+ LAND4551 + SITE+PLACE

Surveying a site (part 2: taking levels)

MethodThe basic principle determining slope or gradient is the same. The tools to do this may vary. We will use a dumpy (see equipment).

A horizontal line is imagined in relation to the ground. In fact this line is more like a plane which can go in any direction at the same level.

The horizontal plane is established with a dumpy which can be levelled, sitting on a tripod. The dumpy comes with a spirit level.

Then measurements are made from this plane to the ground.

Differences between these readings allow the ground contour to be understood and drawn.

Setting up the surveying

Ideally 3 people are needed for surveys: 1 to read the dumpy, 1 to hold a staff, 1 to record the readings.

Decide what positions and frequency of measurements are needed: sometimes a grid (at say 5m. intervals); other times random, but known, positions (called spot heights).

A position is decided for the first reading – this position is called a datum. All the following readings will relate to this position.

The datum needs to be a secure placement, that is unlikely to change: it could be a manhole cover; or the house finished floor level (FFL).

Setting up the dumpy

Choose a position for the dumpy with a good sightline to the datum and also to as much of the ground to be surveyed as possible. Further up the slope is more useful than too low down on the slope. Use the full height of the staff where possible.

Set up the tripod – it has adjustable feet to allow it to be upright on uneven ground. Ensure the tripod is secure, its feet pressed into the ground if possible. Do not disturb the tripod until it is time to move it again.

Attach the dumpy to the tripod. Level the dumpy using the spirit level – the bubble must be centred in its circle. Turn 2 of

the adjustable screws towards or away from each other until the bubble is near the centre. Turn the dumpy 90° towards the untouched screw and adjust this till the bubble is centred.

Reading the measurements

A staff (1 or 3) is placed vertically on a set position (as above: datum, grid or spot height). The dumpy has a telescope which allows readings to be made over a long distance. Focus

the telescope first on the cross hairs using the eyepiece focus. Line up the dumpy roughly with the staff with the sights on the top.

Focus on the staff using the focus on the side.

Read off the staff (sometimes the staff holder can use a pen or finger to verify the point being read).

Each 100mm block is numbered; the bottom half of this (50mm) looks like an E; the top half is in alternate white and red bands.

Record in a standard table.

This reading: 2.22m

Recording the measurements

As mentioned above the first measurement is to a fixed point (a datum). An ideal datum is the finished floor level of the house (FFL). We usually assume the datum to be 100.00metres. This avoids having any negative numbers to confuse our readings.

All other measurements are going to be related to this point, or if you like a difference from this height.

Mark out a grid on the ground (using horizontal measurements). If the ground is an even slope a few spot heights can be chosen. Take readings at each of these points, on the grid or the spots.

First read the height of the staff at the datum. Record in the column “Staff Reading”: the reading is going to be:

100.00 (datum) + reading Then read and record all the other points chosen. If not all measurements can be taken from 1 spot, the dumpy must be shifted. Take a back site from this new dumpy position to the site of the last reading.

Calculating the Reduced Levels (RLs)

You will notice that the readings are all above the points on the ground. This is the difference between the datum and the level of the dumpy. This is called the height of instrument (HI).

When calculating this height of instrument (HI) It needs to be taken off the reading.The HI that counts is the one taken at the datum.