Levelling

Levelling

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Benchmarks and Datums

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​Objectives

To understand the relationship between sea level and Datums.

To understand​ different types of levelling instruments and there uses.

​Sea level

• Newlyn in Cornwall is the point from which all contour lines are measured.

• For example the peak of Ben Nevis is 1,344m above sea level​

• ​A point above sea level is called a Bench mark.​

Multiple Choice

Sea Level is taken form a specific place. Select the correct answer.

• Ordnance Datum Newlyn (ODN) and marks height zero on maps in Britain.

• Ordnance Datum London (ODN) and marks height zero on maps in Britain.

• Ordnance Datum Greenwich (ODN) and marks height zero on maps in Britain.

• Ordnance Datum Newcastle (ODN) and marks height zero on maps in Britain.

​Benchmark

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​What is a Datum?

• ​ A standard position or level that measurements are taken from.​

  On site you will often see a the mark the mark shown here which shows a line 1m above what would be the finished floor level.

• ​There is a similarity between the Benchmark and Datum mark

​Water Level

​​Click on the picture to watch the video.

Spirit Levels

These can be used to check for plumb or level by ensuring the appropriate bubble is centred between the lines.

​Click the link below or copy into your browser to view video.

​https://youtu.be/kVsSswu6g7o

​Laser Levels

• ​Click on the picture to watch the video.

​Surveying Telescope

​This is a slightly more complex instrument that is accurate over long distances

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​https://youtu.be/j8poe2vvD2Q

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​https://youtu.be/YnDA-nAUyl8

Open Ended

Name 4 instruments for transferring a Datum

What are the advantages and disadvantages of each one?