#Step-by-Step Guide to Dumpy Level Survey
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A dumpy
level (also known as a Builder's Level) is an optical instrument used to
establish or check points in the same horizontal plane. It is used in
archaeological surveying to measure horizontal levels, for example to
demonstrate the difference in height at the top and base of a slope such as an
excavated pit or a surviving earthwork.
Equipment The level
'kit' consists of a level head (in box), staff and tripod.
The level head comprises an eyepiece, bull’s eye spirit level, three leveling screws
and a focus for the telescope lens; the base also incorporates a 360 degree
compass.
The 5m staff is in sections. Each ‘block’ represents one centimeter, and each 'E'
represents 5 centimeters. The 10 cm sections alternate back and forth and
between black and white, and the color alternates between black and red for
each meter.
The tripod
is composed of aluminum and plastic, with three extendable/lockable legs and a
base plate with screw fitting with which to attach the level head. There is a
canvas carrying strap and a belt to secure the legs together.
Benchmarks
and Temporary Bench Marks (BM/TBM) find the nearest OS Bench Mark (BM), which
is part of the national height system for mainland Great Britain and forms the
reference frame for heights above mean sea level. Bench Marks are no longer
maintained by the Ordnance Survey (although Fundamental (F) BMs are), but they
should be marked on most maps.
Bench Marks can usually be found on churches, but also on other notable buildings,
houses, bridges etc. The database describes where it is and what type of
benchmark symbol is used (usually carved into stone, the center of the
horizontal groove is the height reference). It is worth finding the nearest BM
to your survey site as soon as possible so that you can establish the best way
to transfer the height from the BM to your site.
If there is
no BM nearby to your site you can establish a Temporary Bench Mark (TBM) at an arbitrary height,
for example 100m (to ensure all heights are positive).
At some
point you will also need to find the nearest BM, to tie your TBM into and then
make your final level calculations.
To set up a TBM: mark an easily identifiable permanent feature nearby – eg. a coloured
brick in a wall (as in the photograph right), or a fence post; a wooden stake
may also be used but check with the landowner (if it is a scheduled monument
this is not an option).Make a
careful note with a precise description describing the location and nature of
the TBM, preferably with a note annotated on a map and a digital photograph (if you have a handheld GPS use this!).
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