Contours spaced close
together indicate a higher % slope.
Contours spaced wider
apart indicate lower % slope.
Different
types of lines should be used for contours of major elevations.
Common
practice is to identify the major elevations lines, or every fifth line, with a
bolder, wider, line.
Contour
Spacing-Intervals
•Another
decision that must be made is the contour interval.
•The
“best” interval depends on the use of the data.
Contour Intervals--cont.
Data Collection
Two Issues On Data Collection
When collecting topo data
there are two important issues:
Ensuring
sufficient data is collected to define the object.
Ensuring
two types of information is gathered for each station:
–Location
ElevationDefining an Object
It is important to remember that topographic
surveys are three dimensional.
To accurately represent an object on a topo map
data must be collected to define the shape, location and changes in elevation
for an objectDefining An Object-Hills and Depressions
How many stations are required to define the
shape of a hill or depression?
Defining An Object-Ditches
Another situations that requires
some thought is how many stations are required to define the cross section of a
ditch and length of a ditch.
Trapezoidal cross section = minimum of 6
Entire ditch = ??
Interpolation
•Drawing contour lines to produce a
topographic map requires the ability to interpolated between points.
•Interpolation is required because contour
lines are lines of constant elevation and the station elevations that are
measured in the field seldom fall on the desired contour elevation.
•Interpolating is finding the proportional
distance from the grid points to the contour line elevation.
Interpolation
•Interpolating can be done by estimation for
low precision maps.
•It should be done by calculation and
measurement for higher precision maps.
•A combination of methods can also be used,
depending on the use of the map.
Interpolating by Estimation
•Study the illustration.
Logic or intuitive reasoning would conclude
that when the grid points are at 102 feet elevation and 98 feet elevation, then
a contour line of 100 feet elevation would be half way in between
Note: that a dashed line has been drawn between the two
points. In topographic surveying it is
assumed that the area between two measured stations is a plane.
Interpolating by Calculation
•Proportional distance is calculated using an
equation.
•For the previous
example this would result in:
•The 100 foot contour
line would be located 0.5 or half of the distance between the two stations.
Interpolation by
Calculation & Estimation
•The same equation is used for all grid
distances and all contour lines.
•Example:
Determine the location of the 96 foot contour line for the illustration.
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