.  This is the default when the results dialog is displayed.

Unassigned - this item changes the results display to show the results for those triangles which have not been assigned to a group.

The Report button allows you to generate a text report window which can be printed, saved or copied to another application.  The report window contains the same information as the Datum Projection Volume Results dialog.  If you have selected an item from the above drop list, the report will only contain the results for that item.  If you wish to report the results for all the groups, you should create an individual report for each.