PENGENALAN KEPADA PETA TOPOGRAFI

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PENGENALAN KEPADA PETA TOPOGRAFI

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Title: PENGENALAN KEPADA PETA TOPOGRAFI

1
PENGENALAN KEPADA PETA TOPOGRAFI

SGBS 1202
Prof. Madya Mustaffa Kamal Shuib
Jabatan Geologi
Tel 79674227
Email mustaffk_at_um.edu.my
27/7/2007

2
PETA TOPOGRAFI

Model 2 matra permukaan Bumi menunjukkan bentuk,
saiz, dan kedudukan relatif fitur-fitur semula
jadi dan buatan manusia pada sekil yang telah
dikecilkan.
(Topographic maps are two dimensional
representations of the earth's surface, showing
the shape, size, and relative position of natural
and man made features at a greatly reduced scale.)

All standard topographic maps should have an
orientation arrow (north), a legend, a scale, and
a method locating things geographically.

A very common method of geographic location is
the latitude-longitude method. The earth is
nearly a sphere and therefore may be considered
to be divisible into 360 degrees. By measuring
latitude and longitude in degree, minutes (1/60th
of a degree), and seconds (1/60th of a minute),
it is possible to pin-point any spot on the
earth's surface. Any point on earth can be
accurately defined in terms of a) its distance
from the Prime Meridian which extends N-S through
Greenwich, England and the poles, and b) its
distance north or south of the equator. Lines
connecting the poles are knows as longitude lines
or meridians however, these lines are not
parallel as they converge at the poles. Lines
parallel to the equator are know as parallels of
latitude.

5
Topografi/rupabumi

Topografi rupa-bentuk permukaan bumi.
Gambaran rupa-bentuk, saiz, dan kedudukkan
relatif fitur-fitur semula jadi dan buatan manusia

Peta topografi menunjukkan jasad timbul (relief)
di dalam beberapa cara atau gaya dan juga
menunjukkan kedudukan dan penentuan sifat-sifat
di atas bumi samada semulajadi atau buatan
manusia.

7
Jasad timbul

Jasad timbul (relief) istilah umum untuk
rupabentuk bumi termasuk ketinggian dan
kecuraman.
Teknik mengambarkan jasad timbul atas peta
Spot height
Garis Kontour
Warna dan lorekkan.

8
Spot heights

Spot heights are shown on maps as a black dot
with the height written next to it. You can see a
spot height in the figure showing the contour
line features. Spot heights give the exact
altitude or height above sea level (asl) of the
particular location or feature.

Contour lines
Contour lines join points of equal height above
sea level. Thus every point along the line has
the same value.
Contour lines provide geographers with
information about the shape and slope of the land
and the height of features above sea level. The
contour or vertical interval is the difference in
height between two adjacent contour lines. This
interval is normally stated in the maps legend
or near the edge of the map.

10
Jasad timbul

Differences in elevation are shown on topographic
maps by means of contour lines, which are lines
connecting points of equal elevation.
The number of contours used depends on the relief
(total difference in elevation) of the area. A
mountainous area will require many more contours
than a flat area of little relief (if the contour
interval is the same for both).

Contour patterns
Each type of topographic feature is represented
by its own distinctive contour pattern. A skilled
user of topographic maps can visualise the shapes
of particular features by studying the patterns
created by the contour lines.

All topographic maps use mean sea level as their
reference line. Imagine an island. The water line
on the beach would appear as a contour line
labeled "0". The rising land surface would be
mapped by contour lines showing increasing
elevation and spaced accord to the contour
interval (see figure 1 below).

Remember
The spacing of the contours on a map shows the
steepness of slopes. Areas where contour lines
are close together have steep slopes. Areas where
the contour lines are widely spaced are very
flat.
The spacing of contour lines on a map also
indicates the shape of a slope. Evenly spaced
contours indicate a uniform slope. When the
spacing of contour lines reading from high to low
decreases, the slope is convex. When the spacing
of contour lines reading from high to low
increases, the slope is concave.

When contouring, keep in mind the following
points
Contour lines never cross or intersect one
another (but may merge to form a single contour
line where there is a vertical cliff).
The closer the contour lines are together, the
steeper the slope.
Contour lines for most areas should be smooth,
curving lines.
Every contour line closes on itself, either
within or outside the limits of the map.
Contour lines point upstream as they cross a
valley.
Basins have hachure marks on the downhill side.

15
Tips for understanding contour lines.

When first looking at a topographic map, it may
appear somewhat confusing and not very useful.
There are a few rules that topographic contours
must obey, however, and once you understand these
rules the map becomes an extremely useful and
easy to use tool.

