Contours and Contouring from Grid Surveys.

To watch the video on this subject got to 'Develop Contour Lines'

This post continues from a previous post ' Grid surveys'

Contours and Contouring from Grid Surveys.

Using the information from our grid survey we can create a contour plan for our site. Contour lines are lines that indicate point of equal height and are use to show the three dimensional shape of the land on a two dimensional drawing. The contour plan will show the slope of the land and allow you to estimate the amount of excavation that will be needed to reached a required level.

1. Draw a scaled down version of your site, the larger the drawing the more accurate you will be however 1:200 or 1:500 is a common scale for a residential Site Plan.

2. Record the R.L. of each grid intersection on the plan.

3. Note down the Contour intervals that you need your contour lines at, generally every 0.500mm for a residential site, starting fro your lowest R.L to you Highest R.L. e.g. 0.500, 1.000, 1.500, 2.000.

4. Have a look at the site plan and find the first two grid points that the first R.L will fall between.

5. Calculate the difference between the two R.L. points e.g. 0.800 - 0.100 = 0.700

6. Measure the actual length of the Scaled down Grid and divide this by the difference between the two R.L. points. e.g. 10mm ÷ 0.700 = 14.285. This is the distance along this line that a R.L difference of 1 metre would be.

7. So next we need to work out the difference between the first R.L. and the required Contour line R.L. e.g. 0.100 - 0.500 = 0.400

8. Now we need to work out how far along from the first R.L. the contour line is so we need to divide the difference between the first R.L. and the required contour line by the millimeters in 1 metre. e.g. 0.400 ÷ 14.285 = 5.714

9. Now you can measure from the lowest first R.L. 5.714mm to the point where it will cross the grid line.

10. Repeat this for every time that the contour line will cross the grid.

11. Once you have found the point where the contour line will cross the grid you can draw a straight line between each point.

12. Naturally the ground it not straight so now you can use a bit of imagination and curve the line to create a more realistic shape.

13. Now that you have a contour plan you can use it to create a section view of your site. Watch this video to see how. Develop a section from Contour Lines.

Grid Surveys

To watch the video on this topic go to 'Grid Surveys'

Congratulations, you have purchased a block of land and now you want to start construction.

However in order to get the plans are drawn you are going to need to get an idea of the slope or fall of your land. This will allow you and your trades, especially the excavation trades, to work out how much material they will have to move in order to get to the levels or depths required.

The grid survey is achieved by taking levels in a grid pattern across the site, recording the levels and booking the Reduced level of each point.

1. Mark out the grid pattern on the site.
1. a grid pattern of 5m x 5m is generally accurate enough for a residential site
2. when marking point on the ground you can use surveyors paint, a brick, chalk or lime, it doesn't really matter as long as you can find the mark again later so you can take the reading.
3. This can be done with a combination of measurement and visual alignment.
4. the setout should be conducted as accurately as possible however these point can be off by 150mm to 200mm without affecting the accuracy of the survey.

2. Make sure that you identify each intersection so you can reference it later. This can be achieved by assigning a letter to grid lines in one direction (say East -West) and a number to the grid line running at 90 degrees (North- South) see below.

3. Using the instrument of your choice take readings at each point and record them on to a booking sheet. See my video ‘Taking Levels’ for more information on how to do this.
4. Make sure that you record where the reading was taken in the remarks column.

The grid survey is now complete but how do we put it to use? See Contours and Contouring from Grid Surveys.