6
Exercise 1
Sounding for Depth Contours
The bathymetry of a lake basin is determined most accurately from a continuous
record of the basin contours with an accurate sonar device (depth sounder or
fathometer). The depth sounder is moved across the lake at a constant speed along
straight lines from known shoreline points. The continous record shows small
irregularities in bottom contours that easily could be missed by manual sounding
methods. The disadvantage of depth sounders is that most are quite inaccurate in water
less than 2 m deep.
Manual soundings of depth are made along transects between shoreline points with
weighted, calibrated lines or, in shallow waters, with metered sounding poles. Where
sediments are soft, the weights or ends of the poles must consist of a plate (30 to 40 cm in
diameter) to prevent appreciable penetration into the sediments.
The depth measurements along transects are then plotted precisely to scale within
the map ofthe shore line. Points of equal depth at convenient intervals (e.g., 1 m, 2 m, or
5 m) then are interpolated between the points of known depth in several directions.
These points of equal depth then are connected by contour lines (isopleths).
EXERCISES
OPTION 1. FIELD TRIP
In this exercise you will construct a simple map of the shore line of a small lake or a section of a
stream and establish the depth contours. From this map, a number of important morphometric
parameters can be calculated and used in subsequent physical, chemical, and biological analyses.
Select a small lake or section of a stream of reasonable size that permits measurements to be made
in the time allotted.
Triangulation Method
1. Establish a base line along a relatively straight portion of the shore line and stake this line at
each end. The base line should be as long (e.g., 30 to 60 m) as the configuration of the shore
line will permit and should be located so that nearly all points on the entire shore line are
visible.
2. Establish a plane table (or other sturdy table) at each end of the base line (points A and B on
Fig. 1.3). Measure the length of the base line accurately. Securely attach to the surface of each
plane table some high-quality transparent or translucent paper (linen cloth or vellum is good,
especially on a rainy day!). The plane table should be level and stable. With a compass,
establish the north~south direction and draw a magnetic north line.
3. One or more persons then proceed to position a stadia rod (or brightly painted pole) at
numerous positions around the shore line of the lake. The distance between the positions is
governed by the configuration of the shore line- the closer the interval, the more accurate will
be the representation of the true shore line.
4. At each position, a sighting is made from each plane table. With a pin set firmly in the plane
table at point A (or B) of the base line, sight along a straight edge (a sighting alidade is
preferable, but homemade cross-hairs and a ruler will do) and draw a line along the straight
edge. Repeat for each position.
5. Remove the paper with the sightings from each table and superimpose the base lines. Adjust
and measure the distance between points A and B on this base line to establish a convenient
Exercise 1
Sounding for Depth Contours
The bathymetry of a lake basin is determined most accurately from a continuous
record of the basin contours with an accurate sonar device (depth sounder or
fathometer). The depth sounder is moved across the lake at a constant speed along
straight lines from known shoreline points. The continous record shows small
irregularities in bottom contours that easily could be missed by manual sounding
methods. The disadvantage of depth sounders is that most are quite inaccurate in water
less than 2 m deep.
Manual soundings of depth are made along transects between shoreline points with
weighted, calibrated lines or, in shallow waters, with metered sounding poles. Where
sediments are soft, the weights or ends of the poles must consist of a plate (30 to 40 cm in
diameter) to prevent appreciable penetration into the sediments.
The depth measurements along transects are then plotted precisely to scale within
the map ofthe shore line. Points of equal depth at convenient intervals (e.g., 1 m, 2 m, or
5 m) then are interpolated between the points of known depth in several directions.
These points of equal depth then are connected by contour lines (isopleths).
EXERCISES
OPTION 1. FIELD TRIP
In this exercise you will construct a simple map of the shore line of a small lake or a section of a
stream and establish the depth contours. From this map, a number of important morphometric
parameters can be calculated and used in subsequent physical, chemical, and biological analyses.
Select a small lake or section of a stream of reasonable size that permits measurements to be made
in the time allotted.
Triangulation Method
1. Establish a base line along a relatively straight portion of the shore line and stake this line at
each end. The base line should be as long (e.g., 30 to 60 m) as the configuration of the shore
line will permit and should be located so that nearly all points on the entire shore line are
visible.
2. Establish a plane table (or other sturdy table) at each end of the base line (points A and B on
Fig. 1.3). Measure the length of the base line accurately. Securely attach to the surface of each
plane table some high-quality transparent or translucent paper (linen cloth or vellum is good,
especially on a rainy day!). The plane table should be level and stable. With a compass,
establish the north~south direction and draw a magnetic north line.
3. One or more persons then proceed to position a stadia rod (or brightly painted pole) at
numerous positions around the shore line of the lake. The distance between the positions is
governed by the configuration of the shore line- the closer the interval, the more accurate will
be the representation of the true shore line.
4. At each position, a sighting is made from each plane table. With a pin set firmly in the plane
table at point A (or B) of the base line, sight along a straight edge (a sighting alidade is
preferable, but homemade cross-hairs and a ruler will do) and draw a line along the straight
edge. Repeat for each position.
5. Remove the paper with the sightings from each table and superimpose the base lines. Adjust
and measure the distance between points A and B on this base line to establish a convenient