See also, Ground Modelling
This tutorial describes how to use Key TERRA-FIRMA to calculate volumes from ground models (.KGM files) and how to calculate cut and fill volumes between ground models. It assumes that you have completed the "Ground Modelling" tutorial or that you already have a good understanding of TERRA-FIRMA's ground modelling features. The tutorial goes on to explore the various ways to graphically present cut and fill data using TERRA-FIRMA's contouring and elevation analysis tools.
Those of you who need only a quick step-by-step guide or a refresher can go straight to the FastTrack section of the tutorial.
"Cut and fill" or "Volumetrics" is the calculation or analysis of landform volumes. It may be desirable to change the existing landform of a site for a number of reasons. For example, a sloping site will require platforms for buildings and car parks, visual screening bunds may be required at the site boundary or a lake may be created. Landform modification is a very common site design requirement and one in which landscape architects are often involved.
The movement of large quantities of material to and from a site can be extremely costly. In order to minimise this cost it is often desirable to design a landform which uses only material found on a particular site. It is important, therefore, to know what volume of material you are starting with and what volume of material the designed landform will require. Ideally the two volumes will be the same. In such a case, no imported material is required, neither does any material need to be exported, thus minimising cost. This ideal situation occurs when the landform designer has balanced the cut and fill.
A "Cut" volume is defined as the volume of material which is excavated below existing site levels. "Fill" volume is the volume of material which is mounded above existing site levels. Cut and fill volumes are expressed in metres cubed (m³).
If you have no contour data to work with you may like to download a sample file so that you can follow the live tutorial. Click on the icon below to download the AutoCAD drawing file gm-base.dwg. Even if you have your own contours you may like to use this file for the purposes of the tutorial before you start work with your own data. There are two download options, you can either download the native AutoCAD drawing file or you can download the smaller compressed version. The Zip file can be uncompressed with a utility such as WinZip.
The drawing file is approximately 2km x 2km square and contains two sets of contours on different layers (see illustrations below). The contours are at 5m intervals in the range 175m to 395m with index contours drawn at 25m intervals. The drawing contains three sets of contour layers:
In addition to the contour information, the drawing includes a number of spot heights that are used to define the flat bottom of the quarry. The spot heights are block objects called "LEVEL".
|Existing and common contours, showing
the landform before modification.
|Design and common contours and spot
heights, showing the proposed landform.
The two contour sets can be viewed as in the illustrations above by selectively turning off the "quarry" layers and the "exist" layers respectively. See the "Object Properties" tutorial to find out about turning layers off and on.
The first stage in doing a cut and fill analysis involves creating the ground models which will be used. Follow the steps below to create two ground models from the sample data or create two of your own ground models. In order to follow the tutorial, you will need to download the GM-BASE AutoCAD file and create two ground models.
You now have all the base material required to complete the cut and fill calculations.
Key TERRA-FIRMA provides a number of methods for calculating volumes. This tutorial covers the two most useful options:
Both of these commands use a prismoidal method for calculating volume. This method is generally accepted to be the most useful and accurate for general purpose volumetrics. You may have noticed that TERRA-FIRMA also offers the option to calculate volumes by a sectional method. This type of volume calculation is most appropriate when the calculated volume is linear in planform. It is usually used for volumetrics calculations relating to road and rail schemes.
The Ground Model information command can be used to display all relevant information about a ground model file (.KGM). A ground model must be created before this command is used. The Ground Model Info dialogue box is shown below. As you can see, this command can be used to calculate the volume below any ground model.
The dialogue box is divided into three sections. The "Ground Model File Specifications" section gives information such as the full path name of the file, the date and time that the file was created and the file size. This section also contains an edit box which allows you to specify the base level for any volume calculation. By default, this value is set to the minimum Z value in the ground model. This ensures that the entire ground model will be used when generating areas and volumes. The sections below show what happens to the calculated volumes when this value is changed. Notice that it is not possible to generate a volume with a base level above the minimum Z value.
