Introduction to
Topographic
Maps
Topographic Maps
• Two dimensional model of the Earth’s
surface (represents 3-D world)
• Topographic maps are also known as
contour maps.
• Show elevation above sea level using
contour lines.
“Real World”
Contour Map
Topographic Maps
• Contour Line –
- line on a map that connects points of EQUAL elevation.
- show elevation and shape of the land
• Relief – Difference between high and low elevations
Topographic Maps
• Contour Interval –
difference in elevation
between each line.
MUST be equal
spacing.
Contour interval =
20 feet
580
560
540
520
Topographic Maps
• Index Contour –
Usually every 5th line is printed darker and has
an elevation printed on it.
Rules for Contours
1. Contour lines never cross
Rules for Contours
2. Contours form closed loops (even if not
shown of the map.
Rules for Contours
3. Contours bend upstream (uphill) when
crossing a stream.
The direction of a river can be found on
a topographic map three ways:
1. Contour lines bend upstream- All rivers flow downstream.
2. Look at the elevation the river flows through. Rivers flow
downhill or from High elevation to a Low elevation.
3. Streams and rivers tend to flow into oceans or lakes
Mill River generally flows toward the
North East
Rules for Contours
4.
The maximum possible
elevation for a hill is “1”
less than what the next
contour “should” be.
90
The highest possible
elevation of the hill is
just below the value
of the next line that is
not shown
80
70
60
50
399
179
239
Closely Spaced Contours
• Steeper Slope (Gradient) – contour lines are
closer together.
Wide Spaced Contours
• Gradual/Gentle Slope (Gradient) – contour lines
are farther apart.
A
B
Depressions
• Contour lines which show a depression,
crater, or sinkhole on a map.
• Shown by dashed lines (hachure marks)
on the inside of a contour line
• The elevation of the first depression
contour is the same as the lowest
regular contour near it.
Rules for Contours
The lowest possible
elevation for a
depression is “1” more
than what the next
contour “should” be.
90
90
80
The lowest possible
elevation of a
depression is just
above the value of the
next line that is not
shown
51
70
60
50
What do
hachured
lines show?
Areas of
depression: such
as a lake or top
of volcano
What are the rules
concerning hachured
lines?
The 1st hachured
line has the same
elevation as the
previous contour
line
Benchmarks
• a location whose
exact elevation is
known and is noted
on a brass or
aluminum plate.
• benchmarks are
shown on maps by an
X with the letters BM
written next to them.
Map Scales
• Indicates the distance on the map compared to
distance in the real world
• Graphical - by a line divided into equal parts and
marked in units of length.
Let’s take a walk up a hill!
We’re now at an elevation of 100
meters.
100m
Let’s keep going!
100m
Now we’re at 200m.
200m
100m
Shall we march on?
200m
100m
We’ve made it to 300m!
300m
200m
100m
On to the peak!
300m
200m
100m
We’re on the peak, but what’s our
elevation?
300m
200m
100m
Any ideas?
Let’s add contour lines for
every 50 meters and see if
that helps.
300m
200m
100m
We know that we are above
350m, but less than 400m.
350m
300m
250m
200m
150m
100m
50m
Let’s head down the hill, it’s
getting late!
350m
300m
250m
200m
150m
100m
50m
Now what’s our elevation?
If you said somewhere
between 200m and 250m you
are right!
350m
300m
250m
200m
150m
100m
50m
Let’s try this again!
350m
300m
250m
200m
150m
100m
50m
What’s our elevation now?
If you said 50m or just under,
you’re right!
350m
300m
250m
200m
150m
100m
50m
Let’s
now look at the
same hill, but the
way we might see it
from an airplane!
Each color change represents a 50
meter increase.
Now, let’s try the same hike! Our
elevation is 0 meters.
Now what is our elevation?
If you said more than 150 meters,
but less than 200 meters your
right!
Let’s go a little higher.
Think you know our elevation
now?
More than 300meters
But less than 350meters
If we were standing on the peak,
what would be our elevation?
• More than 350 meters,
less than 400 meters
Let’s head down hill.
Know our elevation?
More than 100 meters,
less than 150 meters
Gradient
• The slope between any two points on a hill
• Gradient =
Change in Field Value
Distance
Gradient
• A trail is four miles long as measured by
the scale on a map. The beginning of the
trail is at the 1,060 ft contour line and the
end of the trail is at the 960 ft contour line.
Calculate the gradient of the trail.
Gradient =
1060 ft – 960 ft
4 miles
= 25.0 ft/mi
Gradient
Contour Interval = 20 ft
200
0
2
4
miles
6
8
10