Topographic Maps
Topographic Maps – show the topography, or the surface features of the earth.
-- Show natural features
* ex.: rivers, hills, valleys, etc.
-- Show constructed (man-made) features as well
* ex.: buildings, roads, major gas or power lines, etc.
*These maps will also give you information on the height (elevation) of the land, the steepness of the
slopes, and the shape of the land
====================================================================================
Elevation – the height of the land above sea level
-- it is measured from a point that is known as mean sea level
Mean Sea Level – the point midway between the highest & lowest tide levels of the ocean
-- the elevation at this point is 0 (zero)
-- it serves as the reference point for elev. [similar to the equator (lat.) or prime meridian (long.)]
Contour Lines – lines on a topographic map that connect points of equal elevation
-- when going from one contour line to the next the elevation changes (it either goes up or down)
-- the shape of the contour lines will reflect the shape of the land
Contour Interval – The difference in elevation between one contour line and the next
-- contour intervals may differ from one map to another ( 5 ft, 10 ft, 20 ft, 50 ft, 100ft )
-- the contour interval that is used is dependent upon the relief of the land
Relief – The difference in elevation between the highest and the lowest points of the area being mapped
• High Relief : very mountainous, great changes in elevation, large contour interval (ex. 50 ft)
• Low Relief : fairly flat, little change in elevation, small contour interval (ex. 5 ft)
===================================================================================
What does the topography of the right side of the island look like?
What does the topography of the left side of the island look like?
.B
0’
30’
60’ 90’
120’
150’
What is the contour interval of this map?
.A
What is the highest possible elevation found on the map?
What is the elevation of points A & B?
Index Contours – occur at every 5th contour line
-- Darker than the other contour lines (they stand out more and are easier to locate)
-- They are labeled with an elevation (they are used to determine the elevation of the other lines)
50’
Quadrangles – the name given to topographic map sheets. There are 2 main types of Quadrangles.
-- 15’ series
* This map shows 15’ of latitude and 15’ of longitude
* The information is reduced a great amount and as a result this type of map is less detailed
-- 7.5’ series
* This map shows 7.5’ of latitude and 7.5’ of longitude
* The information is not reduced very much and as a result this type of map has greater detail
Symbols used on Topographic Maps
-- Buildings that are occupied
x
BM 1321
-- Buildings that are not occupied
-- School
-- Church
BENCHMARK -- a reference point
on the map used for determining
elevations and establishing exact
locations for surveying purposes. It is a
brass marker that is embedded in a slab
on concrete with information stamped
into it.
Determining Landforms using Topographic Maps
* Closely spaced contour lines Æ rapid change in elevation (steep slope)
* Widely spaced contour lines Æ gradual change in elevation (gradual slope)
* Evenly spaced contour lines Æ The slope increases at the same angle over a great distance (the same slope)
Uniform, steep slope
Contour lines evenly spaced and close together indicate a uniform, steep slope.
The closer the contour lines are to each other, the steeper the slope
Uniform, gentle slope
Contour lines evenly spaced and wide apart indicate a uniform, gentle slope
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Anytime the contour lines form into a “ V – shape “ this indicates a valley.
(a) Valley; (b) Draw
A valley is a course of a stream that has at least a limited extent of reasonably
level ground bordered on the sides by higher ground. The valley generally has
maneuvering room within its confines.
Contours indicating a valley are V-shaped and tend to parallel a major stream
before crossing it. The more gradual the fall of a stream, the farther each contour
parallels it. The V-point of the contour crossing always points upstream.
A draw (sometimes called a gulley) is a less developed course of a stream in
which there is essentially no level ground and, therefore, little or no maneuvering
room within its confines.
The ground slopes upward on each side and toward the head of the draw. Draws
occur frequently along the sides of ridges at right angles to the valleys between
them. Contours indicating a draw are V-shaped, with the point of the V toward
the head (or top) of the draw.
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Often times (although not all the time) you will find a stream or a river in the bottom of the valley.
Two ways to determine the direction of the river flow:
-- Rivers will always flow from a higher elevation to a lower elevation (they will flow downhill).
-- The “ V – shape “ of the contour lines will be pointing upstream. So the river will flow
the opposite way that the point of the “ V “ is pointing.
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------
GRADIENT – The amount that the elevation drops between 2 points that is averaged out into feet per mile
For example: if the elevation drops 80 feet over 4 miles, the gradient would be 20 ft / mile
The gradient tells you the slope of the land between two points. It is calculated by dividing the relief by the path
distance. Path distance is the distance measured along the path travelled (NOT necessarily the straight line distance!).
For example, if you were planning a canoe trip, you may want to know the gradient between your starting point
upstream and your stopping point downstream. If your starting elevation was 50 feet above sea level, and your ending
elevation was 20 feet above sea level, the relief would be (50-20) or 30 feet. Although the straight line distance between
your starting and ending points may only be 2 miles, with the curvature of the river the actual path distance is greater--let's
say 3 miles. The gradient therefore = 30 feet / 3 miles = 10 feet per mile!
Hilltops – will be represented with the contour lines forming into smaller and smaller closed loops.
Contour Interval –
The Darker lines that are labeled with an elevation –
200
The max. elev. that could occur on this map would be –
100
Hill
This is a point or small area of high ground. When you are located
on a hilltop, the ground slopes down in all directions.
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Depressions – (holes in the ground) will be shown using hachure lines
The Rule for numbering hachure lines: When you go UP in elevation and go from a regular contour line
to a hachure line, the elevation will STAY THE SAME.
100
100
** Extra, more advanced topographic mapping information. (You do not need to study this info. for your test) **
Concave slope (you do not need to know this term)
Contour lines closely spaced at the top and widely spaced at the bottom indicate
a concave slope.
Considering relief only, an observer at the top of a concave slope can observe the
entire slope and the terrain at the bottom or concealment from observers or
weapons at or near the top; also, farther up the slope, the climb would be more
difficult. However, a unit attacking up a concave slope would have no cover
Convex slope (you do not need to know this term)
Contour lines widely spaced at the top and closely spaced at the bottom indicate a
convex slope.
An observer at the top of a convex slope would have no observation of most of the
slope or of the terrain at the bottom. But a unit attacking up a concave slope
would have a much greater degree of cover and concealment than on a concave
slope; also, the climb farther up the slope would be easier.
Cliff
A cliff is a vertical or near vertical slope.
When a slope is so steep that it cannot be shown at the contour interval without
the contours coming together, it is shown by a ticked "carrying" contour or
contours. The ticks always point toward lower ground.
(a) Ridge; (b) Spur
RIDGE -- Normally, a ridge is a line of high ground with minor variations along its
crest. The ridge is not simply a line of hills; all points of the ridge crest are
appreciably higher than the ground on both sides of the ridge.
SPUR -- A spur is a short, continuously sloping line of higher ground normally
jutting out from the side of a ridge.
A spur is often formed by two roughly parallel streams cutting draws down the side
of a ridge.
Saddle
SADDLE -- A saddle is a dip or low point along the crest of a ridge.
A saddle is not necessarily the lower ground between two hilltops; it maybe
simply a dip or break along an otherwise level ridge crest.