MONTSERRAT
positions
Subject: Exact locations in Montserrat
Richard Herd of MVO very promptly supplied the following answer to the
question about the exact location of the GPS and EDM points used by MVO.
Date: Tue, 1 Apr 1997 11:12:20 -0400
Subject: from Richard Herd
.... the exact locations of our GPS / EDM pins. Here is a list of them.
They are in Latitude and Longitude projected onto the WGS 1984 ellipsoid.
Heights are relative to this ellipsoid. To obtain heights relative to sea
level add about 40m. Lat and Long are degrees, minutes, seconds.
Site Lat(N) Long(W) Height (in metres)
Brodericks 16 41 56.29 62 11 59.65 146.96
Castle Peak reflector 16 42 32.88 62 10 32.15 666.74
Chances reflector 16 42 28.65 62 10 52.86 816.84
Chances Peak 16 42 33.04 62 10 51.22 875.83
Dagenham 16 42 56.71 62 12 38.76 84.72
FT3 16 42 50.72 62 10 44.66 758.31
Farrells 16 43 13.79 62 10 48.46 495.69
Galways (M37) 16 41 44.81 62 11 15.55 311.33
Galways (Obs) 16 42 6.99 62 10 51.89 544.31
Harris (M18) 16 44 28.96 62 10 4.01 245.73
Hermitage 16 43 17.52 62 10 11.19 438.64
Long Ground 16 43 29.02 62 9 30.49 201.82
Lookout Yard 16 47 23.13 62 11 17.24 132.83
Lower Amersham 16 42 18.62 62 12 24.83 108.77
M27 16 44 59.52 62 13 44.95 37.35
Ogarras (EDM) 16 40 48.96 62 11 14.24 -9.98
Ogarras (M46) 16 40 39.81 62 11 18.34 -27.19
Perches 16 42 32.07 62 9 52.87 749.41
Roches Yard 16 42 14.25 62 9 11.70 313.04
St Georges (M25) 16 43 21.77 62 12 29.79 317.78
Tar River 16 43 12.77 62 9 39.89 255.52
Upper Amersham 16 42 35.29 62 11 39.77 317.37
Whites 16 43 52.52 62 9 34.39 143.36
Windy Hill 16 44 32.23 62 11 26.28 374.36
The occupations we make are usually:
BIGNET - M18, Whites, Tar River, Dagenham, M46
EASTNET - M18, Whites, Tar River, Farrells, Windy Hill
WESTNET - Dagenham, Lower Amersham, Upper Amersham, Galways, M46
NORTHNET - M18, Dagenham, M27, Lookout Yard
CLOSENET - M18, FT3, Hermitage, Chances Peak, Perches
Individual stations may be substituted or dropped depending on activity of
the volcano and weather conditions. Long Ground is quite often substituted
for the Tar River site if there is a threat to personnel and equipment from
pyroclastic flows. At the moment flows are travelling down the White River
region from Galways wall, making the M46 and M37 sites very dangerous.
The technique used is differential GPS where two stations are run
simultaneously. One station is held fixed and the position of the second
calculated relative to that. In our processing, M18 has been chosen as the
fixed station. For the networks above we use rapid-static GPS where one
station is run as a base for a few hours, while the equipment is moved
between the other stations (i.e. roving station). The roving station
occupies the point for about 15 minutes allowing derivation of the point
position to a few mm. The last station occupied by the roving station
becomes the new base and is left running whilst the equipment at the base
station is moved around the other sites. In this manner all possible lines
between the stations are measured. Long occupation GPS experiments (around
20 hours) are also run to give a very good fix on a single point and an
assessment of measurement errors. This is usually done for the high, more
dangerous sites (FT3, Hermitage, Chances Peak and Perches).
EDM measurements are made in a series of triangles and loose lines to
reflectors:
Northern Triangle - Instrument at Windy Hill and St. Georges Hill; target
at Farrells.
Western Triangle - Instrument at Lower Amersham; targets at Dagenham, Upper
Amersham and Chances Peak Steps.
Southern - Instrument at Ogarras EDM and M37; permanent target at Chances
Peak EDM, temporary target at M37.
Eastern Triangle - Instrument at Whites, Long Ground and Tar River;
permanent target on Castle Peak. This triangle has been dropped because
Castle Peak has been engulfed by the lava dome.
Hope this is of some use. Let me know if more details are needed.
...
Regards - Richard Herd, MVO
Montserrat Volcano Observatory
. . . The Electronic Evergreen, courtesy of GEM Radio Network
END
first version: August 8, 1997;
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