Geology Reference
In-Depth Information
A series of rules for contouring has been developed over the years to produce
visually acceptable maps that reasonably represent the surface geometry. The ref-
erence frame for a contour map is defined by specifying a datum plane, such as
sea level. Elevations are customarily positive above sea level and negative below sea
level. The contour interval selected depends on the range of elevations to be depicted
and the number of control points. There should be more contours where more
data are available. The contour interval should be greater than the limits of error
involved. Errors of 20 ft (7 m) are typical in correlating well logs and as a result
of minor deviations of a well from vertical. Uncertainties in locations in surface
mapping are likely to produce errors of similar magnitude. The contour interval
should be small enough to show the structures of interest. The map can be read
more easily if every fifth or tenth contour is heavier. The map should always have a
scale. A bar scale is best because if the map is enlarged or reduced, the scale will
remain correct.
The contour interval on a map is usually constant; however, the interval may be
changed with the steepness of the dips. A smaller interval can be used for low dips.
If the interval is changed in a specific area, make the new interval a simple multiple
(or fraction) of the original interval and clearly label the contour elevations and/or
show the boundaries of the regions where the spacing is changed on the map.
The contours must obey the following rules (modified from Sebring 1958; Badgley
1959; Bishop 1960):
1. Every contour must pass between points of higher and lower elevation.
2. Contour lines should not merge or cross except where the surface is vertical or is
repeated due to overturned folding or reverse faulting. The lower set of repeated
contours should be dashed.
3. Contour lines should either close within the map area or be truncated by the edge
of the map or by a fault. Closed depressions are indicated by hash marks (tic marks)
on the low side of the inner bounding contour.
4. Contour lines are repeated to indicate reversals in the slope direction. Rarely will a
contour ever fall exactly on the crest or trough of a structure.
5. Faults cause breaks in a continuous map surface. Normal separation faults cause
gaps in the contoured horizon, reverse separation faults cause overlapping contours
and vertical faults cause linear discontinuities in elevation. Where beds are repeated
by reverse faulting, it will usually be clearer to prepare separate maps for the
hangingwall and footwall.
6. The map should honor the trend or trends present in the area. Crestal traces, trough
traces, fold hinges and inflection lines usually form straight lines or smooth curves
as appropriate for the structural style.
Mapping by hand usually should begin in regions of tightest control and move
outward into areas of lesser control. It is usually best to map two or three contours
simultaneously in order to obtain a feel for the slope of the surface. The contours will
almost certainly be changed as the interpretation is developed; therefore, if drafting by
hand, do the original interpretation in pencil. The map should be done on tracing
paper or clear film so that it can be overlaid on other maps.
