Gridded Land Grading

Gridded Land Grading

Definition:

Land grading consists of fitting a flat plane through the spot heights presented. CDS uses a least squares solution which minimizes the square of the distance of each spot height with the calculated plane. In most instances however we really want to minimize the amount of cut and fill; taking into account any compaction ratio. To this end we raise or lower the calculated plane to achieve this result.

Entering data manually

In this scenario we wish to grade a simple rectangular field. We also have little in the way of surveying equipment except a tape measure and a dumpy level. The first stage is to layout the grid points on the ground. In this case we have chosen a grid size of 20 meters in both directions. The grid distance can vary both ways. Also the initial smoothness of the ground will effect what is a good grid size to select.

Bring up CDS and open a new job called GridTutorial. The job is initially empty. We will add in the data points later. Under the grading menu select the "Grid Entry & Grading" option. Select the Setup tab and the following dialog will be displayed. In this example we only have the one field in the job so leave the Field Number set at 1. The origin is purely arbitrary but we have used 1000.0,1000.0. In this example the field is an even 100 metres squared and we are using a grid size of 20 metres. Please fill in your dialog to match that to the right. If the grid length is not an even size a short grid is entered along the

edge and internal calculations are adjusted accordingly. The compaction ratio is very important as the dirt we use for fill tends not to go as far as the dirt we have dug up. The heavy machinery compresses the dirt expelling any air that was trapped in the dirt when we dug it up. Once you are happy with this you can press the apply button and the values are stored away.

The grading dialog is designed to work in the background if necessary. Clicking on the view minimizes the dialog and places it on the task bar at the bottom. The dialog can be brought back up by simply clicking on the minimized icon. Once the grid has been defined it is available to be displayed Clicking on the view and doing a zoom extents if necessary will display the following result.

We are now at the stage where we can enter in the spot heights at the grid points. Click on the "Land Grading Entry" tab to get to the appropriate dialog screen. If we first look at the travel screen the Line box tells us which column we are presently adding in data to. The increase or decrease radio button refers whether we are picking up data up the field or down the field.

In this example we are entering the data manually. Make sure that the Manual tick box is ticked. For the present we can ignore the rest of the page. The grid box shows the current position.

Simply entering in data to the value box and pressing the enter key or using the mouse and the store button. The entry point is automatically moved to the next grid point for entry. Once we are finished with the current column we need to enter the new column name in the line box and press the Apply button.

ie In the travel section set the line to "A" and the radio button to increase. Hit apply. In the value box enter in 1.1 and press the "store" button. Enter 1.2 and store etc etc. After entering A6 the grid prompt shows "Goto Next". Enter "B" in the Line entry and press Apply.

Continue entering in the data as shown below. Once you have finished you may click back into the main view. The land grading dialog is minimized. You may need to do a Zoom Extents. You will have a screen such as below:

At this stage it would be worthwhile visually inspecting the entered data for correctness. It may be worthwhile triangulating and contouring the data as an extra check. Once you are happy with the data entry it is time to perform the actual land grading function.

Click on the minimized land grading dialog. Select the Grade tab and you will have the following screen.

Click on the calculate button and you will obtain the grade and cross grade as shown. A rough check on the grades obtained can be obtained by doing sample joins (or inverses) on points lying on different sides of the job.

In this case there is a difference between the cut and fill values as we have specified a compaction ratio of 140%. ie fill times 1.4 should equal cut.

Further down the dialog you will notice there are some grade, cross and height fields. You can use these to set these values. For example if you need to set a minimum or maximum slope for grade or cross grade you can add it in here. Also if you know that you wanted some dirt left over for example you could put in a negative number for the height and press the apply button. The volumes etc will be recalculated accordingly.

We can now print out some reports for use in the field. Pressing the Cut/Fill button produces a report showing the amount of cut and fill needed to be moved in each cell. The color of the text is determined by the setup dialog. Inspecting this report will show up any potential problems.

Pressing the Print Elevations button shows a report showing the height, design height and height difference at each grid point. Again this report should be checked for inconsistencies.

If you now click back into the main view you can see a screen similar to the following. The view now displays the grid and also has enough information to calculate the volumes for each grid entry. If you observe the whole grid there should be as many red as blue squares to indicate the cut and fill is reasonably balanced.

We now have the option of creating a color printout showing the field as height difference bands. Click on the contour menu and select the Surface Parameters dialog. Add in a new surface and select as a height difference surface. This surface uses the height and design heights on each point to calculate a difference at each point. Triangulate the new surface. Again click on the contour menu and surface parameters. Select the Surface Color tab entry. Tick the surface colors and legend tick box's. Also click on the surface difference defaults. We can now get a picture such as that below:

Additional notes:

Most fields to be graded are not in the regular shape as shown. The gridded grading can handle these cases. In some cases you can simply fail to fill in some of the grid points so that the filled in grid points approximates the shape of the field. For a more sophisticated solution you have the option of specifying a closed bounding string. Fill this in in the setup dialog. The program calculates partial grid values as appropriate and displays and reports the changed values.

Also if you have already picked up points not in a grid you can triangulate the surface and use the snap to grid option in the entry dialog to get the appropriate points.