The Ground Model Info dialogue box also contains sections entitled "Active Model" and "Whole Model". The Active Model section shows information which relates to that part of the model which has been defined as "Active" using the Active/Passive Zones command. If no active/passive zones have been defined, the whole model is active and the contents of this section will be identical to that in the Whole Model section.
To calculate the volume below any ground model, simply start the Model Information command, select the ground model file, set the base level and click on the "Calculate Areas And Volumes" button. In a few seconds the results will be displayed in the Ground Model Info dialogue box.
In theory you could, in fact, do a cut and fill calculation based on the above method. Simply calculate the volume below any two ground models of the same extent (or with the same active/passive zones defined) and with the same base levels. Subtract the volume of the existing ground model from that of the proposed ground model. A negative result will give you the volume of cut, a positive value will give you the volume of fill. If both cut and fill are perfectly balanced, the result would be zero.
Although this method will give you a satisfactory cut/fill value, it is impossible to tell how the total is arrived at. For example, a gross figure of 10,000m³ fill might be arrived at by cutting 10,000m³ and then filling 20,000m³ or it may simply be arrived at by filling 10,000m³. Obviously the cost implications of the two possibilities are quite different. The Prismoidal Volumes command should be used where you need more information about a cut/fill calculation.
The Prismoidal Volumes command is specifically designed for cut and fill calculations. The command works by comparing one ground model with another. In most cases this will be one ground model representing the existing landform and another representing the proposed landform. The latter is usually referred to as the "Design Ground Model".
The first thing you see when starting the Prismoidal Volumes command is the Prismoidal Volumes dialogue box, shown on the right. As you can see, the dialogue box offers a couple of options, namely, to create a difference model and to draw an intersection line. Both of these options are covered later in this tutorial.
Follow the next section of the tutorial to generate cut and fill statistics from the two ground models you created earlier.
If you look closely at the command line illustration above you will notice that although the quarry ground model was created entirely by cutting the existing landform, there is small amount of fill recorded in the results. This is largely caused by small differences between the two ground models at the boundary where the two models diverge. If you look at the actual figures involved, the figure given for fill represents about 0.016% of the balance. This is well within the acceptable tolerance for such a calculation.
The Make Difference Model option of the Prismoidal Volumes command enables you to create a ground model which represents the difference between the existing and design models. This is done by subtracting one model from the other. The Z values of the resulting ground model represent the amount of cut or fill. Negative Z values represent cut and positive values represent fill. Where the two ground models are the same the Z values will be zero. When you have created a difference model, you can contour it to produce contours of cut depth and fill height known as isopachytes.Contours can be generated using the Contours command. The difference model could also be used to create colour coded areas on a plan to represent areas of cut and fill. This could be done using the Elevation Analysis command.
To create a difference model during the cut and fill calculation process, simply check the box against the "Make Difference Model" option in the Prismoidal Volumes dialogue box before clicking on the "Start" button. After the report file has been created (assuming you want one), TERRA-FIRMA displays the Create Differential Model dialogue box, illustrated below.
Simply enter a filename such as "difference" in the "File name" edit box and click the "Save" button. Note that you do not need to add a .KGM file extension as this will automatically be done for you.
The Prismoidal Volumes command also enables you to draw an intersection line which marks the intersection of the two ground models (assuming they do intersect). Effectively this line marks the boundary between areas of cut and areas of fill. This line also represents the zero contour (isopachyte) on a difference model.
To draw an intersection line during the cut and fill calculation process, simply check the box against the "Draw intersection line" option in the Prismoidal Volumes dialogue box before clicking the "Start" button. You will notice that when this option is checked, the "Layer" and "Colour" options become available (previously they were greyed out), see the illustration on the right. Use these options to define a layer name and layer colour for the intersection line. A new layer will automatically be created with the specified name and colour settings. The intersection line will be drawn during the calculation process.
