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.
•Another decision that must be made is the contour interval.
•The “best” interval depends on the use of the data.
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:
Defining 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 object
Defining 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 = ??
•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.
•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.