Wednesday, 9 May 2012

Sloping Garden Design - Surveying Levels 1

When you’re designing a sloping garden one of the trickiest parts is measuring and working with the level changes. Before you can start planning the garden layout for a sloping site you need to find out where and how steep the level changes are. The best way of measuring level changes is to use a use an optical site level (usually called a dumpy level) or a laser level and staff.

I’m going to give you a simple way of working out levels if you don’t have access to conventional site surveying equipment. You’ll need at least one plank, a spirit level, 25mm square wooden pegs of different lengths, a metal tape measure, a mallet and a pad and pencil. You’ll also need a blank survey of the existing garden layout and at least one 30/50m tape measure.

When taking a level survey you need to decide on a datum point. This is a flat area the height of which will not change. When surveying a sloping garden I usually use finished floor height inside the front or back door of the house. This will not be changing over the course of the garden build and can always be referred back to at any stage of construction. The datum point is at height 0, or if you are using correct surveying notation start at 100 or 50 to avoid negative measurements.

This example assumes the site slopes down from the datum point. Lay one end of the plank on your datum point and rest the other end on a peg banged into the ground. Adjust the height of the peg by banging it further into the ground or raising it up until a spirit level laid on top of the plank shows that the plank is level. Measure the distance from the underside of the plank to the ground. This measurement is the amount of drop or fall of the land away from the house at this specific point on the site. Now you need to find out the position of the peg in the garden. Measure a straight line from the peg to the house, then measure from this point to end of the house. Mark these measurements on your blank survey and give it a number. Note the distance from the peg to the ground on your pad and give it the same number. You can also use triangulation to locate the position of the peg – see this other post on how to measure using triangulation.

The next step is to place one end of the plank at the base of the first peg, rest the other end on a new peg, level the plank and measure the height of the peg from the ground. Repeat this exercise until you reach the bottom of the slope. The sum of all the peg heights will be the total fall over the length of the garden. You should measure down the slope in at least one other place. Remember to use the same datum point. If the garden has cross falls, ie slopes across its width as well as its length, you will also need to measure these and plot the peg points on your survey.

If you need help designing your garden please feel free to email me You can visit my Web site and see some of my completed projects and get a bit more information.

No comments: