Surveying Techniques I.
The USGS supplies 1:24,000 scale maps for all the U.S. But detailed
topography at larger scales is rare and/or unavailable. At these larger
scales it is usually necessary to prepare an original map based on air
photos or field surveys. The next four lectures deal with field
surveys.
Principles of spatial location
The purpose of a field survey is to accurately locate points in the
field so that their positions relative to each other can be plotted on a
map. Plotting positions of points in the field is determined by three
basic positioning principles:
a) location by three measured sides (Triangulation)
b) location by offset
c) location by intersection
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In each case, the baseline A-B is in a known location (can be mapped); X is the additional point
you are trying to map.
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Triangulation is based on having three measured distances (AB
the base line), AX and BX. Once the location of X is fixed it can
be used to build additional triangles. Points A and B are “known”
points (can be plotted on a map).
X
A
B
Location by offset is based on having a bearing (angle) and distance from a
known point to an unknown point (the known point could be a measured
distance along a known base line AB or it could be a bench mark, for example).
Intersection is based on having two bearings from two known points to an
unknown point (the two known points are usually at each end of a base line
AB). Because distances are not required, this methods works wherever there is a
line of sight (e.g. across lakes, rivers, canyons etc.).
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Planimetric Position.
All of these survey techniques are designed to locate objects in
their correct planimetric position (horizontal distances between
all objects on map are correct). This is not the same as ground
distance, which is affected by slope. Maps are planimetric.
A
Map distance
B
A
Ground distance
B
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Adding The Third Dimension.
The abney level is a small hand-held level designed to measure
angles in the vertical plane. It can be used to measure the heights of
features such as cliffs, trees or buildings, or the slope of the ground.
Once the angle between the object and the observer is obtained,
vertical heights can be determined by using simple trig functions.
Height of building =
Tan 20o x 200 m =
0.346 x 200 m =
72.8 m
20o
200 m
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The Surveyor’s Level (or Transit). This is basically a telescope
mounted on a tripod. The telescope can be leveled, so that the
central cross hair is aligned to a level plane and can be used to
calculate height. Additional upper and lower cross hairs are used to
calculate distance. The telescope is mounted on a swivel allowing
rotation in a horizontal plane.
View
level
upper crosshair
Horizontal plane
central crosshair
lower crosshair
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View through the level:
Graduated staff
8
7.8 feet – used for distance calculation
7
6.8 feet – used for height calculation
6
5.8 feet – used for distance calculation
5
Distance = upper crosshair – lower crosshair x 100 =
7.8 – 5.8 feet = 2 feet x 100 = 200 feet.
(Note: distance calculation varies from level to level).
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Calculating height:
The level gives relative heights – these can be tied to a nearby
bench mark or spot height to give absolute heights.
For
example, if
point C was
a bench
mark at 200
m, then
point B
would be at
202.74 m.
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Total Station Surveying: A
total station is a survey
instrument that can measure
horizontal and vertical angles,
slope, and horizontal and
vertical distances.
Measurements recorded by
the total station will produce
an x, y, and z value. The xvalue represents the easting
ing, the y-value represents the
northing, and the z-value
represents the elevation.
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NORTH
Y
Z
X
EAST
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reflector
The total station
works by firing an
infrared laser beam
at a reflector
mounted on a stadia
rod. The distance
between the total
station and the
reflector is calculated
based on the time
taken for the beam to
reflect back to the
total station.
Total stations were originally developed for the construction industry
– e.g. surveying new roads, laying out building foundations, utility
lines etc..
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Example based on
UTM:
NORTH
ANGLE
TOTAL STATION:
UTM = 3676595m N
672156m E
Y=150m; X=70m
UNKNOWN POINT
UTM=3676595
-150
3676445m N
672156
+70
672226m E
Y
X
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Most total stations have the ability to record survey data as a digital
file, which can be imported to a PC-based GIS program.
MAP
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Why use a total station? Accuracy: total stations are very accurate
(can be around 1 cm horizontal and vertical accuracy over distances
of around 2 miles. GPS can be fairly accurate for horizontal
positioning (e.g. around 1 m), but are less accurate for vertical
position (e.g. 5x less accurate than for horizontal position) (Note:
very expensive GPS systems can obtain 1 cm horizontal accuracy
and, presumably, 5 cm vertical accuracy).
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When do you use a total station? For mapping small areas (the range
of a total station is around 2 miles or so – assuming you have good
lines of sight). A good example would be mapping an archaeological
dig site.
There are many other applications in earth science that require great
accuracy e.g. monitoring cliff erosion, glacier movement, changes in
beach profiles, sand dune movement.. and so on.
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