16
The rules are as follows

1) Every point on a contour line represents the
exact same elevation (

2) Contour lines can never cross one another.
Each line represents a separate elevation, and
you cant have two different elevations at the
same point.
The only exception to this rule is if you have an
overhanging cliff

3) Moving from one contour line to another always
indicates a change in elevation.
To determine if it is a positive (uphill) or
negative (downhill) change you must look at the
index contours on either side (see figure).

19
4) On a hill with a consistent slope, there are
always four intermediate contours for every index
contour. If there are more than four index
contours it means that there has been a change of
slope and one or more contour line has been
duplicated. This is most common when going over
the top of a hill or across a valley (see
figure).
20

5) The closer contour lines are to one another,
the steeper the slope is in the real world. If
the contour lines are evenly spaced it is a
constant slope, if they are not evenly spaced the
slope changes.
6) A series of closed contours (the contours make
a circle) represents a hill. If the closed
contours are hatchured it indicates a closed
depression (see figure).
7) Contour lines crossing a stream valley will
form a "V" shape pointing in the uphill (and
upstream) direction.

Layer colouring
Layer colouring is a simple and effective way of
showing relief on maps. It involves colouring the
area between selected contours in different
colours. Throughout the Heinemann Atlas you can
find examples of this layer colouring. When it is
used in combination with spot heights, and
sometimes landform shading, it can tell you a
good deal about the shape of the land. However,
remember always to use the key to help you
interpret the colours used.
Shading
Shading is a very effective method of
highlighting landform features. The shading makes
the relief features stand out from the map,
creating a three-dimensional effect. You can see
examples of landform shading in the Heinemann
Atlas.

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An understanding of how tographic maps are
constructed is crucial to understanding their
interpretation. We will use the ficticious
Bumluck area to illustrate the construction and
components of topo maps.
Cartoon aerial view of the Bumluck area

The base for most modern maps is the aerial
photograph. (Because Bumluck is ficticeous, we
represent it with an aerial cartoon.)
Cartoon aerial view of the Bumluck area

A map is a scaled two dimensional representation
of a portion of the Earth's surface. The map
makers choose a scaling factor (or shrinkage
factor) that allows them to fit the area of
interest on a chosen piece of paper. The map
scale is usually represented by a ratio or
fraction, such as 110, or 1100000 or 162,500.
Cartoon aerial view of the Bumluck area of
central Utah with a scaled map.

Many maps that are used as the base maps for
geology are constructed by the United States
Geological Survey (USGS). USGS maps usually give
the scale at the bottom of the map, in this case
124000. What this means is that 1 unit of
measure on the paper map is equal to 24000 of
those same units on the actual ground.
In other words, one inch on the map at left is
equal to 24000 inches on the actual ground in the
Bumluck area.
One centimeter on the map is equal to 24000
centimeters on the actual ground.
Another way to look at it is that features on the
map are 24000 times smaller than the real
features on the ground.

Accurately scaling allows us to measure distances
between locations on the map. Measuring distances
is an important aspect for the interpretation of
some geologic features.

For example, we can measure the number of inches
between Bumluck and the top of Bob's Knob on the
map. The distance between Bumluck and Bob's Knob
on the map is 1.5 inches. Multiplying the 1.5
inches by 24000 to figure out the actual distance
on the ground we get
1.5 x 24000 36000
inches on the actual ground between Bob's Knob
and Bumluck. Dividing by 12 to obain feet, we get
36000 / 12 3000
feet between Bob's Knob and Bumluck.
Dividing the 3000 feet by 5280 (the number of
feet in a mile) gives us about 0.6 miles from
Bumluck to Bob's Knob.
There is a simpler way to do this.

In addition to the numerical scale, standard USGS
maps also show several graphical scales. The
graphical scales are represented by divided bars
which represent miles, feet, and kilometers.
Now to solve the earlier problem of finding the
distance from Bumluck to Bob's Knob, we can
simply place a piece of string or ruler between
the two locations and mark the distance- recall
that it was 1.5 inches.
Now bring the string or ruler or paper down to
whichever measure you want, say miles, and read
off the distance.
On the miles bar, we get about 0.6 miles.
On the feet bar we get about 3000 feet.
On the kilometer bar we get about 1 kilometer

Although it is relatively easy to plot the
locations of canyons, mountain tops, rivers,
cities, etc., on a flat map, heights or
elevations that define the shape of landscape
features (mountains, canyons) require special
illustration. We represent elevations with
contour lines. A contour line connects points of
equal elevation.
Cartoon aerial view of Bumluck with contour lines
shown in gray.

Maps which use contour lines to illustrate the
shape of the landscape are called topographic
maps, or "topos" for short. In the United States,
they are produced by the United States Geological
Survey (USGS).
Topographic map of the Bumluck area at right.

Contour lines are critially important to the
interpretation of geological maps. Consequently,
there are a few rules we need to know about
contour lines.
As noted before, contour lines connect points of
equal elevation. They do not rise or fall.
Topographic map of the Bumluck area at right.

