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History: In the times before
GPS, and even before EDM (Electronic Distance Measurement), the
tools of the surveyor were the theodolite and a tape of some kind
for measuring distances.
The main problem was bringing scale
into the mapping project. This was done by measuring accurate
baselines. If you look at the figure "The Principle" to the left,
the baselines BA and BB was first measured with high
accuracy.
Then all the angles marked with
greek letters were measured. Then by trigonometric calculation -
with corrections of differet kind applied - BB was calculated
starting from BA. The answer should match the measured
distance for BB within a certain error limit.
The basis for all serious surveying
and mapping was a system of trigonometric points. The baselines
were very often measurerd at the airports where the terrain was
flat and they mainly existed along the coast. But more control was
needed. So in 1931 it was decided to measure a baseline in Tisleia
in the middle of Southern Norway.
When you reach the co-ordinates of
this cache you are about 20 metres east of the eastern end of this
baseline. There is granite monument / marker at this end of the
baseline. See the picture "The monument" to the left.
If you look closely (and maybe feel with your fingers) you will
find the text N.G.O. (Norges Geografiske Oppmåling - Geographical
Survey of Norway) and Basis Tisleia 1932 chiseled into the granite.
N.G.O is the former name of what today is known as Statens Kartverk
(The State Mapping Authority).
The baseline was measured in 1932. It was used four different
measuring tapes made from invar (an alloy that has almost no
reaction to temperature) with a length of 24 metres, so four
independent measurements of the whole line was made. The baseline
is 6641,777 metres long horizontaly in the plane of
projection (se The Final Calculation at the end
).
There had to be a peg for each 24 m.
These pegs had to sit in solid ground, in the marshes some pegs
went 4,5 metres deep. Slope distances were measured. The heigth
above sea-level for each peg was determined by levelling from
the precision levelling points along the highway Leira-Gol. The
heigth was then used to calculate the horizontal distance between
the pegs.
The Cache is a Lock&Lock type plastic box of the size 10
x 21 x 7 centimetres wrapped in a black plastic bag. It is not
directly visible. It placed on the ground under the branches of a
small spruce (2 metres of heigth) on the southeren side of the four
thin stems. The cache contains stashnote, logg and items for
trading. Also a small bag for the FTF.
The cache may be reached
from the south (Tisleia Valley) or from the north (The Vaset Area).
In both cases on toll roads. There is a vast network of private
toll roads in the area. The terrain around the cache is somewhat
special: wide open spaces with nice views to the mountains
surrounding the area, the nearest about 7 kilomertres
away.
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