Cumbrous to carry, much slower than 4.
6. Box sextant, used as giving angular accuracy to any of the
foregoing; most useful with taping, and in following.
7. Sextant and three points.
The most rapid accurate method is to adopt three points visible all
over the ground (as trees or chimneys) or set up three markers. Find
shape and size of this triangle. Then at any point take two angles
visible between the points, and this fixes position of observer. A
large site may have forty points fixed in two hours thus to about 1
in 1000. For detail and plotting see Petrie, _Methods and Aims in
Archaeology_.
8. Theodolite.
For the most accurate work a theodolite is used, giving points to
about 1 in 5000. It is almost essential for any astronomical meridian
or latitude.
None of these methods necessitate any helper, except 4 which needs
two helpers. The observation is from the point to be fixed in 1, 2,
3, 4, and 7; but it is _to_ the point, needing signals or visible
features on the points, in 5, 6, and 8, and for those methods a large
stock of rods must be taken, and the whole ground gone over, before
the work of observation; such methods take far more time than the
others. The able surveyor will know by instinct how to use all the
inferior methods as supplements to the higher, whenever time demands
and accuracy allows.
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