The change in elevation between adjacent contour
lines is called the contour interval. On standard
USGS topo maps, the contour interval is noted at
the bottom of the map, just below the graphic
scales. Click map for larger image. For Bumluck,
then the contour interval is every 40 feet. Thus,
the elevation increases or decreases by 40 feet
for ever contour line on the map. The National
Geodetic Vertical Datum of 1929 is a really fancy
way of saying Sea Level in the year 1929. Nearly
all elevations in the U.S. are referenced to this
measure of Sea Level. Sea Level changes so we had
to put the year in which it was referenced.

Every fifth contour line is a little thicker than
the standard contour line and is labeled with the
elevation relative to mean sea level. These
thicker contour lines are called index contours.
Because index contours are always every fifth
contour line, the change of elevation between
index contours is always 5 times the contour
interval. So in the case of Bumluck, the contour
interval was 40 feet. Five times 40 is 200.
Therefore, there is 200 feet of elevation
difference between index contours in the Bumluck
map

Closely spaced contour lines indicate steep
topography

Widely spaced contour lines indicate gentle
slopes

When a contour line crosses a canyon or stream,
the line forms a "V" shape which points upstream.

Let's see how much we've learned. Click on map to
expand.
1. Which hill is steeper, Bob's Knob or Blueberry
Hill?
2. What is the contour interval of this map?
3. How high above sea level is the top of
Blueberry Hill?
4. Where is the lowest spot on the map?
5. Which direction does Scum River flow?

1. Which hill is steeper, Bob's Knob or Blueberry
Hill?
Bobs Knob because the contour lines are closer.
2. What is the contour interval of this map?
40 feet from the explanation at the bottom.
3. How high above sea level is the top of
Blueberry Hill?
4923 feet above sea level.
4. Where is the lowest spot on the map?
In the bottom of the Bland Canyon 4500 feet
5. Which direction does Scum River flow?
to the southeast because the contour lines "V's"
point northwest.

Contour lines can be used to reconstruct the
shape of the land from the topographic map. One
of the most geologically useful reconstructions
is called the topographic profile. The side of
the diagram shown in red is a topographic profile
of the western margin of the Bumluck area. It is
a silhouette of what the side of an area would
look like if you cut a giant vertical slice
through the earth.

42
Topographic Profiles

Profiles or cross-sections are constructed across
a contour map to provide a three-dimensional view
of the topography. The profile is actually a
silhouette of the hills and valleys as seen
against the sky. Topographic profiles are easily
made using graph paper and the method depicted in
this linked figure. Note that profiles need to
have both a horizontal scale and a vertical
scale.

A topographic profile can be constructed along
any arbitrary line drawn on the map. Say you
wanted to construct a topographic profile along a
line through Blueberry Hill. Let's call this line
A-A'.

Place a piece of paper so that its edge is on the
A-A' line. Mark the position of A and A' on the
piece of paper. Now make a tick at each point
where a contour line touches the edge of the
piece of paper and mark its elevation.

Now take your marked paper to another sheet of
paper and place it horizontally in the middle of
a new sheet of paper. Mark points A and A' on the
new sheet of paper by making a vertical line at
each point. Trace a horizontal line connecting
the vertical lines from A to A'. The horizontal
line should go along the edge of your ticked
piece of paper.

Now you need to scale the elevations on the
vertical lines at A-A'. But How? Note that there
is only 200' difference between the hightest an
lowest elevation that you encountered on the map
along your topographic profile.

But on the map, the length of the A-A' line is
nearly as long as the 2 mile bar scale. Two miles
is just over 10,000 feet. So the 200 feet of
elevation change is very small (2) of the
change in horizontal distance.

So if we tried to make the vertical scale on the
left piece of paper the same as the horizontal
scale of the map itself, we would barely be able
to see the difference. This problem is commonly
encountered in making cross sections because the
earth is sooooo large compared to the features on
it's surface. Indeed, even the might Himalaya
Range is but a slight wrinkle relative to the
size of the globe! So what do we do?

So we have 200 feet and about an inch to work
with (the length of the vertical lines at A and
A'). Let's arbitrarily let every 1/4 inch or so
equal 100 feet. This is called exaggerating the
vertical scale or vertical exaggeration. We can
calculate the exact amount of exaggeration but we
leave that for a more advanced class.

Place a dot at the elevation on your new vertical
scale that corresponds to the tick on your
horizontal scale (as in the diagram at left).
Now connect the dots. You have just drawn a
topographic profile. This is the first step in
accurately interpreting the subsurface geology.
We can then plot the location of rock bodies on
the topographic profile and use the information
given on the geologic map to predict where these
bodies will occur in the subsurface.