how do you measure depth underground

[jolly geoff]

for example how would you measure the depth of the euro tunnel at the centre as this is also below water as well as earth and what instrument would you use?

[sddsddean]

I am assuming you mean in accesible places, such as tunnels and mines. Well nowadays its all done by electronics. Using a theodolite starting from a known point you can do a 'traverse' (going from point to point) measuring the horizontal and vertical angles and measure distance electronically, which all leads you to your end point with co-ords for northing, easting and height(depth). In the good old days (when I was an engineer!), we didn't have such extravagant gizmos, so you had to to a traverse with a theodolite and tape to get your easting and northing, then use a level and staff to do another traverse to get your height.

[gasman]

The channle tunnle was dug buy laser guided machines using GPS like the Sat nav does And some very clever electronics.

[gen2]

A less accurate method would be to use an altimeter (aneroid barometer). You zero this at the earth's surface (or sea-level) and it will tell you how high you are above that point (if climbing or flying) or how far below it you are (if descending or mining).

[Jenarry]

Ground penatration radar is sometimes used,
as well as messuring the depth of something it will also paint a sonic picture a bit like ultra sound does.

[heathfield]

In mining, the common method used for very many years was to firstly suspend a wire down the mineshaft to measure the depth of the shaft. From the bottom end of this wire, a distance to a point along a tunnel would be determined by using an accurate surveyor's tape measure. A theodolite would be set up to measure any downward or upward angle of the tunnel from the wire to the measured point. Using simple trigonometry, the vertical difference along the tape-measured distance could be calculated. The tape and the theodolite would be moved from point to point along the tunnel, and the sum total of height differences added to, or subtracted from, the length of the suspended wire. By this means the depth from the surface to any position in the tunnel could be determined

[jolly geoff]

thanks everyone you have made an old man happy.