The Contour command can be used to generate contours from any TERRA-FIRMA ground model. The Draw Contours dialogue box is shown below. As you can see, there are a number of options which have to be set in order to generate the particular contours which may be needed in any particular circumstance.
The dialogue box is divided into a number of sections. Use the "Select…" button in the "KT-F Ground Model Information" section to select the ground model from which contours are to be generated. In many cases the correct ground model will already have been selected automatically.
Use the "Main Contours" section to specify the layer and colour of the contours. Also, most importantly, set the value for the contour interval. By default, the contour interval is set to 1, however this may not be appropriate in every case.
The "Highlight Contours" section of the dialogue box is used to specify the layer, colour and interval for the highlight or index contours. Index contours are commonly used on maps to make the contours easier to read.
Finally, the Max Contour and Min Contour values must be entered. By default, these values are set automatically from the range of Z values in the ground model and rounded to the nearest metre. However, if you want a contour interval of 5 metres, you will need to make sure that the Min Contour value is rounded to the nearest 5. For example, if the default Min Contour value is -63 and you want to generate 5m contours, you will need to change this value to -60.
The illustration above shows the result of contouring the difference model created earlier in this tutorial. A 5m contour interval has been used with index contours at 25m intervals.
Follow the steps below to generate contours for the difference model.
In this particular example the contours represent the depth of cut. Since there are no contours greater than zero, no fill is indicated.
If you are not necessarily interested in showing the depth of cut or fill but simply wish to indicate whether an area is in cut or fill, you could use the Elevation Analysis command to create solid colour bands to show this. See the section below for details.
The Elevation Analysis command can be used to calculate the percentage area coverage of different elevation bands within any TERRA-FIRMA ground model. It can also be used to create a three dimensional triangular model of the terrain surface with triangles colour coded by layer for each elevation band specified. These triangles can either be 3D Faces or they can be Solids.
The image on the right was produced by rendering an elevation analysis created from 3D Faces. Six elevation bands were used and the band colours are the defaults set by TERRA-FIRMA. As you can see, this produces a clear visual analysis of ground model heights which works well in both 3D views and in plan. If you are working mainly in plan, it may be more efficient to create the triangles using Solids since they do not need to be rendered to give a solid colour effect. The image below shows the same analysis but this time the elevation band colours have been changed to make the distinction between bands more subtle and to more closely mimic real terrain colours. The highest band uses AutoCAD colour number 53, the others are coloured 63, 73, 83, 93 and 103 for the lowest. The change in colour was achieved simply by changing the layer colour of the various elevation bands with the Layers command, from the Object Properties toolbar. The way AutoCAD colours display may vary depending upon your graphics card and monitor so it's always best to experiment with colours to get the best effect.
In the example above, the elevation bands were set so that each colour represents a 50m elevation difference. The range in elevations within the model is approximately 100m to 400m. The red band represents elevations between 100m and 150m, the yellow band represents elevations between 150m and 200m and so on. You can use the Model Information command, to find the elevation range within any ground model. The elevation range is expressed as Z min, the lowest and Z max, the highest. It's a good idea to do this before starting the Elevation Analysis command so that you can work out the number of bands you need and the range within each band.
The methodology outlined above holds good for most cases where an elevation analysis of a ground model is required. For some specific cases, however, you may need to create an elevation band set where the elevation difference is not the same for all bands. This is true for the presentation of cut and fill areas on a difference model. Follow the sequence below to create a map of cut and fill areas using the difference model created earlier in this tutorial.
If you have followed this tutorial from the beginning you should now have a range of tools at your disposal to both make cut and fill calculations and to present the data statistically and graphically.
If you found this tutorial useful, you might like to consider making a donation. All content on this site is provided free of charge and we hope to keep it that way. However, running a site like CADTutor does cost money and you can help to improve the service and to guarantee its future by donating a small amount. We guess that you probably wouldn't miss $5.00 but it would make all the difference to us.