and the stadia interval is 6.23 feet. In the table under 3°
TOPOGRAPHIC MAPPING
and opposite 26’, note that the multiplier for horizontal
distance is 99.64, while the one for difference in
elevation is 5.98. If the final distance is ignored, the
horizontal distance is
The difference is elevation is
To these figures, add the corrections for focal distance
given at the bottom of the page. For an instrument with
a focal distance of 1 foot, add 1 foot to the horizontal
difference (making a total horizontal distance of 622
feet) and 0.06 foot to the difference in elevation This
makes the difference in elevation round off to 37.4 feet;
and since the vertical angle has a negative (-) sign, the
difference in elevation is recorded as –37.4 feet.
In the first column on the Remarks side of figure
8-10, enter the elevation of each point, computed as
follows. For point 1, the elevation equals the elevation
of instrument station D1 (532.4 feet) minus the
difference in elevation (37.4 feet), or 495.0 feet.
Subtract the difference in elevation, in this case, because
the vertical angle you read for point 1 was negative. For
a positive vertical angle (as in the cases of points 12 and
13 through 17 of your notes), add the difference in
elevation
The remainder of the points in this example were
detailed in a similar reamer except for point 13. When
a detail point is at the same, or nearly the same, elevation
as the instrument station, the elevation can be
determined more readily by direct leveling. ‘That was the
case for point 13. As seen in the vertical-angle column
of the notes, the vertical angle was 0° at a rod reading
of 5.6 feet. Therefore the elevation of point 13 is equal
to the elevation of the instrument station (532.4 feet)
plus the h.i. (4.8 feet) minus the rod reading (5.6 feet),
or 531.6 feet.
In the above example, as you recall, the transit was
initially backsighted to point A and the zeros were
matched This was because the azimuth of D1A was not
known. However, if you knew the azimuth of D1A, you
could indicate your directions in azimuths instead of in
angles right from D1A. Suppose, for example, that the
azimuth of D1A was Train the telescope on A and
set the horizontal limb to read Then when you
train on any detail point, read the azimuth of the line
from D1 to the detail point.
Now you know how to perform and record a
topographic survey, using the transit-tape or
tranSit-stadia methods. Next, we will see how the
draftsman (who also might be you) prepares a
topographic map. To enhance the explanation of
topographic mapping, we will also discuss some
additional field methods the surveyor uses.
REPRESENTATION OF RELIEF
One of the purposes of a topographic map is to
depict relief. In fact, this is the main feature that makes
a topographic map different from other types of maps.
Before you go any further, refresh your memory on the
subject of topographic relief. Relief is the term for
variance in the vertical configuration of the earth’s
surface. You have seen how relief can be shown in a
plotted profile or cross section. These, however, are
views on a vertical plane, but a topographic map is a
view on a horizontal plane. On a map of this type, relief
may be indicated by the following methods.
A relief model is a three-dimensional relief
presentation-a molded or sculptured model, developed
in suitable horizontal and vertical scales, of the hills and
valleys in the area.
Shading is a pictorial method of showing relief by
the use of light and dark areas to suggest the shadows
that would be created by parallel rays of light shining
across the area at a given angle.
Hachures area pictorial method similar to shading
except that the light-and-dark pattern is created by short
hachure lines, drawn parallel to the steepest slopes.
Relative steepness or flatness is suggested by varying
the lengths and weights of the lines.
Contour lines are lines of equal elevation; that is,
each contour line on a map is drawn through a
succession of points that are all at the same elevation. A
contour is the real-life equivalent; that is, a line of equal
elevation on the earth’s surface.
All of these methods of indicating relief are
illustrated in figure 8-12. The contour-line method is the
one most commonly used on topographic maps.
CONTOUR LINES
Contour lines indicate a vertical distance above, or
below, a datum plane. Contours begin at sea level,
normally the zero contour, and each contour line
represents an elevation above (or below) sea level. The
8-12
